How Smell, Taste & Pheromone-Like Chemicals Control You

- Welcome to the Huberman Lab Podcast,

where we discuss science

and science-based tools for everyday life.

[gentle music]

I'm Andrew Huberman,

and I'm a Professor of Neurobiology and Ophthalmology

at Stanford School of Medicine.

This podcast is separate

from my teaching and research roles at Stanford.

It is however, part of my desire and effort

to bring zero cost to consumer information about science

and science-related tools to the general public.

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This month, we've been talking about the senses,

how we detect things in our environment.

The last episode was all about vision,

how we take light and convert that information

into things that we can perceive

like colors, and faces, and motion,

things of that sort

as well as how we use light to change our biology

in ways that are subconscious that we don't realize,

things like mood, and metabolism, and levels of alertness.

Today, we're going to talk about chemical sensing,

we're going to talk about the sense of smell,

our ability to detect odors in our environment.

We're also going to talk about taste,

our ability to detect chemicals

and make sense of chemicals that are put in our mouth

and into our digestive tract.

And we are going to talk about chemicals

that are made by other human beings

that powerfully modulator the way that we feel,

our hormones, and our health.

Now, that last category are sometimes called pheromones.

However, whether or not pheromones exist in humans

is rather controversial,

there actually hasn't been a clear example

of a true human pheromonal effect,

but what is absolutely clear, what is undeniable

is that there are chemicals that human beings make

and release in things like tears onto our skin,

and sweat, and even breath that powerfully modulate

or control the biology of other individuals.

In fact right now, even if you're completely alone,

your chemical environment internally is being controlled

by external chemicals,

your nervous system, and your hormones, and your metabolism

are being modified by things in your environment,

so we're going to talk about those.

It's an absolutely fascinating aspect to our biology,

it's one of our most primordial,

meaning primitive aspects of our biology,

but it's still very active in all of us today.

This episode, believe it or not,

will have a lot of tools, a lot of protocols.

Even though I'm guessing most of you

can probably smell your environment just fine,

that you know what you like to eat and what tastes good,

and what doesn't taste good to you,

today's episode is going to talk about tools

that will allow you to actually leverage

these chemical sensing mechanisms,

including how you smell

not how you smell in the qualitative sense,

but how you smell in the verb sense,

the action of sniffing and smelling

to enhance your sense of smell

and to enhance your sense of taste as well,

believe it or not, to enhance your cognition,

your ability to learn and remember things.

Everything we're going to talk about as always

is grounded in quality peer-reviewed studies

from some excellent laboratories,

I'll provide some resources along the way,

so that means tools and protocols

and also basic information.

You're going to learn a ton of neuroscience

and lot of biology in general.

And I think what you'll come to realize by the end

is that while we are clearly different

from the other animals,

there are aspects to our biology that are very similar

to that of other animals in very interesting ways.

Before we dive into chemical sensing,

I want to just briefly touch on a few things

from the vision episode.

One is a summary of a protocol.

So, I covered 13 protocols last episode,

if you haven't seen that episode, check it out.

Those protocols will allow you to be more alert

and to see better over time if you follow them.

All of them are zero cost,

you can find any and all of them at hubermanlab.com,

there's a link to those videos and tools and protocols,

everything is timestamped.

The two protocols that I just want to remind everybody of

are the protocol of near-far viewing that all of us

regardless of age, should probably spend about five minutes

three times a week, doing some near-far viewing exercises.

So, that would be bringing a pen or pencil

up close to the point where you're about to cross your eyes,

but you don't cross your eyes

and then out at some distance.

And then look beyond that pen

or other object that you're using

off as far as you can into the distance.

It would be great if you could do this on a balcony or deck

and then look way off in the distance

and then bring it back in.

This is going to exercise that accommodation reflex,

the change in the shape of the lens

can help offset a number of things

including myopia, near-sightedness.

The other one is this incredible study

that showed that two hours a day outside,

even if you're doing other things while you're outside

can help offset myopia, nearsightedness.

So, try and get outside,

it's really the sunlight and the blue light, right?

Everyone's been demonizing blue light out there,

but blue light is great

provided it's not super, super bright

and really close to your eyes.

Blue is terrific if it comes from sunlight.

Two hours a day outside

is going to help offset myopia, nearsightedness.

Now, that's a lot of time,

I think most of us are not getting that time,

but since you can do other things like gardening,

or reading, or walking, or running.

If you can get that two hours outside

your visual system and your brain will benefit.

I also would like to make one brief correction

to something that I said incorrectly

in the previous episode.

At the end of the episode, I talked about lutein,

and how lutein may help offset

some moderate to severe age-related macular degeneration.

As well, I talked about how some people

are supplementing with lutein

even though they don't have age-related macular degeneration

with the idea in mind

that it might help offset some vision loss

as they get older.

I said lutein, and lutein was the correct thing to say,

but once or twice, when I started speaking fast

I said leucine and not lutein.

I want to emphasize that leucine,

an amino acid, very interesting,

important for muscle building covered in previous episodes,

but lutein, L-U-T-E-I-N,

is the molecule and compound that I was referring to

in terms of supplementing for sake of vision.

So I apologize, please forgive me I misspoke,

a couple of you caught that right away,

in listening to the episode after it went up

I realized that I had misspoken.

So, lutein for vision,

leucine for muscles, and muscle growth,

and strength, et cetera.

Before we dive into the content of today's episode,

I want to just briefly touch on color vision.

Many of you asked questions about color vision

and color perception.

And indeed color perception is a fascinating aspect

of the human visual system,

it's one of the things that makes us unique.

There are certainly other animals out there

that can detect all the colors of the rainbow,

some can even detect into the infrared, into the far-red

that we can't see,

but nonetheless, human color vision,

provided that somebody isn't colorblind,

is really remarkable.

And if you're interested in color vision

or you want to answer questions

about art or about for instance

why that dress that showed up online a few years ago

looks blue to you and yellow to somebody else.

All the answers to that are in this terrific book

which is "What Is Color?:

50 Questions and Answers on the Science of Color".

I did not write this book, I wish I had,

the book is by Arielle and Joann Eckstut,

that E-C-K-S-T-U-T.

So, it's "What Is Color?:

50 Questions and Answers on the Science of Color".

It's an absolutely fabulous book,

I've no business relationship to them.

I did help them get in contact

with some color vision scientists

when they reached out to me.

And you can know that all the information in the book

was vetted by excellent color vision scientists.

It's a really wonderful and beautiful book,

the illustrations are beautiful.

If you're somebody who's interested in design or art,

or you're just curious about the science of color,

it's a terrific book, I highly recommend it.

If you just look it up online,

there are a variety of places

that will allow you to access the book.

So, let's talk about sensing chemicals

and how chemicals control us.

In our environment,

there are a lot of different physical stimuli.

There is light,

photons, which are light energy

and those land on your retinas

and your retinas tell your brain about them,

and your brain creates this thing we call vision.

There are sound waves,

literally particles moving through the air

and reverberations that create

what we call sound and hearing.

And of course there are mechanical stimuli,

pressure, light touch, scratch,

tickle, et cetera, that lands on our skin

or the blowing of a breeze

that deflects the hairs on our skin,

and we can sense mechanical touch, mechanical sensation.

And there are chemicals,

there are things floating around in the environment

which we call volatile chemicals.

So, volatile sounds oftentimes like emotionally volatile,

but it just means that they're floating around out there.

So, when you actually smell something

like let's say you smell

a wonderfully smelling rose or cake.

Yes, you are inhaling the particles into your nose,

they're literally little particles

of those chemicals are going up into your nose

and being detected by your brain.

Also, if you smell something putrid,

disgusting, or awful, use your imagination,

those particles are going up into your nose

and being detected by neurons that are part of your brain.

Other ways of getting chemicals into our system

is by putting them in our mouth,

by literally taking foods and chewing them,

or sucking on them and breaking them down

into their component parts,

and that's one way that we sense chemicals

with this thing, our tongue.

And there are chemicals that can enter

through other mucosal linings

and other kind of just think damp,

sticky linings of your body.

And the main ones would be the eyes,

so you've got your nose, your eyes and your mouth.

But mainly when we have chemicals coming into our system

it's through our nose or through our mouth.

Although, sometimes through our skin

certain things can go transdermal, not many,

and through our eyes.

So these chemicals,

we sometimes bring into our body, into our biology,

through deliberate action.

We select a food, we chew that food,

and we do it intentionally.

Sometimes they're coming into our body

through non-deliberate action.

We enter an environment,

and there's smoke and we smell the smoke,

and as a consequence we take action.

Sometimes we are forced to eat something

because somebody tells us we should eat it

or we do it to be polite.

So, there are all these ways

that chemicals can make it into our body.

Sometimes however, other people

are actively making chemicals with their body,

typically this would be with their breath,

with their tears,

or possibly, I want to underscore possibly,

by making what are called pheromones,

molecules that they release into the environment

typically through the breath

that enter our system through our nose,

or our eyes, or our mouth

that fundamentally change our biology.

I will explain how smell and taste

and these pheromone effects work,

but I'll just give an example,

which is a very salient and interesting one

that was published about 10 years ago

in the Journal Science.

Science Magazine is one of the three

what we call apex journals.

There are a lot of journals out there,

but for those of you that want to know,

Science Magazine, Nature Magazine, and Cell

are considered the three top kind of apex journals,

they are the most stringent

in terms of getting papers accepted,

they're even reviewed there.

They have about a 95% rejection rate at the front gate,

meaning they don't even review 95%

of what gets sent to them.

Of the things that they do decide to review

then get sent out,

a very small percentage of those get published,

it's very stringent.

This paper came out in Science

showing that humans, men in particular in this study,

have a strong biological response and hormonal response

to the tears of women.

What they did is they had women,

and in this case it was only women for whatever reason,

cry and they collected their tears.

Then those tears were smelled by male subjects,

or male subjects got what was essentially the control,

which was the saline.

Men that smelled these tears that were evoked by sadness

had a reduction in their testosterone levels

that was significant.

They also had a reduction in brain areas

that were associated with sexual arousal.

Now, before you run off with your interpretations

about what this means and criticize the study

for any variety of reasons, let's just take a step back.

I will criticize the study for a variety of reasons too.

One is that they only used female tiers and male subjects,

so it would have been nice for them to also use female tears

and female subjects smelling those,

male tears and male subjects smelling those,

male tears and female subjects smelling those, and so on.

