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

KEYPOINTS

• 01

  Humans have a remarkable sense of smell

  Sobel, an olfaction researcher at the Weizmann Institute, pushes back on the idea that humans smell poorly. We detect mercaptan, the sulfur additive that gives cooking gas its smell, at about 0.2 parts per billion, which no machine matches well. He also notes that we smell through the mouth (retronasal olfaction) as well as the nose. He points out that a widely repeated claim about a billion receptors in a bloodhound's nose turned out to be made up.

• 02

  Smell loss: training has better evidence than supplements

  Olfactory neurons regenerate. But if a head injury completely severs the olfactory nerve at the cribriform plate, smell is lost for good. Sobel's rule of thumb for viral loss, including COVID, is that if smell hasn't returned within about a year to a year and a half, it probably won't. Because olfaction is 'use it or lose it', smell-training programs have strong evidence, more than alpha-lipoic acid, for which the evidence is 'not overwhelming'.

• 03

  Not all 'smells' are olfactory

  The trigeminal nerve also has endings in the nose and throat, so sharp sensations like lemon can come through it even when true smell is gone. 'Pure olfactants' such as coffee and vanilla stimulate only the olfactory nerve. They are a better test of whether real smell has come back.

• 04

  The nasal cycle tracks your autonomic state

  One nostril usually flows more than the other, and dominance alternates about every 2.5 hours on average. The switch becomes very pronounced in sleep. It is driven by the balance of sympathetic and parasympathetic activity. Sobel's lab built a 24-hour wearable 'nasal Holter' that logs each nostril's flow separately. From those recordings alone, they can tell adults with ADHD from those without, and whether they're on Ritalin.

• 05

  Inhaling through the nose shapes cognition

  Sobel's theory is that because the mammalian brain evolved around sniffing, with information arriving in bursts on each sniff, nasal inhalation came to time neural processing in general. In his lab, people performed a visuospatial task significantly better when trials started on inhalation than on exhalation. He and Huberman favor nasal over mouth breathing whenever possible.

• 06

  Smell loss is an early sign of neurodegeneration

  Smell loss is not diagnostic on its own, because there are many causes. But a large decline, such as smell becoming 50% worse within a year, is a bad sign for Parkinson's or Alzheimer's risk. Olfactory neurons are the one place the brain directly touches the outside world. Sobel says smell tests haven't become routine screening mainly because smell hasn't been effectively digitized.

• 07

  We constantly sniff ourselves and others, mostly without knowing it

  Sobel believes nearly all of our chemical social sensing is subconscious. People sample their own body odor all day and appear to compare it with others' odor.

• 08

  Humans may lack a working vomeronasal organ, but chemosignals still work

  In mice, the Bruce effect, named for the British scientist who described it in 1959, makes a recently mated female miscarry on about 80% of exposures to an unfamiliar male's odor. The human vomeronasal (Jacobson's) organ is considered vestigial, though Sobel isn't convinced it is completely non-functional. Human chemosignaling clearly exists through other routes.

• 09

  Menstrual synchrony findings are contested

  Martha McClintock published menstrual synchrony in Nature in 1971 as an undergraduate, then a 1998 follow-up showing that odors collected at different cycle phases had opposite effects on cycle length. Sobel notes that the findings were later widely questioned, partly because statistics on cyclic events are tricky.