Dr. Noam Sobel: How Smells Influence Our Hormones, Health & Behavior | Huberman Lab Podcast

In this episode of the Huberman Lab podcast, Dr. Andrew Huberman hosts Dr. Noam Sobel, a professor of neurobiology at the Weizmann Institute of Science, to explore the intricacies of olfaction and chemosensation. They discuss the remarkable human ability to detect and process smells, often as effectively as dogs, and how this influences our interactions and relationships. Humans subconsciously collect chemical information from others, affecting emotions, stress levels, and decision-making. They delve into phenomena such as the nasal cycle, which reflects autonomic nervous system dynamics, and the potential for smell-based diagnostics and digital olfaction. The conversation also touches on the social and reproductive implications of body odors, the impact of tears on hormone levels, and the potential for future technologies to digitize and transmit smells. Dr. Sobel's groundbreaking research underscores the profound and often underappreciated role of olfaction in human life.

• Dr. Noam Sobel is a professor of neurobiology at the Weizmann Institute of Science.
• His laboratory focuses on olfaction (sense of smell) and chemosensation (response to chemicals in the environment).
• Humans can smell as well as dogs and can sense the chemical world around them effectively.
• Humans process chemical information from people they meet, which influences emotions, decisions, and relationships.
• Tears from others can impact hormone levels.
• Alternation in nostril breathing reflects nervous system dynamics and affects alertness and sleepiness.
• Humans constantly sense their own odors, which influences cognition and behavior.

"Humans can smell things around them as well as dogs can. In fact, humans are incredibly good at sensing the chemical world around them."

• Humans have a remarkable sense of smell, comparable to dogs.

"Every time you meet somebody, you are taking chemicals from that person either from the chemical Cloud that surrounds them or directly from the surface of their body."

• Human interaction involves the exchange and processing of chemical information.

"Tears, yes the tears of others, are impacting your hormone levels in powerful ways."

• Emotional responses such as shedding tears can chemically influence others.

"There is an alternation of ease through which you can breathe through one nostril or the other, and that alternation reflects an underlying Dynamic of your nervous system."

• The nasal cycle alternates nostril dominance, affecting alertness and sleepiness.

• Olfaction involves both orthonasal (through the nose) and retronasal (through the mouth) pathways.
• Airborne molecules travel up the nose to the olfactory epithelium, which contains receptors that transduce chemical signals into neural signals.
• Humans have about 7 million olfactory receptors of approximately 350 different kinds.
• Olfactory signals are transmitted via the olfactory nerve through the cribriform plate to the olfactory bulb and then to various brain regions, including the piriform cortex, amygdala, and hypothalamus.

"Molecules Airborne molecules travel up our nose a distance in the human of about six or seven centimeters to about here where they interact with... the olfactory epithelium."

• The journey of odor molecules to the olfactory epithelium.

"We have about seven million such receptors lining a structure known as the olfactory epithelium."

• The olfactory epithelium contains millions of receptors.

"The olfactory nerve goes through those holes and synapses at the first Target in the brain, which is the olfactory bulb."

• The pathway of olfactory signals from the nose to the brain.

• Olfactory memories are often more robust than other types of memories.
• The first exposure to a smell generates a particularly strong memory representation.
• Olfactory information projects directly to memory-related brain regions like the hippocampus.

"There's something about the first time you experience a smell that generates a particularly robust representation more than other sensory stimuli."

• Initial exposure to a smell creates a strong memory.

"The smell immediately reminded him of an event in his childhood where the same Madeleine appeared."

• Example of the strong connection between smell and memory (Proust effect).

• Humans have a remarkable sense of smell, comparable to sophisticated machines.
• Training can significantly improve one's ability to track scents.
• An experiment demonstrated that humans could follow an odor trail using their sense of smell alone.

"Humans have an utterly remarkable sense of smell. Our detection threshold is 0.2 parts per billion."

• Humans' detection threshold for certain smells is extremely low.

"We brought in participants, naive participants, not graduate students from our lab, completely deprived them of any other sensory input... and they could just do this right off the bat."

• Experiment showing humans' innate ability to follow a scent trail.

• The nasal cycle alternates airflow between nostrils approximately every two and a half hours.
• This cycle is linked to the balance in the autonomic nervous system.
• The nasal cycle becomes more pronounced during sleep.
• Research is ongoing to use the nasal cycle as a marker for diseases and conditions like ADHD.

"The nasal cycle alternates every two and a half hours on average."

• The periodic alternation of nostril dominance.

