Think for a minute about the little bumps on your tongue. You probably saw a diagram of those taste bud arrangements once in a biology textbook: sweet sensors on the tip, salty on both sides, sour in the back, bitter in the back.
But the idea that specific tastes are confined to certain areas of the tongue is a myth that “persists in the collective consciousness despite decades of research debunking it,” according to an article published this month in the New England Journal of Medicine. The concept that taste is limited to the mouth is also wrong.
The old diagram, used in many textbooks over the years, originated in a study published by David Hanig, a German scientist, in 1901. But the scientist wasn't suggesting that the various tastes are separated on the tongue. He was actually measuring the sensitivity of different areas, said Paul Breslin, a researcher at the Monell Chemical Senses Center in Philadelphia. “What he found was that you can detect things at a lower concentration in one part than in another,” Dr. Breslin said. The tip of the tongue, for example, is full of sweet sensors but also contains others.
Map errors are easy to confirm. If you put a lemon wedge on the tip of your tongue, it will taste sour, while if you put a little honey on the side, it will taste sweet.
Taste perception is a remarkably complex process, starting with that first encounter with the tongue. Taste cells have a variety of sensors that signal the brain when they encounter nutrients or toxins. For some flavors, tiny pores in cell membranes let taste chemicals in.
Such taste receptors are not limited to the tongue; they are also found in the gastrointestinal tract, liver, pancreas, fat cells, brain, muscle cells, thyroid, and lungs. We don't generally think of these organs as tasting anything, but they use receptors to sense the presence of various molecules and metabolize them, said Diego Bohórquez, a self-styled gut brain neuroscientist at Duke University. For example, when the gut notices sugar in food, it tells the brain to alert other organs to prepare for digestion.
Dr. Breslin likens the system to an airport preparing for a plane to land.
“Imagine if a plane landed at an airport terminal that wasn't ready,” he said. No one would be willing to drive the plane to the gate, clean it up, or unload the baggage.
Taste, he said, prepares things. It wakes up the stomach, stimulates salivation and sends some insulin into the blood, which in turn transports sugars into the cells. Ivan Pavlov, a Russian physiologist who won a Nobel Prize for his studies on digestion in 1904, demonstrated that pieces of meat placed directly into a hole in a dog's stomach would not be digested unless you dusted the dog's tongue with some beef jerky powder to start. things outside.
Dr. Bohórquez was inspired to look for a gut-brain connection two decades ago, when he was in graduate school and a friend who had undergone bariatric surgery asked him why he no longer hated sunny eggs. Dr. Bohórquez thought that perhaps the taste receptors in her now-weakened gut sensed that she wasn't getting enough nutrients and started signaling to her brain that, hey, eating runny egg yolks would be a good idea now.
He and his colleagues found a connection in the lab. Cells carrying taste receptors in the gut, which he called neuropods, make direct contact with nerve cells that let the brain know that a nutrient is in the gut.
“Taste perception is more complex than just taste buds,” said Dr. Bohorquez.
More recent studies are only making the issue more complex. Umami, a savory taste found in foods such as fish sauce and ketchup, began to be accepted as the fifth taste category by researchers in the late 1980s and early 1990s, nearly 80 years after it was was proposed by Kikunae Ikeda, a Japanese chemist. More than 2,100 research articles on umami are now listed by the National Library of Medicine.
Several years ago, an Australian research team suggested that there might be a special taste receptor for fat. Dr. Breslin and others are studying how taste receptor cells identify fat, information that could be useful in understanding why some people overeat.