The Biology of Taste Perception & Sugar Craving | Dr. Charles Zuker

KEYPOINTS

• 01

  Detection and perception are separate steps

  Zuker, a Columbia neuroscientist and Howard Hughes investigator since 1989, defines detection as tongue cells binding a sugar molecule, before anything is perceived. Perception happens when the brain turns that electrical signal into meaning. He notes the brain is about 2% of body mass yet uses roughly 25 to 30% of the body's energy and oxygen. Because every brain encodes signals slightly differently, your experience of the same stimulus can differ from someone else's.

• 02

  Five basic tastes, each with a built-in meaning

  Sweet (energy), umami (amino acids and protein; examples are seaweed, tomatoes, cheese and MSG) and low salt (electrolytes) are innately attractive. Bitter (likely toxins) and sour (likely spoiled or fermented food) are innately aversive. You are born liking sweet and disliking bitter. Flavor is a broader experience combining taste with smell, texture, temperature and appearance.

• 03

  Fat and metallic may not be true tastes

  Zuker says much of what we call fat taste is probably mechanosensory: cells sensing fat rolling over the tongue rather than a dedicated taste receptor. Metallic taste, like a penny or blood, may be a particular ratio of activity across the existing five lines, the way pressing several piano keys produces a chord.

• 04

  The tongue map is a myth

  Each taste bud holds around 100 receptor cells, and Zuker's lab, which identified receptors for all five tastes (sour most recently), found that virtually every taste bud in the mouth contains all five types. Bitter receptors are somewhat enriched at the back of the tongue, a last line of defense that can trigger gagging before you swallow. The palate is rich in sweet receptors. He suggests the classic map likely came from mistranslated early data.

• 05

  Taste identity and taste valence live in different brain areas

  Sweet and bitter run as separate labeled lines from the tongue through the taste ganglia and brainstem stations to separate areas of taste cortex, in well under a second. Valence (whether it's good or bad) is imposed in separate amygdala targets for sweet and bitter. His lab has engineered mice that recognize sweet without finding it attractive.

• 06

  Activating taste neurons directly creates the full experience

  In mice, silencing sweet cortex neurons erased the perception of sugar. Activating bitter cortex neurons while the animal drank plain water made it gag as though tasting something bitter. In a place-preference test, mice sought out the side of a box where their sweet neurons were stimulated and avoided the side where bitter neurons fired, which shows these signals carry a felt positive or negative state and don't just drive licking.

• 07

  Internal state rewrites what tastes good

  Taste signals pass through several relays because each one lets hunger, thirst or depletion modulate the signal. Low salt is appealing and seawater is repellent, but salt-deprived animals find very concentrated salt (1 molar sodium chloride) highly attractive. Thirst suppresses hunger to conserve water: without water you'd die in about a week, while you can go without food for far longer.

• 08

  Liking is the tongue, wanting is the gut

  Mice without sweet receptors at first drink equally from sugar water and plain water. Within about 48 hours they drink almost only the sugar, finding the bottle by smell, texture or location. Cells in the intestine, not the stomach, sense glucose and signal the vagal (nodose) ganglion and brainstem that energy has arrived. Zuker calls this gut-brain signal the basis of our unquenchable desire for sugar.

• 09

  Why artificial sweeteners don't end sugar cravings

  Sweeteners activate the same tongue receptor as sugar, but the gut sensor is a different molecule that recognizes glucose, not sweeteners. Zuker believes that's why artificial sweeteners have failed to curb our appetite for sugar: they satisfy liking but never tell the brain sugar arrived. A goal for future research is finding ways to activate this circuit without sugar.

• 10

  Processed foods hijack a deliberately slow system

  The gut signal builds slowly, Zuker argues, so that in the wild an animal links its preference only to the richest, most reliable sugar sources after repeated exposure. Highly processed foods arrive pre-broken-down, activating these circuits on a timescale nature never produced and continually reinforcing wanting. He views obesity as a disorder of brain circuits rather than simply of metabolism.