"This is linked to balance in the autonomic nervous system."

• Connection between the nasal cycle and autonomic nervous system balance.

"We built a wearable device that measures airflow in each nostril separately and logs it for 24 hours."

• Development of a device to measure and analyze the nasal cycle.

"We can tell the difference between ADHD and non-adhd adults and whether they are on Ritalin or not just from the nasal airflow recording."

• Potential use of nasal cycle measurements as a diagnostic tool.

• The myth of Bloodhounds having a billion olfactory receptors was debunked.
• Many scientific and medical claims propagate without proper evidence.
• The idea that yoga practitioners can control their nasal cycle was tested and found to be untrue.

"The myth of Bloodhounds having a billion receptors in their nose was totally made up and propagated through the literature."

• Example of how scientific myths can spread without evidence.

"No yoga teacher that we found could willfully switch between left and right nostril flow."

• Testing and debunking the claim that yoga practitioners can control their nasal cycle.

These notes provide an exhaustive overview of the key ideas and topics discussed in the transcript, formatted for clarity and detail.

• The nasal cycle alternates airflow between nostrils and is linked to the autonomic nervous system.
• The periodicity of nasal airflow is approximately two and a half hours.
• Studies are being conducted to determine if stress can shift this nasal cycle.

"Given that the alternating flow through one or the other nostrils reflective of the autonomic nervous system has this two and a half hour periodicity, if I suddenly enter a bout of stress for instance, does it switch because that's reflective of the autonomous nervous system?"

• The speaker is questioning if stress can alter the nasal cycle, indicating a link between stress and autonomic function.

"We have one experiment where we're exposing participants to pain using a cold water hand exposure... and already in so now I'm sharing pilot data with you... it seems that the exposure to cold generates a shift in the nasal balance."

• The experiment suggests that autonomic arousal caused by pain can drive shifts in nasal airflow.

• Olfactory receptors converge in glomeruli in the olfactory bulb.
• The olfactory system is unilateral with a mirror representation on both sides of the brain.
• Alternating nostril airflow is likely not an olfaction story but related to broader brain function.

"I don't think that the nasal cycle is an olfaction story... I think the nasal cycle story is a different story about brain function."

• The nasal cycle is proposed to be more about brain function rather than solely olfaction.

"We think that nasal inhalation is timing and driving a lot of aspects and patterns of neural activity and cognitive processing."

• Nasal inhalation may influence neural activity and cognitive processing.

• Cognitive tasks are performed better during inhalation compared to exhalation.
• Nasal inhalation has a more pronounced effect on cognitive task performance than mouth inhalation.

"In that particular task people performed significantly better on inhalation versus exhalation."

• Cognitive performance is enhanced during inhalation.

"They also did better on mouth inhalation versus mouth exhalation but the difference wasn't as pronounced as it was with nasal inhalation versus exhalation."

• Nasal inhalation is more beneficial for cognitive tasks than mouth inhalation.

• Nasal breathing is beneficial for overall health, including dental health and cognitive function.
• Mouth breathing is associated with more colds and infections.

"Nose breathing is great for your health relative to mouth... I think it's also good for your cognition not only for your dental health."

• Nasal breathing supports health and cognitive function.

• Loss of smell can be an early indicator of neurodegenerative diseases like Parkinson's and Alzheimer's.
• Current olfactory tests are not specific enough for reliable diagnosis.

"Loss of the sense of smell is one of the if not the earliest sign of neurodegenerative disease... there's reduced upsit in Alzheimer's and Parkinson's and a host of other diseases."

• Loss of smell is an early symptom of neurodegenerative diseases.

"Olfaction has not been effectively digitized... the utterly poor control of the stimulus."

• The difficulty in digitizing olfaction limits its use as a diagnostic tool.

• Humans subconsciously use smell for social interactions and identifying individuals.
• Handshaking may involve subconscious olfactory sampling.

"We noticed... people will shake hands and then... touch their nose."

• Handshaking may lead to subconscious sniffing of the hand.

• People tend to form close friendships with individuals who have similar body odors.
• Similarity in body odor can predict friendship.

"Click friends are indeed more similar in their body order than you would expect by chance."

• Body odor similarity is linked to the formation of close friendships.

"People who smell more similar to each other think that the other person is more likely to be their friend."

• Body odor similarity influences perceptions of potential friendship.

• Romantic partners may be chosen based on body odor, which reflects immune system diversity.
• This selection may be linked to the major histocompatibility complex (MHC).