They didn't do that,

they did have a large number of subjects,

so that's good, that adds power to the study.

And they did have to collect these tears

by having the women watch what was essentially

a sad scene from a movie.

They actually recruited subjects that had a high propensity

for crying at sad movies, which was not all women.

It turns out that the people that they recruited

for the study were people who said, "Yes,

I tend to cry when I see sad things in movies."

What they're really trying to do

is get just get tears that were authentically cried

in response to sadness,

as opposed to putting some irritant in the eye

and collecting tears that were evoked by something else

like just having the eyes irritated.

Nonetheless, what this study illustrates

is that there are chemicals in tears

that are evoking or changing the biology

of other individuals.

Now, most of us don't think about sniffing

or smelling other people's tears,

but you can imagine how in close couples,

or in family members,

or even close friendships, et cetera,

that we are often in close proximity

to other people's tears.

Now, I didn't select this study as an example

because I want to focus on the effects

of tears on hormones, per se,

although I do find the results really interesting.

I chose it because I wanted to just emphasize

or underscore the fact

that chemicals that are made by other individuals

are powerfully modulating our internal state,

and that's something that most of us don't appreciate.

I think most of us can appreciate the fact

that if we smell something putrid, we tend to retract,

or if we smell something delicious, we tend to lean into it.

But there are all these ways in which chemicals

are affecting our biology,

and interpersonal communication using chemicals

is not something that we hear that often about,

but it's super interesting.

So, let's talk about smell

and what smell is and how it works.

I'm going to make this very basic,

but I am going to touch on some of the core elements

of the neurobiology.

So, here's how smell works.

Smell starts with sniffing.

Now, that may come as no surprise,

but no volatile chemicals can enter our nose

unless we inhale them.

If our nose is occluded, or if we're actively exhaling

it's much more difficult for smells to enter our nose,

which is why people cover their nose

when something smells bad.

Now, the way that these volatile odors

come into the nose is interesting.

The nose has a mucosal lining,

mucus that is designed to trap things,

to actually bring things in and get stuck there.

At the base of your brain,

so you could actually imagine this

or if you wanted, you could touch the roof of your mouth.

So, right above the roof of the mouth,

about two centimeters is your olfactory bulb.

The olfactory bulb is a collection of neurons

and those neurons actually extend out of the skull,

out of your skull into your nose

into the mucosal lining.

So, what this means in kind of a literal sense

is that you have neurons that extend

their little dendrites and axon-like things,

they're little processes as we call them,

out into the mucus,

and they respond to different odorant compounds.

Now, the olfactory neurons

also send a branch deeper into the brain

and they split off into three different paths.

So, one path is for what we call innate odor responses,

so you have some hard wired aspects

to the way that you smell the world

that were there from the day you were born

and that will be there until the day you die.

These are the pathways and the neurons that respond

to things like smoke,

which as you can imagine there's a highly adaptive function

to being able to detect burning things

because burning things generally means lack of safety

or impending threat of some kind.

It calls for action,

and indeed these neurons project

to the central area of the brain called the amygdala,

which is often discussed in terms of fear,

but it's really fear and threat detection.

So some compounds, some chemicals in your environment

when you smell them, unless you're trained to overcome them

because you're a firefighter

you will naturally have a heightened level of alertness,

you will sense threat,

and if you're in sleep, even it will wake you up.

All right, so that's a good thing,

it's kind of an emergency system.

You also have neurons in your nose

that respond to odorants

or combinations of odorants that evoke a sense of desire

and what we call appetitive behaviors, approach behaviors,

that make you want to move toward something.

So, when you smell a delicious cookie,

or some dish that's really savory that you really like,

or a wonderful orange,

and you say, "Mmm,"

or it feels delicious, or it smells delicious

that's because of these innate pathway,

these pathways that require no learning whatsoever.

Now, some of the pathways from the nose,

these olfactory neurons into the brain

are involved in learned associations with odors.

Many people have this experience

that they can remember the smell

of their grandmother's home,

or their grandmother's hands even,

or the smell of particular items baking,

or on the stove in a particular environment.

Typically, these memories tend to be of a kind of nurturing

sort of feeling safe and protected.

But one of the reasons why olfaction smell

is so closely tied to memory

is because olfaction is the most ancient sense that we have,

or I should say chemical sensing is among the most primitive

and ancient senses that we have,

probably almost certainly evolved

before vision and before hearing.

But when we come into the world

because we're still learning about the statistics of life

about who's friendly and who's not friendly,

and where's a fun place to be

and where's a boring place to be,

that all takes a long time to learn.

But the olfactory system seems to imprint,

seems to lay down memories very early

and create these very powerful associations.

And if you think about it long enough and hard enough

many of you can probably realize

that there are certain smells that evoke a memory

of a particular place, or person, or context.

And that's because you also have pathways out of the nose

that are not for innate behaviors like cringing,

or repulsion, or gagging,

or for that appetitive mmm sensation,

but that just remind you of a place,

or a thing, or a context,

could be flowers in spring,

could be grandmother's home and cookies.

This is a very common occurrence,

and it's a very common occurrence

because this generally exists in all of us.

So, we have pathway for innate responses

and a pathway for learned responses.

And then we have this other pathway,

and in humans it's a little bit controversial

as to whether or not it sits truly separate

from the standard olfactory system

or whether or not it's its own system embedded in there,

but that they call the accessory olfactory pathway.

Accessory olfactory pathway is what in other animals

is responsible for true pheromone effects.

We will talk about true pheromone effects,

but for example in rodents

and in some primates, including mandrills.

If you've ever seen a mandrill,

they have these like big beak noses things,

you may have seen them at the zoo,

look them up if you haven't seen them already,

M-A-N-D-R-I-L-S

mandrills, there are strong pheromone effects.

Some of those include things like

if you take a pregnant female rodent or mandrill,

you take away the father that created those

fetuses or fetus,

and you introduce the scent of the urine

or the fur of a novel male,

she will spontaneously abort or miscarry those fetuses,

it's a very powerful effect.

In humans, it's still controversial

whether or not anything like that can happen,

but it's a very powerful pheromonal effect in other animals.

Another example of a pheromone effect

is called the Vandenbergh effect

named after the person who discovered this effect,

where you take a female

of a given species that has not entered puberty,

you expose her to the scent or the urine

from a sexually competent, meaning post-pubertal male,

and she spontaneously goes into puberty earlier.

So, something about the scent

triggers something through this accessory olfactory system,

this is a true pheromonal effect

and creates ovulation, right and menstruation.

Or in rodents it's an estrous cycle, not a menstrual cycle.

So, this is not to say that the exact same things

happen in humans.

In humans, as I mentioned earlier,

there are chemical sensing between individuals

that may be independent of the nose.

And we will talk about those,

but those are basically the three paths

by which smells, odors impact us.

So, I want to talk about the act of smelling,

and if you are not somebody who is very interested in smell,

but you are somebody who is interested

in making your brain work better,

learning faster, remembering more things,

this next little segment is for you

because it turns out that how you smell,

meaning the act of smelling, not how good or bad you smell,

but the act of smelling,

sniffing, and inhalation powerfully impacts

how your brain functions

and what you can learn and what you can't learn.

Breathing generally consists of two actions,

inhaling and exhaling,

and we have the option of course

to do that through our nose or our mouth.

I've talked on previous episodes about the fact

that there are great advantages to being a nasal breather,

and there are a great disadvantages

to being a mouth breather.

There are excellent books and data on this,

there's the recent book "Breath" by James Nestor,

which is an excellent book

that describes some of the positive effects

of nasal breathing as well as other breathing practices.

There's also the book "Jaws" by my colleagues,

Paul Ehrlich and Sandra Kahn,

with a foreword by Jared Diamond

and an introduction by Robert Sapolsky from Stanford.

So, that's a book chockablock

with heavy hitter authors that describes

how being a nasal breather is beneficial for jaw structure,

for immune system function, et cetera

Breathing in through your nose,

sniffing actually has positive effects

on the way that you can acquire and remember information.

Noam Sobel's group originally at UC Berkeley

and then at the Weizmann Institute

has published a number of papers

that I'd like to discuss today.

One of them, Human Non-Olfactory Cognition

Phase-Locked with Inhalation,

this was published in Nature Human Behavior,

an excellent journal.

Showed that the act of inhaling

[Andrew inhales deeply]

has a couple of interesting and powerful consequences.

First of all, as we inhale

the brain increases in arousal,

our level of alertness and attention increases

when we inhale as compared to when we exhale.

Now, of course with every inhale, there's an exhale,

you could probably double up on your inhales

if you're doing size or something,

physiological size I've talked about these before,

so double inhales [inhales twice]

followed by an exhale [exhales],

something like that.

Or if you're speaking, you're going to change your cadence

and ratio of inhales and exhales,

but typically we inhale, then we exhale.

As we inhale, what this paper shows

is that the level of alertness goes up in the brain,

and this makes sense because as the most primitive

and primordial sense by which we interact

with our environment and bring chemicals into our system

and detect our environment,

inhaling is a cue for the rest of the brain

to essentially to pay attention to what's happening,

not just to the odors

as the name of this paper suggests,

Human Non-Olfactory Cognition Phase-Locked with Inhalation.

What that means is that the act of inhaling itself

wakes up the brain,

it's not about what you're perceiving

or what you're smelling.

And indeed sniffing as an action,

inhaling as an action has a powerful effect

on your ability to be alert,

your ability to attend, to focus,

and your ability to remember information.

When we exhale, the brain goes through a subtle,

but nonetheless significant dip

in level of arousal and ability to learn.

So, what does this mean?

How should you use this knowledge?

Well, you could imagine,

and I think this will be beneficial for most people

to focus on nasal breathing

while doing any kind of focused work that doesn't require

that you speak, or eat, or ingest something.

There's a separate paper published

in the journal of neuroscience that showed

that indeed if subjects, human subjects,

are restricted to breathing through their nose,

they learn better than if they have the option

of breathing through their mouth,

or a combination of their nose and mouth.

These are significant effects

in humans using modern techniques from excellent groups.

So, sniffing itself is a powerful modulator

of our cognition and our ability to learn.

You can imagine all sorts of ways

that you might apply that as a tool.

And I suggest that you play with it a bit

that if you're having a hard time staying awake and alert,

you're having a hard time remembering information,

you feel like you have a kind of attention deficit,

nonclinical of course,

nasal breathing ought to help,

extending or making your inhales more intense ought to help.

