Essentials: How Smell, Taste & Pheromones Shape Behavior

0:00Welcome to Huberman Lab Essentials, where we revisit past episodes for the most potent and actionable science -based tools for mental health, physical health, and performance. 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. 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.

0:39And we are going to talk about chemicals that are made by other human beings that powerfully modulate the way that we feel our hormones and our health. Now, that last category are sometimes called pharaohmones. However, whether or not pharaohmones exist in humans is rather controversial. There actually hasn't been a clear example of a true human -fermonal 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.

1:21There are things floating around in the environment, which we call volatile chemicals. 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. There are literally little particles of those chemicals are going up into your nose and being detected by 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 a thing our tongue so these chemicals we sometimes bring into our body into our biology through deliberate action we select the food we chew that food and and we do it intentionally.

2:15Sometimes they're coming into our body through non -deliberate action. We enter an environment and they're smoke and we smell the smoke and as a consequence, we take action. 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, our eyes, or our mouth, that fundamentally change our biology. 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, showing that humans, men in particular in this study, have a strong biological response and hormonal response to the tears of women.

3:11What 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 sailing. 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. They actually recruited subjects that had a high propensity for crying at sad movies, which was not all women. What they were really trying to do is 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.

4:08Nonetheless, what this study illustrates is that there are chemicals in tears that are evoking or changing the biology of other individuals. 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.

4:52But 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. As many of you know, I've been taking AG1 daily for more than 13 years. However, I've now found an even better vitamin mineral probiotic drink. That new and better drink is the new and improved AG1, which just launched this month.

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6:43AGE1 uses the highest quality ingredients in the right combinations and they're constantly improving their formulas without increasing the cost. So I'm honored to have them as a sponsor of this podcast. If you'd like to try AGE1, you can go to drinkag1 .com slash huberman to claim a special offer. Right now, AGE1 is giving away an AGE1 welcome kit with 5 free travel packs and a free bottle of vitamin D3 K2. Again, go to drinkag1 .com slash huberman to claim the special welcome kit with 5 free travel packs and a free bottle of vitamin D3K2. Smell starts with sniffing. That may come as no surprise, but no volatile chemicals can enter our nose unless we inhale them.

7:25If 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 mucosolining 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, right above the room, wow, about two centimeters is your ol' factory 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.

8:12So what this means in kind of a literal sense is that you have neurons that extend their little little dendrites and acts only like things, their little processes, as we call them, out into the mucus, and they respond to different and 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 hardwired 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.

9:11It calls for action, and indeed, these neurons project a central area of the brain called the amygdala, which is often discussed in in terms of fear, but it's really a fear and threat detection. 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 a pettative 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, that's because of these innate pathways, these pathways that require no learning whatsoever.

9:50Now, 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 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. So we have pathway for innate responses and a pathway for learned responses.

10:36And 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, fair -mone effects. For example, in rodents and in some primates, including mandrels, If you've ever seen mandrel, they have these like 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 mandrels There are strong pheromone effects.

11:17Some of those include things like if you take a pregnant female rodent or mandrel 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. Another example of a pheromone effect is called the Vandenberg 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 this scent or the urine from a sexually competent, meaning post -puberadol male, and she spontaneously goes into puberty earlier.

12:10So 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 ester cycle, not a menstrual cycle. So this is not to say that the exact same things happen in humans. and humans, as I mentioned earlier, there are chemical sensing between individuals that may be independent of the nose, 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's very interested in smell, but you are somebody who's interested in making your brain work better, learning faster, remembering more things.

12:55This 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. Nome Sobles Group, originally at UC Berkeley and then at the Weitzmann Institute, has published a number of papers that I'd like to discuss us today, one of them, human non -alphactory cognition, phaselocked with inhalation, this was published in Nature, Human Behavior, and Excellent Journal. As we inhale, what this paper shows is that the level of alertness goes up in the brain.

13:39And 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, phased 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.

14:31When we exhale, the brain goes through a subtle but nonetheless significant dip in level of arousal and ability to learn. How should you use this knowledge? Well, you could Imagine, and I think this would 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 is 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.

