In the 1930s, scientists discovered a heavier form of water. So-called “heavy water” (D2O) weighs more because the nucleus of each of its two hydrogen atoms contains not just a proton but a neutron as well. Known as deuterium, heavy hydrogen causes subtle differences in heavy water—from small increases in boiling and freezing points to a roughly 10% increase in density.
Now, an international team of researchers has confirmed another difference long rumored to be true: Heavy water tastes sweet. “It’s a very gentle sweetness,” says study author Masha Niv, a taste scientist at the Hebrew University of Jerusalem. “It’s not like Sweet’n Low.”
Heavy water won’t work as a sweetener. Indeed, in large quantities, it’s lethal. But the work, recently reported in Communications Biology, could inspire a deeper understanding of how the sweet taste receptor works.
Heavy water’s flavor has rarely been formally tested as part of an experiment. Harold Urey, who discovered deuterium, tasted heavy water in 1935 and reported with a colleague in Science that it was no different than regular water. And a more recent study relied on human sensory studies alone to investigate. But anecdotally, some chemists have reported that heavy water tastes sweet—including Niv’s coauthors, chemists Pavel Jungwirth and Phil Mason of the Czech Academy of Sciences in Prague. So they and collaborators decided to put that sweetness to the test.
They first purchased commercial heavy water and distilled it further to remove impurities. The team then conducted a series of sensory tests to determine whether volunteers could distinguish heavy water and regular water. In each case, volunteers sampled three drops—two of heavy water and one of regular water, or vice versa—and then tried to identify the odd water out. In a smell test, only 9 out of 25 people chose correctly, about a third, which is expected by chance. But in a taste test, 22 out of 28 did. And in a taste test with noses plugged, over half did, suggesting that taste receptors on the tongue were indeed picking up the subtle flavor.
Through other taste tests, the team found that as the ratio of heavy water to regular water in a mixture increased, so too did the sweetness. They also learned that heavy water added to the sweetness of other sweeteners, but had no effect on the savory taste of umami and diminished the bitterness of low levels of quinine.
Sugars and other sweeteners activate the taste receptor known as TAS1R2/TAS1R3. To learn whether heavy water does the same, the team offered volunteers samples of heavy water as well as heavy water mixed with lactisole, a molecule that blocks the sweet receptor. Eighteen of 25 volunteers said the mixture without lactisole tasted sweeter, suggesting that heavy water indeed activates the same taste receptor as sugar. “How can such a small difference in water activate specifically this particular receptor?” says Niv. “It is really strange.”
To confirm the finding, the team studied human cells engineered to express the sweet taste receptor. They again found that heavy water activated the receptor, while lactisole blocked activation. They also offered heavy and regular water to mice, but the animals showed no preference. This finding was not unexpected, says Niv, as humans can taste some sweeteners, such as aspartame, that mice cannot.
Finally, the researchers built models to begin exploring how heavy water might interact with the sweet taste receptor. The exact structure of the receptor is unknown, but based on similar receptors, the models suggest several possible mechanisms. For example, heavy water may limit the mobility of the receptor, holding it in a more activated position. “Once we know what exactly within this receptor is responsible for this activation, this will allow us to better understand the sweet taste receptor, and then suggest additional ways of activating it,” says Niv.
“They are providing a lot of good evidence that indeed [heavy water] does have a sweet taste and that sweet taste—like all of those other chemically diverse sweet tasting stimuli—is working via this same receptor,” says neurobiologist Steven Munger, director of the Center for Smell and Taste at the University of Florida in Gainesville, who was not involved in the study. (Munger doubts these findings will lead to new artificial sweeteners, as the exact mechanism is not clear and the public’s appetite for synthetic ingredients is waning.)
“How exactly this heavy water actually activates the sweet receptor—that is pretty puzzling to me,” says molecular biologist Peihua Jiang of the Monell Chemical Senses Center in Philadelphia, who hosted the study’s lead author, Natalie Ben Abu of The Hebrew University of Jerusalem, at the center during preliminary stages of the work. He’d like to see a 3D structure of the receptor made through a technique such as cryogenic electron microscopy so that researchers can better visualize how it interacts with heavy water, as well as sugar, artificial sweeteners, and sweet-tasting proteins.
Niv hopes additional modeling work will help uncover further clues. If models suggest that heavy water interacts with a particular region on the receptor, she’ll engineer slight tweaks in that region and then test whether activation is blocked. “Once we get there,” she says, “then we’ll know that we really cracked it.”
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