Now, this isn't really about chemical sensing per se,

but here's where it gets interesting and exciting.

If you are somebody

who doesn't have a very good sense of smell,

or you're somebody who simply wants to get better

at smelling and tasting things,

you can actually practice sniffing.

I know that sounds ridiculous,

but it turns out that simply sniffing nothing.

So, doing something like this.

[Andrew sniffs deeply]

I guess the microphone sort of has a smell [sniffs],

I guess my pen doesn't have a smell.

[Andrew sniffs deeply]

It turns out that doing a series of inhales,

and of course each one is followed by an exhale,

10 or 15 times and then smelling an object

like an orange or another item of food,

or even the skin of somebody else

will lead to an increase in your ability

to perceive those odors.

Now, there are probably two reasons for that.

One reason is that the brain systems

of detecting things are waking up

as a mere consequence of inhaling.

Okay, so this is sort of the olfactory equivalent

of opening your eyes wider in order to see, more or less.

Okay, last episode I talked about

how opening your eyes wider

actually increases your level of alertness,

it's not just that your level of alertness

causes your eyes to be open wider.

Opening your eyes wider

can actually increase your level of alertness.

Well, it turns out that breathing more deeply

through the nose, wakes up your brain

and it creates a heightened sensitivity

of the neurons that relate to smell.

And there's a close crossover,

I'm sure you know this, between smell and taste.

If any of you have ever had a cold

or you have for whatever reason

you've lost your sense of smell,

you become what they call anosmic,

your sense of taste suffers also.

We'll talk a little bit more

about why that is in a few minutes,

but as a first protocol,

I'd really like all of you to consider

becoming nasal breathers

while you're trying to learn, while you're trying to listen,

while you're trying to wake up your brain in any way

and learn and retain information,

this is a powerful tool.

Now, there are other ways

to wake up your brain more as well.

For instance, the use of smelling salts.

I'm not recommending that you do this necessarily,

but there are excellent peer reviewed data

showing that indeed, if you use smelling salts,

which are mostly of the sort that include ammonia,

ammonia is a very toxic scent,

but it's toxic in a way that triggers this innate pathway,

the pathway from the nose to the amygdala,

and wakes up the brain and body in a major way.

This is why they use smelling salts

when people pass out,

this is why fighters used to use

or maybe sometimes still use smelling salts

in order to heighten their level of alertness,

this is why powerlifters will inhale smelling salts.

They work because they trigger the fear

and kind of overall arousal systems of the brain,

this is why I think most people probably shouldn't

use ammonia or smelling salts to try and wake up,

but they really do work.

If you've ever smelled smelling salts

and I have, I tried this,

they give you a serious jolt,

it's like six espresso

infused into your bloodstream all at once,

you are wide awake immediately

and you feel a heightened sense of kind of desire to move

because you release adrenaline into your body.

Now, inhaling through your nose

and doing nasal breathing is not going to do that,

it's going to be a more subtle version of waking up your system

of alerting your brain overall.

And for those of you that are interested in having a richer,

a more deep connection

to the things that you smell and taste,

including other individuals perhaps not just food,

practicing or enhancing your sense of sniffing,

your ability to sniff

might sound like a kind of ridiculous protocol,

but it's actually a kind of fun and cool experiment

that you can do.

You just do the simple experiment of taking for instance

an orange, you smell it,

try and gauge your level of perception

of how orange-ish it smells,

or lemony, lemonish, lemony, I don't know

is it lemonish or lemony?

Lemony it smells, then set it away,

do 10 or 15 inhales

[Andrew inhales and exhales]

followed by exhales of course,

or just through the nose.

[Andrew breaths rapidly]

I'm not going to do all 10 or 15.

And then smell it again,

and you'll notice that your perception of that smell,

the kind of richness of that smell

will be significantly increased.

And that's again, for two reasons,

one, the brain is in a position to respond to it better,

your brain has been aroused by the mere act of sniffing,

but also the neurons that respond

to that lemon odor, that lemony or odor

are going to respond better.

So, you can actually have a heightened experience

of something, and that of course will also be true

for the taste system.

You also can really train your sense of smell

to get much, much better.

When Noam Sobel's group was at Berkeley

I happened to be a graduate student around that time,

and every once in a while I'd look outside

and there would be people crawling around on the grass

with goggles on, gloves on,

and these hoods on with earmuffs.

And they looked ridiculous,

but what they were doing is they were actually

learning to follow scent trails.

So, in the world of dogs

you have sight hounds that use their eyes

in order to navigate and find things,

and you have scent towns that use their nose.

And the scent hounds are remarkable,

they can be trained to detect a scent.

These are the sniffing,

you know, the bomb sniffing

and the drug sniffing dogs in airports.

There are now dogs actually that can sniff

out COVID infections with a very high degree of accuracy,

they can be trained to do that.

There's something about the COVID

and similar infections that the body produces

probably in the immune response,

some odors and the dogs are I think as high as 90%

in some cases, maybe even 95% accuracy,

just remarkable.

There are theories that dogs can sniff out cancer,

this stuff all exceeds statistical significance.

It's still a little bit mysterious in some ways,

but you may not ever achieve

the olfactory capabilities of a scent hound,

but what Noam Sobel's lab did is they had people

completely eliminate their visual experience

by having them wear dark glasses or goggles,

so they couldn't see, and they couldn't hear,

they couldn't sense anything with their sense of touch,

they had thick gloves on.

But they had these masks on

where just their nasal passages were open

and people could in a fairly short amount of time

learn to follow a chocolate scent trail on the ground,

which is not something that most people want to do,

but what they showed using brain imaging, et cetera

in subsequent studies is that the human brain,

you can learn to really enhance your sense of smell

and become very astute in distinguishing

whether or not one particular odor

or combinations of odors

is such that it's less than,

or more than a different odor for instance.

Now, why would you want to do this?

Well, if you like to eat as much as I do,

one of the things

that can really enhance your sense of pleasure

from the experience of ingesting food

is to enhance your sense of smell.

And if you don't have a great sense of smell,

or if you have a sense of smell

that's really so good that it's always picking up bad odors,

we'll talk about that in a minute.

Well, then you might want to tune up your sense of smell

by doing this practice of 10 or 15 breaths,

excuse me, sniffs, not breaths,

sniffs and then interacting with some food item

or thing that you're interested in smelling more of.

So, these could be the ingredients that you're cooking with,

I really encourage you to try and really smell them.

You sometimes hear this

as kind of a mindfulness practice

like ooh, really smell the food, really taste the food.

And we always hear about that

as kind of a mindfulness and presence thing,

but you actually can increase the sensitivity

of your olfactory and your taste system by doing this.

And it has long-term effects,

that's what's so interesting.

This isn't the kind of thing

that you have to do every time you eat.

You don't have to be the weirdo in the restaurant

that's like picking up the radish

and like jamming it up your nostrils, please don't do that.

You don't have to necessarily smell everything,

although it's nice sometimes to smell the food

that you're about to eat and as you eat it,

but it has long-term effects

in terms of your ability to distinguish

and discriminate different types of odors.

And these don't even have to be very pungent foods

it turns out,

the studies show that doesn't have

to be some really stinky cheese,

you know, there are cheese shops

that I've walked into where like I just basically gag,

I can't handle it, I just can't be in there,

it just overwhelms me.

Other people, they love that smell.

So, you have to tune it to your interest and experience,

but I think even for you fasters out there,

everybody eats at some point,

everybody ingests chemicals through their mouth.

And one of the ways that you can powerfully increase

your relationship to that experience

and make it much more positive

is through just the occasional practice

of 10 or 15 sniffs of nothing,

which almost sounds ridiculous like how could that be?

But now, you understand why,

it's because of the way that the sniffing action increases

the alertness of the brain

as well as increasing the sensitivity of the system.

No other system that I'm aware of in our body

is as amenable to these kinds of behavioral training shifts

and allow them to happen so quickly.

I would love to be able to tell you

that just doing 10 or 15 near-far exercises with a pen

or going outside for 10 or 15 seconds each morning

is going to completely change

the way that you see the world.

But it actually isn't the case,

you actually, it requires more training,

a little bit more effort in the visual system.

In the olfactory system,

and your smell system, and in your taste system

just the tiniest bit of training and attention,

and sniffing, inhaling can radically change

your relationship to food

such that you actually start to feel very different

as a consequence of ingesting those foods

as well as becoming more discerning about which foods

you like and which ones you don't like.

And we're going to talk about that

because there's a really wonderful thing

that happens when you start developing a sensitive palate

and a sensitive sense of smell

in a way that allows you to guide your eating

and smelling decisions,

and maybe even interpersonal decisions

about who you spend time with, or mate with, or whatever,

in a way that is really in line with your biology.

In fact, how well we can smell and taste things

is actually a very strong indication of our brain health.

Now, that's not to say

that if you have a poor sense of smell

or a poor sense of taste, that you're somehow brain damaged

or you're going to have dementia,

although sometimes early signs of dementia

or loss of neurons in other regions of the brain

related to say Parkinson's can show up first

as a loss of sense of smell.

Again, it's not causal,

and it's certainly not the case that every time

you have a sudden loss of smell

that there's necessarily brain damage,

I want to be very about that,

but they are often correlated.

There's also a lot of interest right now

in loss of sense of smell

because one of the early detection signs of COVID-19

was a loss of sense of smell.

So, I just briefly want to talk about loss of sense of smell

and regaining sense of smell and taste

because these have powerful implications for overall health.

And in fact can indicate something about brain damage

and can even inform how quickly we might be recovering

from something like a concussion.

So, our olfactory neurons,

these neurons in our nose that detect odors

are really unique among other brain neurons

because they get replenished throughout life,

they don't just regenerate, but they get replenished.

So regeneration is when something is damaged and it regrows,

these neurons are constantly turning

over throughout our lifespan,

they're constantly being replenished,

they're dying off and they're being replaced by new ones.

This is an amazing aspect of our brain

that's basically unique to these neurons,

there's one other region of the brain

where there's a little bit of this maybe,

but these olfactory neurons

about every three or four weeks they die.

And when they die, they're replaced

by new ones that come from a different region of the brain,

a region called the subventricular zone.

The name isn't as important,

but as the phenomenon,

but these neurons are born in the ventricle,

the area of your brain that's a hole that contains...

