Sweet, salty, sour and bitter? No, the textbook taste map of the tongue is a myth

The traditional taste map of the tongue that is taught in school is a myth. The idea that tastes like salt and sweet are perceived in neatly defined areas of the tongue is wrong, or at best, a simplistic interpretation of how this intricate sensory network really works. See for yourself — put some grains of salt on the tip of your tongue, which is sweet flavors are supposedly perceived. No doubt you will be able to taste the saltiness. Science has shown that our sense of taste is much more complex than the taste map in textbooks — what’s more, it goes beyond the mouth.

A scientific review published in the New England Journal of Medicine has reexamined how exactly the mouth perceives taste and recalled that other organs, such as the intestine, also have taste receptors. “It is time to let go of old ideas, such as the myth of the tongue taste map (which persists in the collective consciousness despite decades of research debunking it) and the notion of taste as limited to the mouth. Research reveals that downstream signaling of extraoral taste receptors regulates our physiological balance long after conscious gustation has faded,” states endocrinologist Josephine M. Egan, from the Laboratory of Clinical Investigation at the U.S. National Institute on Aging, in the article.

The journey of the sense of taste begins when chemicals, food flavorings, reach the mouth and activate taste receptors. These cells — which are buried in the taste buds — are the ones that recognize the primary sensory qualities, the five flavors: sweet, salty, bitter, sour and umami. Humans can have up to 4,500 taste buds, and each of them has about 60 taste receptors. Once activated, these sensory cells send information about the flavor to the brain, which in turn integrates it with the stimuli coming in from our sense of smell and the information conveyed by the trigeminal nerve (in charge of registering texture, temperature or pain, for example). This is the complex process of taste perception, explains Egan.

According to Egan, the taste map that appeared in school textbooks — with the primary tastes concentrated in specific areas — “is not accurate.” “Taste receptor cells sensitive to each tastant are present in taste buds across the tongue, meaning that there is no taste map representing regions of the tongue corresponding to specific tastes,” she argues.

José Manuel Morales, member of the Otology commission of the Spanish Society of Otorhinolaryngology and Head and Neck Surgery (SEORL-CCC), agrees that the taste map is “quite simplistic.” “It comes from a misinterpretation of a German article from the early 20th century. Yes, there are different sensitivity thresholds in different areas of the tongue, but that was interpreted as exclusive areas. And it is not like that. Taste buds are present across the tongue, and they all perceive everything. But, depending on these stimulation thresholds, some flavors or others are perceived more,” he explains.

The “evolutionary gatekeeper”

The perception of a flavor is subjective, but taste qualities have meaning for the body. Taste is, in Egan’s words, a kind of “evolutionary gatekeeper” for the substance that enter our body: it guides humans towards tasty foods that contain energy and away from dangerous substances. For example, sweet preference is “innate, developed well before birth, and consuming sweet tastants triggers satisfaction through central reward pathways,” says Egan. While bitter and acidic flavors, on the other hand, detect potentially toxic substances, which is why people tend to have an aversion to them.

Dietary patterns also shape taste, adds Egan: “Western diets rich in fat and carbohydrates change the proteomic landscape of the tongue, and obese, diabetic mice and their offspring have an increased preference for sweet stimuli.” But the researcher clarifies that a direct connection between obesity and taste perception in humans has not yet been demonstrated.

Our sense of taste does not end in the mouth, either. There are extraoral receptors that, although they do not directly perceive flavors like the tongue, are also activated in one way or another when different taste stimuli reach them. “Researchers have discovered diverse roles for extraoral taste receptors, such as regulating male fertility and protecting tissue in the pulmonary vasculature. The gut has emerged as a site for exploring the involvement of taste receptors and their downstream signaling pathways in appetite, nutrition, and disease,” scientist explains in the article.

Taste is not an isolated sense in the mouth. Morales points out, in fact, that “the most important thing for the perception of flavors” is the “interrelation between smell and taste.” “In order for you to perceive the nuances of a flavor, you need your sense of smell,” he argues.

Beyond the mouth

Regarding the taste receptors beyond the mouth, Morales clarifies: “It is not that we have taste buds in other parts of the body. But there are receptors in the cells that are similar to those in the cells of the salivary glands and that can be activated with the same elements. And these can influence the secretion of dopamine, the feeling of greater or lesser satiety or the secretion of insulin.”

But this does not mean that the intestine, for example, can perceive a specific flavor: “Receptors in the intestine do not transmit the sensation of taste, but rather trigger a process of regulation or deregulation of insulin or something else.” He explains: “A patient with stomach cancer, who has a gastrostomy [a feeding tube that goes directly to the stomach], does not via their mouth. If we were able to perceive flavor in the intestine, that patient could tell if what was reaching them was sweet or salty. And they can’t. Because taste, as we all understand it, is detected in the mouth. Receptors in other parts of the body have another function.”

Diego Bohórquez, a neuroscientist at Duke University, discovered in animal studies that cells that detect glucose in the intestine not only identify it, but also distinguish it from other sweeteners and communicate that information to the brain using different neurotransmitters. “When the mouse is stimulated, it chooses to consume the glucose because that brings it calories and the sweetener does not. And when we turn off these cells in the intestine — not on the tongue — the mouse literally becomes blind and cannot distinguish between a sweetener and a sugar,” he says.

By way of comparison, the researcher point to people’s preference for sugary soft drinks over their low-calorie counterparts. “We instinctively prefer sugar more than sweetener. What’s fascinating is that before it was believed that it was only happening in the tongue and, in reality, it is the intestine that is guiding that decision-making,” he says.

In this context, he says, the tongue “simply creates an initial representation, like an alert.” If it is something toxic, the tongue will say no and will not consume it. But once the mouth accepts food, the intestine has to be “trusted,” says Bohórquez. “The intestine has to detect, discern, communicate and react to that stimulus. And we are realizing that the intestine not only changes with respect to the stimulus, but often, it also anticipates the stimulus. The change in the gut’s sensory ability is what causes us to crave a certain food,” he adds.

Thus, stimulating the taste receptors on the tongue is not the same as the person not receiving the sugar stimulus in the intestine, Bohórquez explains: “There are some references to chewing gum causing reflux in many people, and it causes reflux because it is receiving the sweetness here [in the mouth] and the intestine is preparing itself, but it does not receive the sugar. So, it gets irritated.”

More flavors

Researchers are still investigating the limits of taste, how big a role is played by each organ, and how to construct a map of taste. There is even talk of new taste modalities, such as the taste of fat. Although there is debate on this issue.

“This is like a palette of colors, with the three basic ones you can obtain the whole chromatic range,” says Morales. “Flavors come from a combination of the different flavor qualities of food. A fatty or metallic flavor is a combination of the five basic ones. Is spicy a taste modality? Not for me. Taste sensitivity is a combination of chemical sensations but also of perception through the trigeminal nerve, which gives us the sensation of temperature and texture. If spiciness gives you a trigeminal sensation, I wouldn’t identify it as a taste”

Bohórquez also highlights the subjectivity of perception: “The beauty of biology is that diversity. With a combination of five things and the variability in each of the receptors, that diversity creates such a diverse representation of the world for each individual, which is completely unique. What is sweet for you is completely different from what is sweet for me, even though we are talking about a chemical that is completely standardized.”

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