15:13Now 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 of body in a major way. This is why they use smelling salts when people pass out. They work because they trigger the fear and kind of overall arousal systems of the brain.

15:51This is why I think most people probably shouldn't use ammonia or smelling salts to try and wake up, but they really do work. Now, inhaling through your nose and doing nasal breathing, 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, 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, do 10 or 15 inhales, Followed by excels of course or just through the nose.

16:34I'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. So you can actually have a heightened experience of something and that of course will also be true for the taste system. I'd like to take a quick break and thank one of our sponsors, Element. Element is an electrolyte drink that has everything you need and nothing you don't. That means the electrolytes sodium magnesium and potassium in the correct ratios, but no sugar. We should all know that proper hydration is critical for optimal brain and body function.

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17:44There are a bunch of different great tasting flavors of element. I like the water melon, I like the raspberry, I like the citrus, basically I like all of them. If you'd like to try element, you can go to drinkelement .com slash huberman to claim an element sample pack with the purchase of any element drink mix. Again, that's drink element spelled LMNT, so it's drinkelement .com slash huberman to claim a free sample pack. You also can really train your sense of smell to get much, much better. 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.

18:20In fact, how well we can smell and taste things is actually a very strong indication of our brain health. 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 re -grows. 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 really interesting because other neurons in your cortex, in your retina, in your cerebellum, they do not do this.

19:05They 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. 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.

19:45Here 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 response 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 mentioned, 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 cribiform plate.

20:29It's like a Swiss cheese type plate where they're going through, and if you get a head hit, that bone, the cribiform and plate shears 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 read out by which you might evaluate whether or not you're regaining some of your sensory performance.

21:08Of 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, Ophactory Disfunction 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 Asma 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. What they discuss is, and I'll just read here briefly, because 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's no standard treatment for patients with the loss of smell.

21:50Now I'm paraphrasing post -injury. olfactory training has shown promise for beneficial effects. 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. As a last point about specific orders and compounds that can increase arousal and alertness.

22:30And 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.

23:09You can also damage your vision. You've ever teared up because you inhaled something that was really possibly even the surfaces of your eyes. So please be very, very careful. Sents 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 neural circuitry 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 a rousal end alertness and attention and that cue you for optimal learning, aka focus, those are very general mechanisms.

24:00They involve very basic molecules like adrenaline and epinephrine, same thing, actually adrenaline and epinephrine. The number of stimuli, whether it's peppermint or ammonia or a loud blast, the number For a 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, place 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. Let's talk about taste, meaning how we sense chemicals in food and in drink.

24:39There are essentially five, but scientists now believe there may be six things that we taste alone or in combination. They are sweet taste, salty taste, bitter taste, sour taste, and umami taste. Most of you have 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. 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.

25:35You know, that high school textbook diagram that you know, sweet is in one part of the tongue and sour is in another and bitter is in another. They are completely intermixed along your tongue. 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. The gustatory nerve from the tongue goes to the nucleus of the solitary tract and then to the stalemus and to insular cortex. And it is an insular cortex, this regenerate cortex that we sort out and make sense of and perceive the various tastes.

26:09Now 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 that sour, that sweet, that's bitter, that's umami. And that's an assessment that's made by the cortex. Now, what do these different five receptors in code 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.

27:03So 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 bodies. Bitter receptors are there to make sure we don't ingest things that are poisonous. The bitter receptors create a what we call labeled line a unique trajectory to the neurons of the brainstem that control the gag reflex. If we taste something very bitter, it automatically triggers the gag reflex. PUTRED smells will also evoke these same neurons. The umami receptor isn't sensing savory because the body loves savory.

27:50It's because savory is a signal for the presence of amino acids. The presence of amino acids in our gut and in our digestive system and the presence of fatty acids is essential. The sour receptor. Why would we have a sour receptor? It's there and we know it's there to detect the presence of spoiled or fermented food. 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.

28:32Now, what's this sixth sense within the taste system? Not sixth sense generally, but within the taste system. What's this punitive possible sixth receptor? 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. I want to talk about the tongue and the mouth as an extension of your digestive tract. We are essentially a series of tubes, and that tube starts with your mouth and heads down into your stomach.