How Smell, Taste & Pheromone-Like Chemicals Control You

The speaker, who introduces himself as Andrew Huberman, a professor of neurobiology and ophthalmology at Stanford School of Medicine, places this episode in a month-long series on the senses. The previous episode covered vision: how light becomes perceptions such as color, faces and motion, and how light also affects mood, metabolism and alertness without our noticing. This episode turns to chemical sensing.

Three topics are laid out. The first is smell, the detection of odors in the environment. The second is taste, the detection of chemicals placed in the mouth and digestive tract. The third is chemicals made by other humans that change how we feel and affect our hormones and health. That last category is sometimes called pheromones, and the episode is careful about the term from the start.

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

The claim the episode treats as certain is narrower: humans make and release chemicals in tears, on the skin, in sweat and even in breath, and these chemicals strongly affect other people's biology. It adds that even someone who is alone has their nervous system, hormones and metabolism shaped by chemicals in their surroundings. Chemical sensing is called one of our most primordial aspects of biology, yet still very active.

The episode promises practical protocols. One plays on the word "smell": how you smell in the verb sense, the act of sniffing, as a way to sharpen smell and taste and to improve learning and memory. It says the material rests on peer-reviewed studies from strong laboratories, and that one takeaway will be how similar parts of human biology are to that of other animals.

Before getting to chemical sensing, the episode recaps the vision episode. That episode covered 13 zero-cost protocols, which are listed with timestamps at hubermanlab.com. Two are repeated here.

Near-far viewing: about five minutes, three times a week, at any age. Bring a pen or pencil close to the point just before your eyes cross, then move your focus to a far distance, ideally from a balcony or deck, and back again. This is meant to exercise the accommodation reflex, the change in the shape of the lens, and is said to help offset problems including myopia (nearsightedness). Time outdoors: one study is cited as showing that two hours a day outside, even while doing other things, can help offset myopia. The benefit is credited to sunlight and its blue light. The episode argues that blue light has been unfairly demonized and is helpful when it comes from sunlight rather than from a very bright source close to the eyes. It concedes two hours is a lot of time, but notes it can be combined with gardening, reading, walking or running.

There is also a correction. In the vision episode, lutein was discussed as possibly helping offset moderate-to-severe age-related macular degeneration. Some people also take it without that condition, hoping to limit vision loss with age. When speaking fast, the speaker sometimes said "leucine" instead. The episode clarifies that leucine is an amino acid relevant to building muscle, while lutein, spelled L-U-T-E-I-N, is the compound meant for vision. It notes that a few listeners caught the slip.

Responding to listener questions on color vision, the episode notes that some animals see colors humans can't, including into the infrared and far-red. Still, it calls human color vision remarkable in people who are not colorblind. It recommends "What Is Color? 50 Questions and Answers on the Science of Color" by Arielle and Joann Eckstut, which it says answers questions such as why the online dress looked blue to some people and yellow to others. The speaker states he has no business relationship with the authors. He did connect them with color vision scientists, who vetted the content.

The episode sets chemicals beside the other physical stimuli the senses detect. Photons land on the retina and the brain builds vision. Sound waves create hearing. Mechanical stimuli such as pressure, light touch, scratch, tickle or a breeze bending skin hairs create touch. Then there are volatile chemicals, where "volatile" just means floating in the environment. Smelling a rose, a cake or something putrid means literally inhaling particles of those chemicals, which neurons that are part of the brain then detect.

Several routes of entry are listed. The main ones are the nose and the mouth, where chewing or sucking food breaks it into components the tongue senses. Chemicals can also enter through other damp mucosal linings, mainly the eyes. A few can pass through the skin, though the episode notes that not many do.

Exposure can be deliberate, as when we choose and chew a food. It can be unplanned, as when we walk into a room, smell smoke and react. It can also be social, as when we eat something to be polite. Finally, other people actively produce chemicals in their breath and tears. The episode stresses that they may "possibly" also produce pheromones, molecules released mainly through the breath that enter through our nose, eyes or mouth and change our biology.

The main example of chemical communication between people is a paper published about 10 years ago in Science. The episode explains that Science, Nature and Cell are considered the three top journals. It says they turn away about 95% of submissions without review and publish only a small share of the rest.

In the study, women cried and their tears were collected. Male subjects then smelled either the tears or a saline control. Men who smelled the tears, which had been evoked by sadness, showed a significant drop in testosterone. They also showed reduced activity in brain areas associated with sexual arousal.

The episode warns against over-reading the result and criticizes the design itself. The study used only female tears and male smellers. The full set of comparisons was missing: female tears smelled by women, male tears by men, male tears by women, and so on. On the plus side, the sample was large, which adds statistical power.

Recruitment also mattered. To get tears truly caused by sadness rather than by an eye irritant, the researchers had participants watch a sad movie scene. They specifically recruited people who said they tend to cry at sad movies, which, as the episode notes, does not describe all women. Even with these limits, the episode concludes that the study shows chemicals in tears changing another person's biology.

The episode explains why it chose this example. The point is not mainly tears and hormones, though it finds the results interesting. The point is that chemicals from other people strongly shape our internal state, and that we are often close to other people's tears in couples, families and close friendships. People readily notice pulling back from a bad smell or leaning toward a delicious one, but chemical communication between people gets far less attention.

The episode keeps the neurobiology basic. Smell starts with sniffing: volatile chemicals can't enter the nose unless we inhale them. That is also why covering the nose or breathing out makes smelling harder. The nose has a mucus lining designed to trap incoming particles. The olfactory bulb sits about two centimeters above the roof of the mouth, at the base of the brain. Its neurons reach out of the skull into that mucus lining, extending dendrites and other small branches that respond to different odorant compounds.

Olfactory neurons also send a branch deeper into the brain, where it splits into three paths. The first handles innate, hard-wired responses that last from birth to death. These include detecting smoke, which signals danger. These neurons project to the amygdala, which the episode describes as handling fear and threat detection more broadly than fear alone. Such smells raise alertness and can wake you from sleep, unless training overrides the response, as for a firefighter. The same innate system also has neurons tuned to odors that draw us toward something, such as a cookie, a savory dish or an orange, with no learning required.

The second path handles learned associations with smells. One example is remembering the smell of a grandmother's home or hands, or of food baking. The episode says these memories often carry a sense of safety and nurture. It links smell's tie to memory to its age, correcting itself mid-sentence to say chemical sensing is among the most ancient senses and almost certainly evolved before vision and hearing. Young people take a long time to learn who is friendly and which places are safe, but the olfactory system seems to lay down powerful associations very early.