It's not an empty hole,

it's a hole basically that contains cerebral spinal fluid.

Well, there's a little subventricular zone,

there's a little zone below, sub ventricles.

And that zone, if you are exercising regularly,

if your dopamine levels are high enough,

those little cells there are like stem cells.

They are stem cells and they spit out

what are called little neuroblasts,

those little neuroblasts migrate

into the front of your brain and then shimmy,

they kind of move through

what's called the rostral migratory stream.

They kind of shimmy along

and land back in your olfactory bulb,

settle down and extend little wires

into your olfactory mucosa.

This is an ongoing process of what we call neurogenesis

or the birth of new neurons.

Now, this is really interesting

because other neurons in your cortex, in your retina,

in your cerebellum, they do not do this,

they are not continually replenished throughout life.

But these neurons, these olfactory neurons are,

they are special.

And there are a number of things

that seem to increase the amount

of olfactory neuron neurogenesis.

There is evidence that exercise, blood flow,

can increase olfactory neuron neurogenesis.

Although, those data are fewer

in comparison to things like social interactions,

or actually interacting with odorants of different kinds.

So, if you're somebody who doesn't smell things well,

you have a poor sense of smell,

your olfactory system doesn't seem very sensitive,

more sniffing, more smelling is going to be good.

And then the molecule dopamine, this neuromodulator,

that is associated with motivation and drive.

And in some cases, if it's very, very high with mania,

or if it's very, very low with depression or Parkinson's,

but for most people where dopamine

is in essentially normal ranges

dopamine is also a powerful trigger of the establishment

of these new neurons and their migration

into the olfactory bulb and your ability to smell.

Now, you don't want to confuse correlation with causation,

so if you're not good at smelling

does that mean you have low dopamine?

No, not necessarily.

If you have low dopamine,

does that mean that you have a poor sense of smell?

No, not necessarily.

Some people who take antidepressants of the sort

that impact the dopamine system strongly like Wellbutrin

will report a sudden, meaning within a couple of days,

increase in their ability to smell particular odors,

and it's a very striking effect.

Some people when they are in a new relationship

because dopamine and the hormones,

testosterone and estrogen are associated with novelty

and the sorts of behaviors that often are associated

with new relationships those three molecules;

dopamine, testosterone, and estrogen kind of work together.

And oftentimes people will say

or report when they're newly in love

or in a new relationship that they're just obsessed with,

or they just so enjoy the scent of another person

so much so that they like to borrow

the other person's clothing

or they'll sniff the other person's clothing

or they can even just in the absence of the person

they can imagine their smell and feel a biological response,

something that we'll talk more about.

So, these neurons turnover throughout the lifespan

and as we age, we actually can lose our sense of smell.

And it's likely, I want to underscore likely,

that that loss of sense of smell as we age

is correlated with a loss of other neurons

in the retina, in the ears, a loss of vision,

loss of hearing, loss of smell, loss of the sense apparati

which our neurons is correlated with aging.

So, what we've been talking about today

is the ability to sense these odors,

but what I'd like to do is empower you with tools

that will allow you to keep these systems tuned up.

Last time, we talked about tuning up

and keeping your visual system tuned up and healthy

regardless of age.

Here, we're talking about really enhancing

your olfactory abilities, your taste abilities as well

by interacting a lot with odors, preferably positive odors,

and sniffing more, inhaling more,

which almost sounds crazy,

but now you understand why.

Even though it might sound crazy

it's grounded in real mechanistic biology

of how the brain wakes up and responds to these chemicals.

Now, speaking of brain injury,

olfactory dysfunction is a common theme

in traumatic brain injury for the following reason,

these olfactory neurons as I mention

extend wires into the mucosa of the nose,

but they also extend a wire up into the skull.

And they extend up into the skull

through what's called the cribriform plate,

it's like a Swiss cheese type plate

where they're going through.

And if you get a head hit,

that bone, the cribriform plate,

sheers those little wires off

and those neurons die.

Now, eventually they'll be replaced,

but there's a phenomenon by which concussion

and the severity of concussion

and the recovery from a head injury

can actually be gauged in part,

in part, not in whole,

but in part by how well or fully one recovers

their sense of smell.

So, if you're somebody that unfortunately

has suffered a concussion,

your sense of smell is one readout

by which you might evaluate whether or not you're regaining

some of your sensory performance.

Of course, there will be others

like balance, and cognition, and sleep, et cetera.

But I'd like to refer you to a really nice paper

which is entitled Olfactory Dysfunction

in Traumatic Brain Injury: the Role of Neurogenesis

the first author is Marin, M-A-R-I-N.

The paper was published

in Current Allergy and Asthma Report, this is 2020.

I spent some time with this paper, it's quite good,

it's a review article,

I like reviews if they're peer-reviewed reviews

and in quality journals.

And what they discuss

is and I'll just read here briefly

'cause they said it better than I could,

"Olfactory functioning disturbances are common

following traumatic brain injury, TBI,

and can have a significant impact on the quality of life.

Although there is no standard treatment for patients

with the loss of smell."

Now I'm paraphrasing, "Post-injury olfactory training

has shown promise for beneficial effects.

Some of this involves,"

they go on to tell us the role of dopamine,

dopaminergic signaling, as I mentioned before,

but what does this mean?

This means that if you've had a head injury

or repeated head injuries that enhancing your sense of smell

is one way by which you can create new neurons.

And now, you know how to enhance your sense of smell

by interacting with things that have an odor very closely,

and by essentially inhaling more,

focusing on the inhale to wake up the brain

and to really focus on some of the nuance of those smells.

So, you might do for instance a smell test

by which you smell something like a lemon,

put it down, do 10 inhales or so,

smell again, et cetera.

You might also just take a more active role

in trying to taste and smell your food,

and taste and smell various things.

I mean, please don't ingest anything

that's poisonous that you're not supposed to be ingesting,

but you know what I mean,

really tuning up this system,

I think is an excellent review,

we're going to do an entire episode

all about the use of the visual system in particular,

but also the olfactory system for treatment

of traumatic brain injury, as well as other methods.

But I wanted to just mention it here

because a number of people asked me about TBI.

And here again, we're in this place where the senses

and our ability to sense these chemicals

through these two holes in the front of our face,

our nostrils is a powerful readout

and way to control brain function

and nervous system function generally.

Just a quick note about the use of smelling salts,

I have a feeling that some of you may be interested in that

and its application.

If you are interested in that,

I recommend you go to the scientific literature first

rather than straight to some vendor

or to the what do they call it these days?

Costello bro science, he says,

bro science, the bro science.

You can go to this paper, which is excellent

and is real science, which is Acute Effects of Ammonia

Inhalants on Strength and Power Performance in Trained Men.

It's a randomized controlled trial, it was published

in the Journal of Strength and Conditioning Research

in 2018, and it should be very easy to find.

I will provide a link to the so-called PubMed ID,

which is a string of numbers,

and we'll put that in the caption

if you want to go straight to that article,

does show a significant what they call,

this is what the words they use literally in quotes,

"psyching up effect through the use

of these ammonia inhalants

and a significant increase in maximal force

in force development in a variety of different movements."

So, for those of you that are interested

in ammonia inhalants, so-called smelling salts,

that might be a good reference.

The other thing I wanted to talk about

with reference to odors is this myth

which is that we don't actually smell things in our dreams,

that we don't have a sense of smell.

That's pure fiction, I don't know who came up with that,

it's very clear that we are capable

of smelling things in our sleep.

However, when we are in REM sleep,

rapid eye movement sleep,

which is the sleep that predominates

toward the second half of the night

our ability to wake up in response to odors is diminished.

It's not absent, but it's diminished.

If smoke comes into the room, we will likely wake up

if the concentration of smoke is high enough

regardless of the stage of sleep we're in,

but in REM sleep we tend to be less likely

to smell, to sniff.

And that actually was measured in a number of studies

that sniffing in sleep is possible.

So, if you put an odor like a lemon

underneath someone's nostrils

in the early portion of the night, they will smell,

and they will later...

They will sniff, excuse me,

whether or not they smell or not, I guess depends on them

and when they showered last,

but they will definitely sniff

and they will report later, especially if you wake them up

soon after that, they had a dream

or a percept of the scent of a lemon for instance.

Later in the night, it's harder for that relationship

to be established,

it's likely that because of some of the paralysis

associated with rapid eye movement sleep,

which is a healthy paralysis, so-called sleep atonia,

you don't want to act out your dreams in REM sleep

that there is a less active tendency to sniff.

And actually this has real clinical implications,

the ability to sniff

in response to the introduction of an odor

is actually one way in which clinicians assess

whether or not somebody's brain is so-called brain dead.

That's not a nice term, but brain dead,

or whether or not they have the capacity

to recover from things like coma

and other states of deep unconsciousness,

or I guess you'd call it subconsciousness.

So, what will happen is if someone has an injury

and they're essentially out cold,

the production of a sniffing reflex,

or a sniffing response to say a lemon

or some other odor presented below the nostrils

is considered a sign that the brain is capable of waking up.

Now, that's not always the case, but it's one indication.

So just like you could use mechano sensation,

so, a toe pinch for instance,

you know, or scraping the bottom of somebody's barefoot

to see if they're conscious,

or shining light in their eyes,

these are all things that you've seen

in movies and television,

or maybe if you've seen in real life as well.

Well, odors and chemical sensing is another way

by which you can assess

whether or not the brain is capable of arousal.

And actually olfactory stimulation

is one of the more prominent ones

that's being used in various clinics.

As a last point about specific odors and compounds

that can increase arousal and alertness,

and this was simply through sniffing them

not through ingesting them.

There are data, believe it or not, there are good data

on peppermint and the smell of peppermint,

minty type sense,

whether you like them or not will increase attention,

and they can create the same sort of arousal response

although not as intensely

or as dramatically as ammonia salts can for instance.

By the way, please don't go sniff real ammonia,

you could actually damage your olfactory epithelium

if you do that too close to the ammonia.

If you're going to use smelling salts

be sure you work with someone

or you know what you're getting and how you're using this.

You can damage your olfactory pathway

in ways that are pretty severe,

you can also damage your vision.

If you've ever teared up because you inhaled something

that was really noxious, that is not a good thing,

it doesn't mean you necessarily cause damage,

but it means that you have irritated the mucosal lining

and possibly even the surfaces of your eyes,

so please be very, very careful.

