Think for a moment about the little bumps on your tongue. You may have seen a diagram of the arrangement of taste buds in your biology textbook: sweet sensors at the tip, salty sensors on either side of that, sour sensors behind those, and bitter sensors at the very back.
But the idea that certain tastes are localized to specific areas of the tongue is a myth that “persists in the collective consciousness despite decades of research proving it to be false,” according to a review published this month in the New England Journal of Medicine. Also false is the idea that taste is limited to the mouth.
This old diagram, used in many textbooks for years, comes from a study published in 1901 by German scientist David Hanig. But the scientist wasn't suggesting that the different tastes are separate on the tongue. He was actually measuring the sensitivity of different regions, according to Paul Breslin, a researcher at the Monell Chemical Senses Center in Philadelphia. “What he found was that some parts can detect substances at lower concentrations than other parts,” Breslin said. For example, the tip of the tongue is densely populated with sweet sensors, but it also contains other sensors.
It's easy to see the errors in the map: if you place a slice of lemon on the tip of your tongue it tastes sour, but if you place a little honey on the side it tastes sweet.
Taste perception is an incredibly complex process that begins with the first contact with the tongue. Taste cells have a variety of sensors that send signals to the brain when they encounter nutrients or toxins. For some tastes, taste chemicals enter through tiny pores in the cell membrane.
These taste receptors aren't just found on the tongue: They're also found in the digestive tract, liver, pancreas, fat cells, brain, muscle cells, thyroid and lungs. Though these organs aren't generally thought of as tasting anything, they use their receptors to sense the presence of different molecules and metabolize them, said Diego Bojorquez, a self-described gut-brain neuroscientist at Duke University. For example, when your intestine notices that your food contains sugar, it tells the brain to alert other organs to prepare for digestion.
Dr. Breslin likens the system to an airport getting a plane ready to land.
“Think about what would happen if a plane landed in an airport terminal that wasn't ready,” he says: There would be no one there ready to guide the plane to the gate, clean it, or unload the baggage.
Taste prepares things, he said: It wakes up the stomach, stimulates saliva production, and sends small amounts of insulin into the bloodstream, which then carries sugar to the cells. Ivan Pavlov, a Russian physiologist who won the Nobel Prize in 1904 for his work on digestion, showed that if you put a chunk of meat directly into a dog's stomach pit, it won't be digested unless you start the digestion process by dusting the dog's tongue with dried meat powder.
Dr. Bojórquez was inspired to explore the gut-brain connection 20 years ago, when, as a graduate student, a friend who had had bariatric surgery asked her why she no longer disliked fried eggs. Dr. Bojórquez suspected that perhaps her weakened gut taste receptors had detected that she wasn't getting enough nutrition and were sending signals to her brain that she would now prefer the yolk of a soft-boiled egg.
He and his colleagues discovered a connection in the lab: Taste receptor-bearing cells in the gut, which he called the neuropods, make direct contact with nerve cells that signal the brain that nutrients are in the gut.
“Taste perception is more complex than just taste buds,” says Dr. Bojorquez.
Recent research has only made things more complicated. Umami, the savory taste found in foods such as fish sauce and ketchup, began to be recognized by researchers as a fifth taste category in the late 1980s and early 1990s, about 80 years after Japanese chemist Kikunae Ikeda coined the term. The National Library of Medicine now lists more than 2,100 research papers on umami.
A few years ago, a team of Australian researchers suggested that there might be a special taste receptor for fat. Dr. Breslin and his colleagues are studying how taste receptor cells identify fat, information that may help explain why some people overeat.