Thirst is one of the fundamental urges an organism can have. After all, water is the solvent that we rely on to transport essential molecules and other particles around our bodies to keep us alive. So it's perhaps a bit suspicious that water doesn't seem to have any flavor. Shouldn't we have evolved a taste for this essential substance?
Well, perhaps we have. Several new studies seem to indicate not only that water does, in fact, have a taste, but that we might have taste buds that respond directly to it, reports Science.
We now know that there are at least five basic tastes: salty, sour, sweet, bitter and umami. If there is a sixth taste for water, then we should be able to see evidence for it somewhere in the mouth or tongue. Yuki Oka, a neuroscientist at the California Institute of Technology in Pasadena, and colleagues have sought to solve the issue once and for all. They've devised a multifaceted experiment that seeks to identify any water-sensing taste receptor cells (TRCs) on the tongues of lab mice.
How to test for a new taste receptor
The team first used a technique known as genetic knockout to silence different types of TRCs, in order to eliminate the possibility that those cells might accidentally detect some other taste within the water besides the water itself. They then flushed the rodents’ mouths with water to see if any cells were still responding. It turned out that the acid-sensing sour TRCs were still firing vigorously, indicating they these cells might have a dual purpose, capable of distinguishing water or acidic fluids.
Lab mice lacking any sour TRCs were then given a choice of drinking water or a clear, tasteless, synthetic silicone oil, and these mice took far longer to choose water.
After that, the experiment got even weirder. Other lab mice were then bred specifically to express light-sensitive proteins in their acid-sensing TRCs, which made their cells fire in response to light from a laser. In other words, these mice were designed so that if they did have a sense for water, that this sense would respond in exactly the same way to a specific type of blue light. After shining a stream of blue light into the enclosures of the mice, researchers then watched as the mice attempted to drink the light. In fact, some thirsty mice licked the light stream as many as 2,000 times every 10 minutes, probably because the light was not quenching their thirst like water would.
Whether or not the mice were actually tasting the water rather than merely sensing it in some other way is still an open question. It's impossible to know what the experience was actually like for the mice; it's entirely possible that their sense for water is not tied to flavor, even though taste buds provide the sensation.
So if you're on the side of the debate that contends that water is flavorless, you might still be correct. But water isn't senseless, at least it doesn't appear that way according to this research. The study also opens up the possibility that our tongues and tastebuds might be doing more than merely providing us with a sense of taste. If this study doesn't prove that water is the sixth flavor, it might nonetheless be the first evidence of a whole other sense entirely: a water sense.