Scents like peppermint,

like these ammonia smelling salts,

the reason they wake you up

is because they trigger specific olfactory neurons

that communicate with the specific centers of the brain,

namely the amygdala and associated neurocircuitry

and pathways that trigger alertness

of the same sort that a cold shower or an ice bath,

or a sudden surprise,

or a stressful text message would evoke.

Remember, the systems of your body that produce arousal,

and alertness, and attention,

and that cue you for optimal learning, aka focus.

Those are very general mechanisms,

they involve very basic molecules like adrenaline

and epinephrin same thing actually,

adrenaline and epinephrin.

The number of stimuli, whether it's peppermint or ammonia,

or a loud blast,

the number of stimuli that can evoke

that adrenaline response and that wake up response

are near infinite.

And that's the beauty of your nervous system,

it was designed to take any variety of different stimuli

placed them into categories,

and then evoke different categories

of very general responses.

Now, you know a lot about olfaction

and how the sense of smell works,

here's another experiment that you can do.

I'll ask you right now.

Do you like, hate, or are you indifferent

to the smell of microwave popcorn?

Some people, including one member of my podcast staff

says it's absolutely disgusting to them,

they feel like it's completely nauseating.

I don't mind it at all,

in fact, I kind of like it.

I think the smell of a microwave popcorn

is kind of pleasant.

I don't particularly like it,

but it's certainly not unpleasant.

Some people have a gene that makes them sensitive

to the smell of things like microwave popcorn

such that it smells like vomit.

I probably don't have that gene

because I find the smell

of microwaved popcorn pretty pleasant.

Some people hate the smell of cilantro,

some people ingest asparagus,

and when they urinate they can smell the asparagus

in a very pungent way,

other people can't smell it at all.

These are variants in genes that encode

for what are called olfactory receptors.

Each olfactory sensory neuron expresses one odorant gene,

one gene that codes for a receptor

that responds to a particular odor.

If you don't have that gene

you will not respond to that odor.

So, the reason why some people find the smell

of microwave popcorn to be very noxious, putrid in fact,

is because they have a gene that allows them

to smell the kind of putrid odor within that.

Other people who lack that gene just simply can't smell it,

so we are not all the same

with respect to our sensory experience.

What one person finds delicious,

another person might find disgusting.

I'll give a good example

which is that I absolutely despise

Gorgonzola and blue cheese, absolutely despise it,

it smells and tastes

like dirty moldy socks to me.

Some people love it, they crave it,

actually, some people get a visceral response to it,

and we will talk about how certain tastes

can actually evoke very deep biological responses,

even hormonal responses

when we talk about taste in a few minutes.

But there are these odors,

for instance in popcorn

it's the molecule 2-acetyl-1-pyrroline,

not proline but pyrroline,

that gives off to some people like me

a toasted smell as the sugars in the kernels heat,

but the compound is also found

in things like white bread and jasmine rice,

which don't have as pungent an odor,

but some people smell that and it smells like cat urine.

Now, there are scents like musky scents and musty scents

that are secreted by animals like skunks

and other animals of the so-called Mustelidae family.

So, these would be ferrets and other animals that can spray

in response to fear, or if they just want to mark a territory

because they want to say that's mine.

Dogs incidentally have scent glands that they rub on things,

cats have them too.

This musty odor, some people find actually quite pleasant,

some people find it to be very noxious

and that will depend of course on the concentration, right?

I'll never forget the first time

Costello got sprayed by a skunk and it was awful.

I actually don't mind the smell of skunk at a distance,

it's actually a little bit pleasant,

I admit it's a little bit pleasant to me.

I don't think that makes me too weird

because if you ever read the book

"All Quiet on the Western Front" about World War I,

there's a description in there about the smell of skunk

at a distance being mildly pleasant,

so the author of that book probably shared

a similar olfactory profile to me, or I to them rather,

but some people find even the tiniest bit

of the smell of skunk or musk to be noxious or awful.

Now, of course in high concentrations, it's really awful.

And unfortunately, poor Costello,

he was like literally red-eyed and just snorting,

and it was awful.

There's a joke about dogs that says that dogs

either get skunked one time and never again

or 50 or a hundred times.

Costello has been skunked no fewer,

I'm not making this up,

has been skunked no fewer than 103 times.

And that's because if he sees something

or hears something in the bushes, he just goes straight in,

he does not learn.

But if you like this, that musty scent or musky scent,

well that says something about the genes that you express

in your olfactory neurons,

it is completely inherited.

And if you don't like that scent,

if it's really noxious

or you have this response to microwave popcorn,

well, that means you have a different compliment,

a different constellation if you will

of genes that make up for these olfactory sensory neurons

and the receptors that they express.

Let's talk about taste.

Not whether or not you have taste or you don't have taste,

there's no way for me to assess that,

but rather how we taste things,

meaning how we sense chemicals in food and in drink.

There are essentially five,

but scientists now believe there may be six things

that we taste alone or in combination,

they are sweet tastes,

salty tastes, bitter tastes,

sour tastes, and umami taste.

Most of you probably heard of umami by now,

it's U-M-A-M-I.

Umami is actually the name for a particular receptor

that you express on your tongue

that detects savory tastes,

so it's the kind of thing in braised meats.

Sometimes people can even get the activation of umami

by tomatoes or tomato sauces.

What are each of these tastes

and taste receptors responsible for?

And then we'll talk about the sixth,

maybe you can guess what it is,

I don't know if you can guess it now.

I couldn't guess it, but of the five tastes

each one has a specific utility or function.

Each one has a particular group of neurons in your mouth,

in your tongue, believe it or not,

that responds to particular chemicals

and particular chemical structures.

It is a total myth, complete fiction,

that different parts of your tongue

harbor different taste receptors.

You know, that high school textbook diagram

that you know sweet is in one part of the tongue

and sour is in another,

and bitters in another, complete fiction,

just total fiction related to very old studies

that were performed in a very poorly controlled way,

no serious biologists

and certainly no one that works on tastes

would contend that that's the way

that the taste receptors are organized,

they are completely intermixed along your tongue.

If you have heightened or decreased sensitivity

to one of those five things I mentioned;

sweet, salty, bitter, umami, or sour

at one location in your tongue,

it likely reflects the density of overall receptors

or something going on in your brain,

but not the differential distribution of those receptors.

So, the sweet receptors are neurons that express

a receptor that respond to sugars,

in the same way that you have cones, photoreceptors,

in your eye that respond to short,

medium, or long wavelength light,

meaning blueish, greenish, or reddish light.

You have a neuron, or neurons plural,

in your tongue that respond to sugars.

And then those neurons, they don't say sweet,

they don't actually send any sugar into the brain,

they send what we call a volley,

a barrage of action potentials of electrical signals

off into the brain.

It's an amazing system.

So, all these receptors in your tongue

make up what are called the neurons

that give rise to a nerve,

a collection of wires, nerve bundles

of what's called the gustatory nerve,

it goes from the tongue

to the so-called nucleus of the solitary tract.

And some of you requested names,

I usually don't like to include too many names

for sake of clarity,

but the gustatory nerve from the tongue

goes to the nucleus of the solitary tract

and then to the thalamus and to insular cortex.

You don't have to remember any of those names

if you don't want to,

but if you want mechanism, you want neural circuits,

that's the circuit, gustatory nerve from the tongue,

nucleus of the solitary tract in the brainstem,

then the thalamus, and then insular cortex.

And it is in insular cortex,

this regenerate cortex that we sort out

and make sense of and perceive the various tastes.

Now, it's amazing because just taking a little bit

of sugar or something sour

like a little bit of lemon juice

and touching it to the tongue

within 100 milliseconds, right?

Just 100 milliseconds, far less than one second,

you can immediately distinguish

ah, that's sour,

that sweet, that's bitter, that's umami,

and that's an assessment that's made by the cortex.

Now, what to these different five receptors encode for?

Well, sweet, salty, bitter, umami, sour,

but what are they really looking for?

What are they sensing?

Well, sweet stuff signals the presence of energy, of sugars.

And while we're all trying

or we're told that we should eat less sugar

for a variety of reasons,

the ability to sense whether or not a food

has rapid energy source

or could give rise to glucose is essential

so we have sweet receptors.

The salty receptors, these neurons are trying to sense

whether or not there are electrolytes

in a given food or drink.

Electrolytes are vitally important

for the function of our nervous system,

and for our entire body,

sodium is what allows neurons to fire.

What allows them to be electrically active.

We also need potassium and magnesium,

those are the ions that allow the neurons to be active.

So the salty receptors,

the reason that they are there

is to make sure that we are getting enough,

but not too much salt,

we don't want to ingest things that are far too salty.

Bitter receptors are there

to make sure we don't ingest things that are poisonous.

How do I know this?

How can I say that?

Even though I was definitely not consulted

at the design phase, how can I say that?

Well, the bitter receptors

create a what we call labeled line,

a unique trajectory to the neurons

of the brainstem that control the [gags],

the gag reflex.

If we taste something very bitter

it automatically triggers the gag reflex.

Now, some people like bitter taste,

I actually liked the taste of bitter coffee,

children generally like sweet tastes

more than bitter tastes,

but even babies if they taste something bitter,

they'll just immediately spit it up,

it's like the gag reflex.

Putrid smells will also evoke the same neurons,

so some people are very sensitive,

they have a very sensitive

or low threshold vomit reflex,

you're going to and there was somebody in my lab early on.

And we never did this intentionally, and we're just laughing

'cause it was so dramatic.

How we would have a discussion,

someone would say something about something kind of gross,

appropriate for the workplace, but nonetheless gross,

we are biologists,

would say something and they would say, "Stop, stop stop,

I'm going to throw up."

You know and some people have a very low threshold

quick gag reflex.

Other people don't, other people have a very stable stomach,

they don't, you know,

they rarely, if ever vomit.

The umami receptor isn't sensing savory

because the body loves savory,

it's because savory is a signal

for the presence of amino acids.

And we'll talk more about this,

but the presence of amino acids

in our gut and in our digestive system,

and the presence of fatty acids is essential,

there is in fact, no essential carbohydrate or sugar.

Now, I'm not a huge proponent of ketogenic diets

nor am I against them, I think it's highly individual,

you have to decide what's right for you,

but everybody needs amino acids to survive,

the brain needs them and we need fatty acids,

especially to build a healthy brain during development,

you need amino acids and fatty acids.

