May 1, 2023 • 3hr 10min

Dr. Noam Sobel: How Smells Influence Our Hormones, Health & Behavior

Huberman Lab

In this episode, Dr. Andrew Huberman interviews Dr. Noam Sobel, professor of neurobiology at the Weizmann Institute of Science. Dr. Sobel is a leading expert on the human olfactory system and chemosensation. The discussion covers the incredible capabilities of human smell, how we use chemical signals to interact with others, the links between smell and behavior/physiology, and cutting-edge research on digitizing odors.

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Key Takeaways

Humans have an incredibly powerful sense of smell, able to detect certain odors at concentrations as low as 0.2 parts per billion
The human olfactory system consists of about 7 million olfactory receptors in the nose that detect odor molecules and send signals to the olfactory bulb in the brain
Humans can improve their ability to track scents and follow odor trails with practice, performing nearly as well as dogs in some cases
There is an alternating nasal cycle where airflow shifts between the left and right nostrils about every 2.5 hours, linked to autonomic nervous system balance
Humans constantly smell themselves and others, using chemical signals to gain information and influence behavior/physiology
Body odor similarity may play a role in friendship formation and social bonding
Human tears contain chemical signals that can lower testosterone and aggression in men
Loss of smell is often an early sign of neurodegenerative diseases like Alzheimer's and Parkinson's
Digitizing and transmitting smells electronically may be possible in the future, with applications for communication, food, and medical diagnostics

Introduction

In this episode, Dr. Andrew Huberman interviews Dr. Noam Sobel, professor of neurobiology at the Weizmann Institute of Science. Dr. Sobel is a leading expert on the human olfactory system and chemosensation. The discussion covers the incredible capabilities of human smell, how we use chemical signals to interact with others, the links between smell and behavior/physiology, and cutting-edge research on digitizing odors.

Topics Discussed

Structure and Capabilities of the Human Olfactory System (6:46)

Dr. Sobel explains the basic structure of the human olfactory system:

Humans have about 7 million olfactory receptors lining the olfactory epithelium in the nose
There are about 350 different types of olfactory receptors in humans
Odor molecules bind to receptors, which send signals via the olfactory nerve to the olfactory bulb in the brain
From the olfactory bulb, signals are sent to other brain regions like the piriform cortex, amygdala, and hypothalamus

He emphasizes that humans have an incredibly sensitive sense of smell, contrary to popular belief:

"Humans have an utterly remarkable sense of smell. To put that a bit into sort of, you know, things that are tangible. So, for example, mercaptans, which are added to cooking gas so that we smell it...our detection threshold, that is the level at which we can detect it, is 0.2 parts per billion."

Human Odor Tracking Abilities (29:07)

Dr. Sobel describes experiments showing humans can track odor trails almost as well as dogs:

Participants could follow scent trails buried under grass while crawling on hands and knees
With practice, humans could track scents as fast as they could crawl
Having two separate nostrils improved tracking performance compared to one combined nostril

"Within average of four days, the rate limiting factor became the speed at which they could crawl. So as fast as you could crawl, you could scent track."

The Alternating Nasal Cycle (39:25)

Dr. Sobel explains the alternating nasal cycle:

Airflow shifts between the left and right nostrils about every 2.5 hours on average
This cycle is linked to balance in the autonomic nervous system
The shift becomes very pronounced during sleep
This provides a non-invasive way to measure autonomic nervous system balance

"You're walking around with a marker on balance in your autonomic nervous system. And we do nothing with it."

Smell and Neurodegenerative Disease (54:47)

Loss of smell is often an early sign of neurodegenerative diseases:

Olfactory loss can precede other Parkinson's symptoms by 10+ years
There are theories that some neurodegenerative diseases may enter the brain through the olfactory system
Olfactory testing could potentially be used as an early screening tool

Congenital Anosmia (1:00:12)

Dr. Sobel discusses congenital anosmia - being born without a sense of smell:

Affects about 0.5% of the population
Often not diagnosed until age 14 on average
Associated with shorter lifespan and reduced social/romantic contacts
May be linked to lack of or underdeveloped olfactory bulbs

Social Chemosignaling (1:06:19)

Humans constantly exchange chemical signals that influence behavior and physiology:

People often sniff their hand after shaking hands with someone
We may use body odor similarity to form friendships
Fear produces detectable odors that can influence others' emotions

"We're finding more and more mechanisms where we do this. And the one I'm referring to now, for one example, is we started looking at handshaking. Handshaking is this really odd behavior."

Body Odor and Friendship Formation (1:15:07)

Dr. Sobel describes research on how body odor similarity may influence friendship:

People who become fast friends tend to have more similar body odors
An electronic "nose" could predict friendship potential between strangers based on body odor similarity
We constantly smell ourselves and others, often subconsciously

"People who smell more similar to each other think that the other person is more likely to be their friend, is more likely to be a nice person, et cetera, et cetera. So we could actually predict friendship using the electronic nose."

Odors and Romantic Attraction (1:22:02)

Dr. Sobel discusses how odors may influence mate selection:

Humans may prefer partners with different immune system genes, detectable through odor
This could promote genetic diversity in offspring
More research is needed on human pheromones and mate selection

Tears as Chemical Signals (2:12:11)

Dr. Sobel describes research on human tears as chemical signals:

Women's emotional tears, when sniffed by men, lowered testosterone levels by about 14%
Tears also reduced brain activity in regions associated with sexual arousal
Tears may act as a chemical signal to reduce male aggression

"When you sniff them, you have a pronounced reduction in testosterone within about 20 minutes, half an hour...about a 14% drop in free testosterone."

Digitizing Smell (2:52:12)

Dr. Sobel discusses efforts to digitize and transmit smells electronically:

His lab has developed algorithms to predict odor similarity of molecular mixtures
They've created "olfactory metamers" - different molecules that smell the same
Recently transmitted the smell of violets over the internet in a proof-of-concept
Could enable adding smells to online communication, VR experiences, etc.

"We now solve that problem. We can predict the odor of any molecular mixture. We can say how it's going to smell."

Future of Olfactory Digitization (3:05:50)

Dr. Sobel speculates on future applications of digitized smell:

Could enable new forms of medical diagnostics
May allow constant health monitoring through odor analysis
Still in early stages, comparable to 1950s black and white TV

"When olfaction digitization is brought to the equivalent of vision and audition that you have now, then it will be in medical diagnosis. You'll have...an electronic nose in your bathroom...and it will be doing diagnostics all the time."

Conclusion

This wide-ranging discussion highlighted the incredible capabilities of the human sense of smell and how we use chemical signals to interact with our environment and each other. Dr. Sobel's research reveals that humans are constantly engaged in chemical communication, often subconsciously influencing behavior, emotions, and physiology. Emerging technologies to digitize smell could open up new frontiers in communication, food experiences, and medical diagnostics. Overall, the conversation emphasized how central our often-overlooked sense of smell is to human biology and behavior.