"Romantic order preferences in humans are influenced by Body order and this is linked to MHC."

• Body odor and MHC play a role in romantic attraction.

• In mammals, olfaction plays a crucial role in reproductive behavior.
• The Bruce effect in mice demonstrates the influence of odor on pregnancy.

"If you expose a pregnant Mouse... to the order of the non-stud male, she will miscarry the pregnancy."

• Odor can trigger miscarriage in mice, indicating a strong link between olfaction and reproduction.

• The human vomeronasal organ is considered vestigial and non-functional.
• However, some researchers believe it may still play a role in human olfaction.

"The going notion is that the human Jacobson organ or Romanus organ is vestigial... I'm just not sure that it's not."

• The functionality of the human vomeronasal organ remains uncertain.

• A high percentage of human pregnancies end in miscarriage, many of which are unexplained.
• There may be a remnant of the Bruce effect in humans.

"Humans have an enormous number or ratio of spontaneous miscarriage... there’s a statistical backdrop or setting if you will force something like a Remnant Bruce effect in humans."

• The high rate of unexplained miscarriages in humans suggests a possible olfactory influence.

• URPL is diagnosed after two consecutive unexplained miscarriages.
• Some couples in the study experienced up to 12 consecutive unexplained miscarriages.
• Medical tests fail to identify the cause of these losses.

"Patients and participants in a study of people who or couples who are experiencing what is referred to as unexplained repeated pregnancy loss."

• URPL is a highly emotional and difficult condition for affected couples.

"This is an emotional difficult place to be and these are couples who are losing their pregnancy for no apparent reason."

• Hypothesis: There may be an olfactory-related cause similar to the Bruce effect in mice.

"We hypothesized that perhaps here there's something akin to a Bruce type effect."

• Investigated if women experiencing URPL could identify their spouse's body odor better than control women.
• Women with URPL showed significantly higher acuity in identifying their spouse's body odor.

"Women who experience repeated pregnancy loss can identify their husbands or their spouses by their body odor with much greater acuity than the typical person."

• fMRI studies showed significant brain response differences to stranger male body odor in women with URPL, particularly in the hypothalamus.

"There was a difference in response to stranger male body odor between the two groups, and it was in the hypothalamus."

• The term 'pheromone' is controversial and often misapplied to humans.
• Humans emit chemo signals that influence each other's physiology and behavior.

"Humans definitely emit chemo signals from their body and these chemo signals influence other humans and influence their behavior."

• The 'smell of fear' is a well-studied example, where fear-induced body odor increases arousal in others.

"Humans emit a particular body odor when they're in a state of fear and this body odor influences other humans in effect increasing their autonomic arousal."

• Scent marking and olfactory memory play roles in human relationships and emotional states.
• The smell of a partner's clothing can evoke positive emotions and reduce anxiety during separation.

"It's not uncommon for romantic partners when one is traveling or away for the other partner to smell their article of clothing in order to bring about positive connotations of the other partner."

• Olfactory cues are deeply integrated into human bonding and emotional experiences.

"There’s something in the breath of romantic partners that’s hopefully appetitive not aversive as well as in children."

• Hexadecanal, a chemo signal, affects aggression differently in men and women.
• In men, hexadecanal lowers aggression; in women, it increases aggression.

"Hexadecanal consistently reduced aggression in men... Hexadecanal increased aggression equally significantly in women."

• The effect is hypothesized to be related to evolutionary roles in parental behavior.

"Paternal aggression is often directed at you... maternal aggression is often protective."

• Hexadecanal is abundant in baby head odor, suggesting an evolutionary mechanism for reducing paternal aggression and increasing maternal protectiveness.

"Hexadecanal is the most abundant semi-volatile in baby heads."

• The phenomenon of menstrual cycle synchronization among co-housed women is debated.
• Initial studies suggested olfactory cues could synchronize cycles, but later studies have called this into question.

"She collected sweat from donor women and deposited it on the upper lip of recipient women... one extended the cycle in recipients and one shortened the cycle in recipients."

• The statistical complexity of cyclic events makes this phenomenon difficult to study conclusively.

"Statistics of cyclic events are surprisingly complicated... the findings were since called into question quite widely."

• Emotional tears may serve as a chemical signal rather than just ocular maintenance.
• Exposure to emotional tears reduces testosterone levels in men.

"When you sniff them you have a pronounced reduction in testosterone within about 20 minutes half an hour."

• Tears are odorless but still have a significant physiological effect when inhaled.