And the sour receptor, why would we have a sour receptor?

So, that we could have those really like sour candies?

I think they've gotten more and more sour over the years.

I admit I don't eat candy much,

but I do have a particular weakness

for like a really good really sour like gummy peach

or they if the gummy cherries are dipped

in whatever that sour powder,

so I was a kid who I admit it,

I liked the LIK-M-AID thing,

I'd like drink the powder.

Please don't do this, don't give this garbage to your kids,

but I liked it, it was tasty,

but sour receptors are not there

so that you can ingest gummy sour gummy peaches

or something like that, that's not why the system evolved,

it's there and we know it's there

to detect the presence of spoiled or fermented food.

Fermented fruit has a sour element to it,

and fermented things while certainly some fermented foods

like sauerkraut, and kimchi, and things of that sort

can be very healthy for us

and are very healthy in reducing inflammation,

there's great data on that,

pro quality microbiome, et cetera.

Fermented fruit can be poisonous, right?

Alcohols are poisonous in many forms to our system

and the sour receptor bearing neurons

communicate to an area of the brainstem

that evokes the pucker response,

closing of the eyes and essentially shutting of the mouth,

and cringing away.

I think cringe is like a thing now,

my niece, whenever I seem to say something or do something

it's either an eye-roll, a cringe, or both in combination.

So the sour, the sweet,

the salty, the bitter, and the umami system,

were not there so that we could have this wonderful pallet

of foods that we enjoy so much,

they'll allow us to do that, but they're there to make sure

that we bring in certain things

to our system and that we don't ingest other things.

Now, what's the sixth sense

within the taste system?

Not sixth sense generally, but within the taste system.

What's this putative possible sixth receptor?

I already kind of hinted at it

when I talked about fatty acids,

there are now data to support the idea

although there's still more work that needs to be done

that we also have receptors on our tongue that sense fat.

And that because fat is so vital for the function

of our nervous system and the other organs of our body

that we are sensing the fat content in food,

maybe this is why I can only eat half,

but no less than half of a jar of almond butter

or peanut butter in one sitting.

I just can't, unless it's not salted,

in which case, it makes no sense to me.

But it's remarkable how that texture,

and also the flavor, but that texture of fat.

I love butter, I am guilty,

and Costello is definitely guilty of eating pats of butter

from time to time, I have no guilt about this.

People eat pats of cheese,

why shouldn't we eat a pat of butter?

If you think that's gross

then maybe I have a greater abundance

of the fat receptors in my tongue,

maybe I have a fat tongue than you do.

But nonetheless, the ability to sense fat here in our mouth

seems to be critical,

you can imagine why that is.

I want to talk about the tongue

and the mouth as an extension of your digestive tract.

I know that might not be pleasant to think about,

but when you look at it

through the lens that I'm about to provide,

it will completely change the way you think

about the gut brain

and about all the stuff that you've heard

in these recent years about oh, we have this second brain,

it's all these neurons in our gut,

I've been chuckling through these last few years

as people have gotten so excited about the gut brain,

not because of their excitement,

I think that excitement is wonderful,

but we always knew that the nervous system

extended out of the brain and into the body,

and people seem kind of overwhelmed

and surprised by the idea that we have neurons in our gut

that can sense things like sugars and fatty acids.

And I think those are beautiful discoveries,

don't get me wrong.

Diego Bohorquez's lab out of Duke University

has done beautiful studies showing that

within the mucosal lining of our gut

we have neurons that sense fatty acids,

sugars, and amino acids,

and that when we ingest something

that contains one or two or three of those things,

there's a signal sent via the vagus nerve

up into what's called the nodose ganglion,

N-O-D-O-S-E,

and then into the brain where it secretes dopamine

which makes us want more of that thing,

it makes us more motivated to pursue

and eat more of that thing,

that's either fatty, or umami, savory,

or has a sweet taste,

any one or two or three of those qualities,

independent of the taste.

Now, I think those are beautiful data,

but we know that this thing, the mouth.

And for those of you listening

I've just got my couple of fingers in my mouth,

that's why I sound like I've got something in my mouth.

This thing in the front of our face,

we use it for speaking,

but it is the front of our digestive tract.

We are essentially a series of tubes

and that tube starts with your mouth

and heads down into your stomach.

And so, that you would sense

so much of the chemical constituents

of the stuff that you might bring into your body

or that you might want to expel and not swallow

or not interact with by being able to smell is it putrid?

Does it smell good?

Does it taste good?

Is this safe?

Is it salty?

Is it so sour that it's fermented and it's going to poison me?

Is it so bitter that it could poison me?

Is it so savory that, mmm,

yes. I want more and more of this.

Well, then you'd want to trigger dopamine,

that's all starting in the mouth.

So, you have to understand that you were equipped

with this amazing chemical sensing apparatus,

we call your mouth and your tongue.

And those little bumps on your tongue

that they call the papillae,

those are not your taste buds.

Surrounding those little papillae

like little rivers are these little dents and indentations.

And what dents and indentations do in a tissue

is they allow more surface area,

they allow you to pack more receptors.

So, down in those grooves are where all these little neurons

and their little processes are

with these little receptors

for sweet, salty, bitter, umami, sour,

and maybe fat as well.

And so, it's this incredible device

that you've been equipped with,

that you can use to interact

with various components of the outside world

and decide whether or not you want to bring them in or not.

Just as you can lose those olfactory neurons,

if you happen to get hit on the head

or you have some other thing,

maybe it was an infection that caused loss

of those olfactory sensory neurons,

you can also lose taste receptors in your mouth.

If you've ever eaten something that's too hot,

not spicy hot, but too hot,

you burn your tongue, you burn receptors.

It takes about a week to recover those receptors.

For some people it's a little bit more quickly,

but if you burn your tongue badly

by ingesting a soup that's too hot

or a beverage that's too hot,

you will greatly reduce your sense of taste

for essentially all tastes.

And that's because those neurons sit very shallow

beneath the tongue's surface,

and so that if you put something too hot on,

you literally just burn those neurons away.

Luckily those neurons also can replenish themselves.

Those neurons are of the peripheral nervous system,

and like all peripheral system neurons

they can replenish or regenerate.

So, if you burn your mouth in about a week or so

hopefully sooner you'll be able to taste again.

In fact, everybody's ability to taste

is highly subject to training.

You can really enhance your ability to taste

and taste the different component parts of different foods

simply by paying attention to what you're trying to taste,

this is an amazing aspect of the taste system.

I think more than any other system,

the taste system and perhaps the smell system as well

can be trained so that you can learn to pick out the tones,

if you will of different ice cream,

or different beverages.

I'm somebody who, you know,

I don't drink much alcohol,

I'll occasionally have a drink or something,

but a while ago I got to taste a bunch of different

white tequilas, these are different kinds of tequilas

that are, they're not brown, they're white.

And I sort of assumed that all tequila was disgusting,

that was my assumption before doing this.

And then I tasted a couple of white tequilas

and I realized oh, those aren't aren't too bad.

I tasted a few more,

and then pretty soon I could really start to detect

the nuance and the difference.

Now, I haven't had a tequilas in a long time,

now I sort of tend to not drink at all these days,

but in a very short period of time like a couple of days

I got very good at detecting which things I liked

and I could start to pick out tones.

So, I'm not a wine drinker,

but for those of you that are,

you know, you hear about oh, it has floral tones,

or berry tones, or chocolate tones.

You know, some of that is just kind of menu-based

and kind of marketing-based silliness

designed to get you excited

about what you're about to ingest.

But some of it is real,

and for people that are skilled

in assessing wines or assessing foods.

I'm much more of an eater than a drinker,

you can really start to develop a sensitive palate,

a nuanced palette through what we call top-down mechanisms.

This olfactory cortex that takes these five,

maybe the sixth fat receptor too,

information and tries to make sense

of what's out there in the world.

And what its utility is, is it good? Is it bad?

Do I want more of it or less than it?

That neural circuitry is unlike other neural circuitry

in that it seems very amenable

to behavioral plasticity for whatever reason,

and we could talk about what those reasons might be.

You know, it's interesting sometimes

to think about how your taste literally, chemical taste,

is probably very different than that of other people,

how a food tastes to you is probably very different

than how it tastes to somebody else.

The same probably cannot be said of something

like vision or hearing, unless you're somebody

who has perfect pitch or your color vision is disrupted,

or you're a mantis shrimp,

chances are when we look at the same object

two people are seeing more or less the same object

or perceiving it in a very similar way.

There are experiments that essentially establish that.

Now we, have taste receptors

and a lot of those tastes receptors,

their chemical structures are known,

they come with fancy names like the T1R1 or the T1R2,

which were identified as the sweet and umami receptors.

So, what's interesting is that this umami flavor

is the savory flavor rather

that's sensed by umami receptors is very close

to the receptor that detects sweet things.

Similarly, bitter is sensed

by a whole other set of receptors.

Now, there's a fun naturally occurring experiment

that will forever change the way that you look at animals,

and the way certainly that I think about dogs

and Costello in particular.

Carnivorous large animals like tigers

and some grizzly bears for instance,

we know that they have no ability to detect sweet,

they don't actually have the receptors

for sweet on their tongue,

but their concentration of umami receptors

of their ability to detect savory

is at least 5,000 times that which it is in humans.

In other words, if I eat a little piece of steak,

or Costello eats a little piece of steak,

that steak probably tastes much,

much more savory than it does to me.

So dogs, and tigers, and bears, et cetera,

they're going to taste savory things and smell savory things

with a much higher degree of sensitivity,

but they can't taste sweet things.

Other large animals, which are mostly herbivores

like the panda bear for instance.

It's hard to believe that thing is even a bear,

I got nothing against pandas,

I just think that they get a little bit too much

of the limelight frankly.

So, no vendetta against Panda, save the pandas,

I hope they replenish all the pandas,

but pandas in all their whatever

have no umami receptors, they can't taste savory,

but they have greatly heightened density of sweet receptors.

So, there they are eating these whatever bamboos all day

or not bamboozle, but bamboos all day

and they can taste things that are very sweet

with a much higher degree of intensity.

And in general, animals that are more gentle

that are herbivores, excuse me.

Or animals that have the propensity for aggression,

that's where you really see the divergence

of the umami receptor

because it's associated with meat and amino acids.

