A study finds that acidic taste receptor cells in the taste buds of the tongue also taste plain water, although for what purpose remains unknown.

A camel drinks water in the desert near the Red Sea, Egypt.

Even the textbooks will have to change: from small, children learn that water is colorless, odorless and tasteless. Of course, we have always known that some waters taste better to us than others, but also that this is due to the substances they have dissolved, and not to the solvent itself. Now, a new study confirms something that many scientists had been suspecting, and that is actually water, and not only its content, also stimulates the taste cells of our tongue as do other flavors.

The taste of water or its lack of it is something that has intrigued us since ancient times. Aristotle, who seems to have thought about absolutely everything that a human mind can think, did not miss the question of whether we are really capable of tasting water. The Greek philosopher came to the conclusion that “the natural substance water itself tends to be tasteless.” In the twentieth century, the idea that water has no flavor took hold, and that sometimes it seems different to us depending on what we eat because it brings us a kind of memory of the last flavor we have tasted before drinking.

But something kept the scientists puzzled. When we feel a liquid on the skin, it is our touch system that is responsible; blindly, we are unable to guess whether what has fallen on our arm is water or fruit juice. But inside the mouth we do notice the difference, thanks to the cells of the taste buds that are specialized in detecting one or the other of at least five basic flavors: salty, sweet, bitter, sour and umami (tasty). Today we know that these receptors, although specific for a particular flavor, are actually blind, and that it is the brain that interprets the flavor according to the zone activated by the signal. But what about water? Is it simply its absence of taste that we recognize, or is there some taste receptor that is also activated?

The taste of water

The latter is a possibility that cannot be dismissed lightly, without showing that we do not have receptors for water and that therefore it does not send a neuronal taste signal to the brain. And even less considering recent research that has discovered how some insects and amphibians do have nerve cells that are activated by water. For example, the fruit fly does this thanks to a protein called PPK28.

“We imagined that mammals could also have machinery in the taste system for detecting water,” says Yuki Oka, a biologist at the California Institute of Technology (Caltech, USA). Working with mice, Oka and his colleagues first measured the electrical signals sent to the brain by the five basic flavors through the nerves that carry the sense of taste. And as they describe in their study, published in the journal Nature Neuroscience, discovered that pure water also triggers a signal in these nerves. According to the first author of the study, Dhruv Zocchi, “this was exciting, because it implied that some taste cells were capable of detecting water.”

But what are these taste cells that seem to taste water? To find out, the scientists used drugs and genetic engineering to separately turn off receptor cells for different flavors on the mice’s tongues. When, for example, they blocked salty taste receptors, the animals’ nerves continued to send signals to the brain in response to other flavors, but not salt. And then they ran into an unexpected result: “To our surprise, when we silenced the cells from the sour taste, the response to water was also completely blocked,” says Oka. “The results suggest that the water is detected through the cells of the acid taste“.

Mice that drink light

To confirm their results, the researchers resorted to an ingenious system. Using a newly developed technique called optogenetics, they modified the acid taste receptors in mice to activate with blue light. They then removed the water from the bottles and replaced it with a laser of this color that turned on when the mice tried to drink. The result was that the animals went to the bottle to try to drink the light as if it were water. And they kept doing it over and over again, since the laser stimulated their receptor cells, making them believe that they were tasting the water, but of course they were never satisfied; some animals sucked the cannula from the bottle up to 2,000 times in ten minutes.

The study leaves a big question in the air: why do the acid taste receptors also serve to taste water? Oka and his collaborators do not yet have an answer, but they do have a hypothesis: since saliva is acidic, the biologist speculates, perhaps the cells responsible for this taste detect that we are drinking because the water reduces the acidity in our mouth. Always assuming, of course, that the mechanism of mice also applies to humans, something that must be confirmed.

But Zocchi adds another possible approach, and that is that so far we have misinterpreted it, and actually what until now we believed were receptors of acid taste are not really such, but cells to taste water that also respond to substances such as lemon juice: “it may be that the acid cells are not directly linked to the unpleasant acidity that we perceive, but instead induce a different type of flavor when stimulated , like that of water ”, he suggests.

Before changing textbooks, more research will be needed, but for now we can affirm that a water only tastes like water without the picky eater coming to remind us that water is insipid.