"Tears are completely odorless... and yet when you sniff them you have a pronounced reduction in testosterone."

These notes comprehensively cover the key ideas discussed in the transcript, providing an exhaustive overview suitable for study purposes.

• The study observed a significant effect of tears on lowering testosterone levels.
• An independent group from South Korea replicated the testosterone effect with consistent results.
• MRI studies showed a reduction in brain activity in the hypothalamus and fusiform gyrus during arousing states.

"The testosterone effect was replicated by an independent group from South Korea, with the same numbers."

• Demonstrates the robustness of the original findings.

"MRI studies showed a pronounced effect on brain activity, particularly a dampening effect in the hypothalamus and fusiform gyrus."

• Indicates a neurological basis for the observed hormonal changes.

• The study initially had three components: testosterone effect, brain activity, and behavior.
• The behavioral component showed a weaker effect, focusing on arousal ratings from pictures.
• A lab in Holland challenged the study, particularly the behavioral effects.

"The original study had three components: testosterone effect, brain activity, and behavior. The behavioral effect was significant but weaker."

• Highlights the multi-faceted approach of the original study.

"A lab in Holland didn't like our tear story because they built a career on the notion that emotional tears are uniquely human."

• Illustrates the scientific debate and differing viewpoints.

• The Holland lab attempted to replicate only the behavioral part and failed.
• The original researchers offered to collaborate on a replication, but the offer was declined.
• A rebuttal was published to address the failed replication attempt.

"The Holland lab tried to replicate only the behavioral part and failed. They did it very wrongly in the paper."

• Points to methodological differences as a reason for failed replication.

"We offered to fund a graduate student to come over and do the study together, but they refused."

• Shows the original researchers' commitment to scientific integrity.

• Emotional tears are not unique to humans; similar effects were observed in rodents.
• Mouse pup tears lower aggression in adult male mice.
• Dogs emit emotional tears when reuniting with their owners, affecting human oxytocin levels.

"Mouse pup tears lower aggression in adult male mice in a smell-dependent way."

• Demonstrates the cross-species occurrence of emotional tears.

"Dogs emit emotional tears when reuniting with their owners, influencing oxytocin levels in humans."

• Highlights the emotional and biochemical connection between humans and dogs.

• The idea that smell can signal nutrient content is intriguing but not well-studied.
• Processed foods often lack a direct relationship between smell, taste, and nutrient content.
• There is potential for olfactory cues to indicate nutritional value.

"I don't know if the nutrient value of food is systematically encoded in odor, but it's a really good idea."

• Suggests a potential area for future research.

"Processed foods often lack a direct relationship between smell, taste, and nutrient content."

• Points to a gap in current understanding and food industry practices.

• Humans are more similar in their olfactory perception than commonly believed.
• Cultural misconceptions arise from outliers and poor language application to olfaction.
• Similarity matrices can quantify perceptual similarities across individuals.

"Humans are incredibly similar to one another in their olfactory perception, with a correlation of about 0.8."

• Refutes the notion of extreme subjectivity in smell perception.

"The misconception arises from outliers and poor language application to olfaction."

• Explains why people believe in subjective olfactory experiences.

• Efforts are underway to digitize smell, including projects by Google and other companies.
• The goal is to predict and recreate smells using algorithmic frameworks.
• Initial proof of concept includes transmitting smells like violets over IP.

"Google has an offshoot startup called Osmo trying to digitize smell with a significant investment."

• Indicates the serious commercial interest in this technology.

"We generated olfactory metameres, mixtures with zero molecules in common but smell exactly the same."

• Demonstrates a breakthrough in understanding and recreating smells.

• Potential applications include medical diagnostics and enhanced sensory experiences.
• High-resolution olfactory digitization could lead to early disease detection.
• Long-term goals include practical devices for everyday use.

"Every disease will have an odor, and high-resolution olfactory digitization will be used in medical diagnostics."

• Envisions a future where smell plays a crucial role in healthcare.

"We're talking about achieving the olfactory equivalent of 1956 black and white TV, with long-term goals of 4K quality."

• Sets realistic expectations for the current state and future potential of the technology.

• The conversation highlighted the importance and complexity of olfaction research.
• The guest expressed gratitude for the opportunity to share their work and insights.
• Future collaborations and continued research are anticipated.

"Thank you for your interest in science and for giving me the opportunity to share our work."

• Shows appreciation for the platform and audience.

"Future collaborations and continued research are anticipated."

• Indicates ongoing commitment to advancing the field.