And where you see the enhancement of the sweet receptors

for animals that eat a lot of plants and fruits,

and they probably taste very different to them

than they do to you and me.

And, so it's interesting to note

that animals that eat meat, that eat other organisms

can actually extract more savory experience from that.

What does this mean for you?

All right, do you associate yourself as a tiger,

or a grizzly bear, or a panda, or a combination of both?

Most people are omnivores.

However, you may find it interesting that people

that for instance eat a pure carnivore type diet

or a keto diet where they are ingesting a lot of meat,

so therefore are sensing a lot of umami flavors.

And I realized not everyone who's keto eats meat,

but those who do that will develop a more sensitive palate

and likely there are some data, although early data,

craving for umami-like foods.

Whereas people that eat a more plant-based diet

are likely developing a heightened sensitivity

and desire for, and maybe even dopamine response to sugars

and plant-based foods.

Now, this is my partial attempt to reconcile

the kind of online battle that seems to exist

between plant-based versus animal-based,

purely plant-based or purely animal-based diets.

I think most people are omnivores,

but it's kind of interesting to think

that the systems are plastic

such that people might want more meat

if they eat more meat,

people might want more plants

if they eat enough plants for a long period of time.

And this might explain some of the chasm that exists

between these two groups.

Now, this is not to say anything about the ethical

or the environmental impacts of different things,

I don't even want to get into that

because the meat people say that the plant-based diets

have as much a negative impact

as the plant people say that the meat based diets,

that's a totally different discussion.

What I'm talking about here is food craving and food seeking

and one's ability to detect these umami, savory flavors

is going to be enhanced by ingesting more meat

and less activation of the sweet receptors.

So in other words, the more meat you eat

the more you're going to become like a tiger, so to speak.

And the more that you avoid these umami flavors and meats

and the more that you would eat plant-based foods

and in particular sweet foods,

the more you will likely suppress that umami system

and that you will have a heightened desire for,

appetite for and sensing of sweet foods

or foods that contain sugars.

What I'm about to tell you is going to seem crazy,

but is extremely interesting

with respect to taste and taste receptors.

Remember, even though we can enjoy food

and we can evolve our sense

of what's tasty or not tasty, depending on life decisions,

environmental changes, et cetera,

the taste system just like the olfactory system

and the visual system

was laid down for the purpose

of moving towards things that are good for us

and moving away from things that are bad for us,

that's the kind of core function of the nervous system.

Well, taste receptors are not just expressed on the tongue,

they are expressed in other cells and other tissues as well.

Some of you may be able to imagine foods

that are so delicious to you

that they make your entire body feel good.

Or foods that are so horrifically awful to think about

let alone taste, that they create a whole body shuddering

or kind of repellent-type response

where you just either cringe or turn your face away

even in the absence of that food.

That's sort of how I feel about pungent, Gorgonzola cheese.

If you like Gorgonzola cheese, I don't judge you,

I just, that's an individual difference.

I happen to love certain foods,

I do like savory foods very much.

I, when I think about them,

they just they make me feel good.

And I'm oftentimes not even associating

with the taste of those foods,

it feels almost like a visceral thing.

Well, it turns out that some of the taste receptors

extend beyond the tongue,

that they actually can extend

into portions of the gut and digestive system.

And if that's not strange enough,

turns out that some of the taste receptors

are actually expressed on the ovaries and the testes.

So, what that means is that the gonads,

the very cells, and tissues, and organs in our body

that make up the reproductive axis

are expressing taste receptors.

Okay, so how do we interpret this?

Does this mean that when you eat something

that's very savory or very sweet for instance,

that it's triggering activation

of the ovaries or of the testes?

Well, it's possible.

Now, how those molecules,

those chemical molecules would actually get there

isn't clear, the digestive track does not run directly

to the testes or to the ovaries.

But nonetheless, what this means is that chemical sensing

of the very things that we detect on our tongue

and that we call taste in quotes in food

is also evoking

cellular responses within the reproductive gonads.

Now, whether or not this underlies the positive association

that we have with certain foods isn't clear,

but I'd be remiss if I didn't point out the obvious,

which is that the relationship between the sensual nature

of particular foods

and sensuality generally and the reproductive axis

is something that's been covered in many movies,

there are entire movies that are focused

on the relationship between for instance,

chocolate and love and reproductive behaviors,

or certain feasts of meat

and their wonderful tastes

and the kind of sensuality around feasts

of different types of foods,

but in general, it's the sweet and the savory,

rarely is it the sour or the bitter, the salty or the fat.

And not surprisingly perhaps,

it is the T2Rs and the T1Rs,

the receptors that are associated with the sweet

and with the umami, the savory flavors

that are expressed not just on the tongue

and in portions of the digestive tract,

but on the gonads themselves.

So, what does this mean?

Does this mean that eating certain foods

can stimulate the gonads?

Maybe, there's no data that immediately

support that right now, but this is an emerging area.

If you'd like to read more about this

there's a great review,

entitled Taste perception: From the tongue to the testis,

although they do also talk about the ovaries.

Why they didn't include that in the title

is I think a reflection of the bias of the author.

The author indeed not incidentally is Feng Li,

last name L-I.

It's a very interesting paper published

in Molecular Human Reproduction.

You can find it easily online, it's downloadable,

I'll also provide a link to it.

I just think it's fascinating

that these taste receptors are expressed in other tissues.

And I should mention that they're expressed

in tissues of other areas of the body as well,

including the respiratory system,

but the richest aggregation

or concentration of these receptors

for umami and sweet of course is on the tongue,

but also on the gonads.

And I think it does speak

to the possible bridge between

what we think of as a sensory

or a sensual experience of food

and the deeper kind of visceral sense within the gut,

and maybe even within the gonads as well

of something that we find extremely pleasurable,

or even appetitive that we want to move toward it.

We are actually going to return to that general theme

in the discussion about touch sensation.

Some people for instance,

when they touch certain surfaces like furs,

or sheep skins, or velvet,

or soft, smooth surfaces

it feels good elsewhere in their body,

not just at the point of contact with that surface.

And similarly, if there's the...

How about this one?

The screech of chalk on a chalkboard, it's a sound,

but it has a very strong visceral component,

or sandpaper, like fingernails on a chalkboard,

not the sound, but the feeling, right?

Exactly, so our whole nervous system is tuned

to either be drawn toward appetitive,

or repelled by aversive behaviors, right?

So there's this push-pull that exists,

and what I'm referring to in terms of these receptors

on the tongue that are also expressed on the gonads

is yet another example of what at least in this case

seems to be an appetitive thing,

a desire to move toward certain foods

and maybe even the experiences

that are associated with those foods.

I want to talk about a particular aspect

of food and a chemical reaction in cooking

called the Maillard reaction.

Some of you have probably heard of the Maillard reaction,

it's spelled M-A-I-L-L-A-R-D.

The D is silent, so don't call it the Maillard reaction,

and it's not the Maillard reaction,

it is the Maillard reaction.

And the Maillard reaction is a reaction

that for the aficionados is a non-enzymatic browning.

The other form of non-enzymatic browning is caramelization,

although when you hear caramel, carmel,

I think it's caramel.

You think sweet, and indeed caramelization is a sugar-sugar

chemical interaction that leads to a kind of nicely toasted

not burnt, but nicely toasted sweet taste.

Whereas the Maillard reaction is that really savory reaction

that occurs when you have a sugar amino acid reaction.

Remember, we have neurons in our gut,

but also neurons in our tongue

and neurons deep in the brain that are comparing the amount

of sugar to savory.

Okay, and the Maillard reaction is very interesting

for you chemists out there,

this is going to be way too elementary.

And for you non-chemists

it's probably going to be a little bit of a reach,

but just bear with me, all these chemicals that we sense

have a different structure,

it's like hydrogens, and oxygens,

and aldehyde groups, and all these things.

And basically the Maillard reaction

involves what's called a free aldehyde.

If you didn't like chemistry, don't worry about it,

it's basically got a group there that kind of sits open

that allows it to interact

with other things and actually through the use of heat

and the process that we call brazing,

which I'll talk about in a moment

you create a what's called a ketone group.

Now, most people now have heard of ketones

'cause they think about the ketogenic diet,

but a ketone group is actually a chemical compound

that can be used for energy,

and that's why people say you can use ketones for energy,

but if you've ever actually encountered ketones,

if you for instance, get liquid ketones,

a ketone ester, and you smell it,

what does it smell like?

It smells a little bit like an alcohol,

but it has a kind of savory taste,

even when you smell it.

Okay, there are other smells that have these tastes too,

but for the Maillard reaction

which could be created for instance

like if you took a piece of meat,

or if you're not a meat eater

if you took tomatoes and you cook them in a pan

and you cooked it nice and slow till it's simmered

and almost started to brown and burn a little bit.

Usually if I do it burns, I'm not a good cook

as Costello points out a lot,

but it gets that like almost tangy, very umami-like flavor.

And sometimes it will even stick to the pan,

if you scrape it off

and actually you can taste it in your mouth

as you're cooking it.

That's the Maillard reaction,

that's that free aldehyde group,

and that's the production of a ketone group.

When you smell ketones, it smells very much like that.

Okay, some people talk about the ketones

will produce like fruity breath.

And that's true if people are really far into ketosis,

their breath has a kind of fruity odor,

that's a little bit of a different thing.

So, the relationship between smell and taste

is a very, very close one.

And this is why when people drink wine

they often will inhale and then sip,

some of that is just kind of like pomp

and circumstance frankly, they make a big deal of it,

but they can sense things with their mouth.

The combination of odor receptors

being activated in a particular way,

and taste receptors in the mouth being activated

in a particular way, triggers the activation

of multiple brain areas that are associated with taste,

and circuitry within the body

that's associated with the behaviors

that relate to that taste like leaning toward it,

or leaning away from it

depending on whether or not it's appetitive or aversive.

So the Maillard reaction is a very interesting reaction

involving this sugar amino acid thing,

but really it's what it's doing is heating up food

such that the amino acids are more available

literally in their chemical form

for detection by the neurons.

This is a phenomenon that occurs

in other domains of the taste system.

For instance, a lot of what's happened

with highly processed foods

is that manufacturers have figured out

how to trigger more dopamine response

by ingestion of these sugary foods and created textures,

and created essentially design of foods for two purposes.

I'm not out to completely demonize processed food,

I did that in a previous episode,

but processed foods are really designed to take foods

that ordinarily would spoil, that would have a shelf life

and extend their shelf life

to turn foods which are not a commodity into a commodity.

