A long-standing, unresolved puzzle concerns the taste of heavy water. Regular water has no distinct taste but rumors indicate that heavy water tastes sweet. Why is this so if heavy water, D₂O, is practically identical to ordinary water, H₂O?

A team of scientists, led by Masha Niv at Hebrew University of Jerusalem’s Institute of Biochemistry, Food Science and Nutrition and Pavel Jungwirth at the Czech Academy of Sciences, found answers to these questions using cell-based experiments, human subjects, biological models, and molecular dynamics simulations. Their research was published in Communications Biology. They found that heavy water tastes sweet to humans but not to mice, and this “taste” is mediated by humans’ sweet taste receptor.

Heavy water differs slightly from normal water. It is called “heavy” because of the 10% additional density it has when compared to hydrogen. It also freezes and boils at slightly higher temperatures than regular water. How do these minor physicochemical differences contribute to such a difference in taste?

To answer this question, the team conducted heavy water taste tests with people and with biological models. Participants received 3 samples of water—two were ordinary water and the third was heavy water. They were asked to sniff the water, then to taste the water while their noses were closed with clips, and finally to sip the water without nose clips.

The findings: heavy water could be distinguished from regular water based on taste alone. Further, heavy water tasted sweeter than ordinary water to humans, but not to rodents. When the researchers added lactisole, which inhibits sweet taste receptor T1R2/T1R3, to the heavy water sample, it decreased the sweet taste for humans and blocked the activation of sweet taste receptor at the cell level. This demonstrated to the researchers that the sweet taste receptor T1R2/T1R3, which is activated by sugars and artificial sweeteners, is also activated by heavy water.

While you won’t find a bottle of heavy water near the cinnamon and almond milk at your local Starbucks anytime soon, “a better understanding of our sweet taste receptors may lead to new ways of eliciting sweet sensation, without the added sugar,” shared Niv.

These findings may have future implications in the medical field, as well. The fact that heavy water elicits responses of our sweet taste receptors is significant since those receptors are not only located on our tongues but in other tissues, as well. Niv also said, “our sweet taste receptor belongs to a very important family of receptors called GPCRs. GPCRs are important drug targets and deeper insights into their activation mechanisms may deepen our understanding of this useful family of proteins.”

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CITATION: Ben Abu, N., Mason, P.E., Klein, H. et al. Sweet taste of heavy water. Commun Biol 4, 440 (2021). https://doi.org/10.1038/s42003-021-01964-y