Something could be stored and used essentially as a tradable

purchasable, sellable resource.

In doing that they change,

they've also decided to change the texture

so that you want to chew more of them.

Like I have this thing

I don't know what it is for those Triscuit crackers.

I don't know why are those things so good?

It's probably the texture, got those layers,

they're just kind of perfectly salty.

I haven't had one in a long time,

so I bet if I had one now it wouldn't taste as good

as I'm imagining it.

But those combinations of texture, smell, and taste

are what combine to activate these different brain areas

that make you really want to desire something.

And the people who make foods,

processed foods in particular,

are phenomenally good at figuring out

what drives the dopamine system and makes you want more

of these things either because of the way they taste

and/or because of the way they trigger neurons

in your gut that have nothing to do with taste

that simply make you desire more of the food.

In other words, many of the foods that are processed foods

make you desire more of them,

it's impossible to eat one chip kind of thing.

Not because they taste good,

but because in your gut they're activating the neurons

that activate dopamine

which make you seek more of those foods

independent of blood sugar or anything else.

So, you may actually be eating more particular foods

not because they taste good,

but because they feel good on your tongue and mouth,

and because the neurons in your gut

which are totally independent of conscious taste

are triggering the release of dopamine

which is a molecule that makes you seek more of,

and do more of anything

that led to the ingestion of that food.

There's a fun experiment that you can do,

which is to completely invert your sense of sweet and sour,

there's actually a way to do this readily.

When I was a post-doc,

I used to have a journal club at my house,

people would come over in the evening once a month,

and we would read a paper,

typically the weirdest paper we could find

and we would eat food and hang out,

that's what nerds did and do for fun,

so that's what we did.

And one time someone brought what's called miracle berry.

Okay, so this isn't some psychedelic plant medicine thing,

miracle berry, you can purchase online,

it's relatively inexpensive.

It actually causes a change

in the configuration of taste receptors

such that when you eat something sour, it tastes sweet.

And so what's really wild is you ingest miracle berry,

and then you bite into a lemon,

maybe even the lemon and peel

and it tastes as sweet as a peach.

And this effect lasts several hours.

Definitely, you know, check any warnings,

I don't know what sort of warnings these,

a miracle berry carries,

but I'm sure there's always something, you can imagine.

There are a number of papers on miracle berry

or miracle fruit it's called,

but it changes your perception of sour

at a perceptual level,

but it does that by changing the activity

of the receptors in the mouth and tongue.

Now, this is important as a principle and it's underscored

by experiments that have been done by for instance

Charles Zuker's lab at Columbia University,

where they've essentially genetically engineered animals

such that the bitter receptor is swapped

with the sweet receptor,

or the sweet receptor is swapped with the bitter receptor.

And what they show is that the actual food,

the experience on the tongue

drives different pathways in the brain.

Here's what they did, they essentially took mice

and swapped out the sweet receptor

and put in a bitter receptor.

And then what they found is that

whereas normally mice would actively seek out

and even work for sugar water, sucrose,

they really like that.

If they replace the sweet receptor with the bitter receptor,

the mice would avoid sugar water.

And the reverse was also true,

that mice would drink a bitter solution avidly,

they liked a bitter solution

if they swapped out the bitter receptor for sweet receptor.

What this means is that our entire experience

of what we taste is dependent

on how we experience that taste the level of the tongue.

And so, you're hopefully not going to do genetic engineering

of your taste receptors,

but if you'd like to do this sort of experiment

you actually can do it very easily using miracle fruit,

the instructions of how much to ingest, et cetera,

any safety concerns are usually on the package

and should be easy to find.

And there's a lot of science to support how this works,

it's kind of a fun experiment that anyone can do

and will completely change your perception

of any food that you're accustomed to eating.

In fact, you can figure out how much sweet

or the sense of sweetness

is contributing to your experience of a food,

even if you don't think of it as a sweet food

through this miracle fruit experiment.

You could take miracle fruit,

you could eat a slice of pepperoni pizza or cheese pizza,

which perhaps normally to you would taste just like pizza,

and you'll notice it tastes very different.

What you are detecting is how much the sense of sweet

was contributing to that particular flavor.

Now, I'd like to return to pheromones.

As I mentioned earlier,

true pheromonal effects are well-established in animals.

And one of the most remarkable pheromone effects

that's ever been described

is one that actually I've mentioned before on this podcast,

but I'll mention again just briefly,

which is the Coolidge effect.

The Coolidge effect is the effect

of a male of a given species,

in most cases, it tended to be a rodent or a rooster mating.

And at some point reaching exhaustion

or the inability to mate again

because they just simply couldn't for whatever reason.

The Coolidge effect establishes that if you swap out the hen

with a new hen, or the female rat or mouse with a new one

then the rat or the rooster

spontaneously regains their ability to mate,

somehow their vigor is returned,

the refractory period after mating that normally occurs

is abolished and they can mate again.

It turns out that the Coolidge effect

runs in the opposite direction too.

I did not know this, but I recently learned of a study,

it was actually done in hamsters, not in in mice,

but it turns out that females also will,

female rodents will mate to exhaustion.

And at some port...

At some point, excuse me,

they will refuse to mate any longer

unless you swap in a new male.

And then because mating in rodents

involves the female being receptive,

there are certain number of behaviors

that mean that tell you that she's willing

and wanting to mate, so-called lordosis reflex.

Then if there's a new male,

she will spontaneously regain the lordosis reflex

and the desire to mate.

And how do you know this?

How do we know it's a pheromonal effect?

Well, this recovery of the desire and ability to mate

both in males and in females

can be evoked completely by the odor

of a new male or female,

it doesn't even have to be the presentation

of the actual animal.

And that's how you know

that it's not some visual interaction

or some other interaction,

it's a pheromonal interaction.

Now, as I mentioned earlier, pheromonal effects

in humans have been debated for a long period of time.

We are thought to have a vestigial,

meaning a kind of shrunken down

miniature accessory olfactory bulb called Jacobson's organ,

or the vomeronasal organ.

Some people don't believe that Jacobson's organ exists,

some people do, there is anatomical evidence for it

in some cadavers.

It sits not very high up in the brain

or where your olfactory bulb is,

but it's actually in the nasal passages,

so there's like little dents

as you go up through your nasal passages,

and there is evidence of something that's vomeronasal-like.

Vomeronasal is the pheromonal organ,

they call it Jacobson's organ if it's present in humans,

kind of tucked into some of the divots in the nasal passage.

Even if that organ, Jacobson's organ isn't there

or is not responsible

for the chemical signaling between individuals,

there is chemical signaling between human beings.

As I mentioned earlier, the effect of tears

in suppressing the areas of the brain

that are involved in sexual desire

and testosterone of males, that's a concrete result,

that's a very good result published by an excellent group

with no pre-existing bias going in,

that's just what they found.

There is also evidence

both for and against

chemical signaling between females

in terms of synchronization of menstrual cycles.

Now, the original paper on this was published in the 1970s

by McClintock, and it essentially said

that when women live together in group housing, dormitories,

and similar that their menstrual cycles were synchronized

and that was due to what was hypothesized

to be pheromonal effects.

Over the years,

that study has been challenged many, many times.

The more recent data point to the idea

that there is chemical-chemical signaling between women

in ways that impact the timing of the menstrual cycle,

but that depending on whether or not

some of the women are in the ovulation phase,

the ovulatory phase of that cycle

or whether or not they are in the follicular phase,

the phase when the follicle is maturing

before the egg actually obviates.

So two separate phases of the 28 day menstrual cycle

will either lengthen or shorten the menstrual cycle

of the person that smells those women.

Translated into English what that means

is that it is very likely it seems

that something, maybe pheromones,

but maybe some other chemical

that is independent of pheromones is being conveyed

between women that are housed together

or spend a lot of time together

to shift their menstrual cycle,

but it doesn't necessarily mean that they synchronize.

So for instance, if one woman is in the follicular phase

of the menstrual cycle,

it might shorten or delay ovulation.

Excuse me, it might accelerate ovulation in another woman,

whereas if somebody is in the ovulatory phase

of their cycle, it might lengthen the menstrual cycle

out so that they, the woman who smells that person's scent

or who smells her sweat,

we still don't know the origin of the chemical

would ovulate later.

So, all of this is to say

is that chemical-chemical signaling is happening

from females to males through tears, we know that.

Is that a pheromonal effect?

Well, by the strict definition of a pheromone,

a molecule that's released from one individual

that impacts the biology of another individual, yes.

But in terms of identifying what the pheromone is in tears,

that's still unknown,

it's not clear what the chemical compound is.

So, we're reluctant as scientists

to call it a true pheromonal effect.

The menstrual cycle and the synchronization

of the menstrual cycle effect seems to hold up

under some conditions.

But in some cases,

there's a kind of clash of menstrual cycles that's created

by chemicals that are emitted

from one female to another.

So, there are many examples of this in humans,

for instance, people can recognize

the t-shirt of their mate.

If you give...

This experiment has been done many times,

I know it's been challenged a number of times,

but the data are pretty good by now that if you offer,

you take a collection of women

who are in stable relationships with somebody,

you offer them the smell of a hundred different shirts

and they can very readily pick out

their significant others scent.

Okay, that's pure olfaction, that's not pheromonal,

but nonetheless is a remarkable degree of discrimination,

olfactory discrimination.

You can dilute their partner's scent

down to the point where they themselves

can't consciously detect the difference

between the sweat or the t-shirt

of a hundred different t-shirts or so,

they might say, "I don't really smell the difference,

but I think it's this one.

Yeah, this one belongs to the person that I've been with."

And they are much greater than chance

at detecting the t-shirt

or identifying the t-shirt correctly.

So, there's no question really

that there is chemical-chemical signaling between humans,

the question is whether or not

it's truly pheromonal in basis.

Now, you'll notice that a lot of the examples I gave

aside from the one of tears is women detecting the scents

of men or of other women.

And it turns out that there are a number of papers,

the best one I think

that I could find is published in Physiology and Behavior

in 2009, it's a review

entitled Sex Differences and Reproductive Hormone Influences

on Human Odor Perception

by Doty, D-O-T-Y, and Cameron.

I encourage you to check out this review

it's available free as a download,

we'll provide a link to it,

you can get the full PDF if you want.

But it does seem that women are better at detecting