This may come as a bit of a shocker, but technically speaking, not all water on Earth is made up of H2O molecules.
Less than a century ago, the discovery of the hydrogen isotope deuterium – 2H, but often simplified to D – revealed the existence of another kind of water with the chemical formula 2H2O or simply D2O.
Here's how they differ. A typical hydrogen atom contains one proton within its nucleus. The deuterium isotope, however, has a neutron in addition to the proton, giving the hydrogen atom a greater mass. Hence, water formed with this type of heavy hydrogen is usually called... heavy water.
Aside from that one key difference between H2O and D2O – which gives heavy water about 10 percent greater density than regular water – these two water types are chemically the same, although deuterium does exhibit slightly different bonding behavior to regular hydrogen (which is also known as protium, by the way).
Because of that altered bonding behavior – which can affect bodily chemistry if you ingest deuterium in D2O – scientists generally say it's not a great idea to drink heavy water, at least not in high doses.
Small amounts are considered harmless for humans, however, and are in fact often administered to participants in scientific experiments.
Due to such incidental consumption, now going back almost a century, there's been a longstanding question over whether heavy water tastes the same as regular drinking water – or whether its subtle isotopic variation yields a different taste that people may be able to perceive.
"There is anecdotal evidence from the 1930s that the taste of pure D2O is distinct from the neutral one of pure H2O, being described mostly as 'sweet'," an international team of researchers led by first authors and biochemists Natalie Ben Abu and Philip E. Mason explains in a new study.
"However, Urey and Failla [the former being Harold Urey, the scientist who discovered deuterium] addressed this question in 1935 concluding authoritatively that upon tasting 'neither of us could detect the slightest difference between the taste of ordinary distilled water and the taste of pure heavy water'."
But was that conclusion a tad premature? Ben Abu and Mason say that Urey and Failla's unequivocal opinion on the topic effectively stifled further research in this area for much of the next century, at least in terms of human taste-testing.
Tests in rats have shown that too much heavy water consumption can be fatal to the animals, but evidence for whether rats can taste the difference remains unclear.
In the last two decades or so, advancements in our understanding of human taste receptors have prompted a reopening of old cases like this – and in their new research, Ben Abu, Mason, and their team can finally confirm that there really is something a bit different about the taste of heavy water.
"Despite the fact that the two isotopes are nominally chemically identical, we have shown conclusively that humans can distinguish by taste (which is based on chemical sensing) between H2O and D2O, with the latter having a distinct sweet taste," explains senior author and physical chemist Pavel Jungwirth from the Czech Academy of Sciences.
In a taste-testing experiment with 28 participants, most people were able to distinguish between H2O and D2O, and tests with mixed amounts of the waters revealed that greater proportions of heavy water were perceived as tasting sweeter.
In tests with mice, however, the animals did not seem to prefer drinking heavy water over regular water, although they did show a preference for sugared water – suggesting that in mice, D2O does not elicit the same sweet taste that people can perceive.
Other taste tests conducted by the team suggest why this is so, indicating that human taste receptivity to D2O is mediated by the taste receptor TAS1R2/TAS1R3, which is known to respond to sweetness in both natural sugars and artificial sweeteners.
Experiments in the lab with HEK 293 cells confirmed the same thing, showing robust responses in TAS1R2/TAS1R3 expressing cells when exposed to D2O.
In addition, computational modeling with molecular dynamics simulations revealed slight differences in the interactions between proteins and H2O versus D2O, which the team says needs further study to fully explain, but accords with previous research, and provides another example of nuclear quantum effects in chemical systems, including that of water.
"Our findings point to the human sweet taste receptor TAS1R2/TAS1R3 as being essential for sweetness of D2O," the authors conclude.
"At a molecular level, this general behavior may be traced back to the slightly stronger hydrogen bonding in D2O vs H2O, which is due to a nuclear quantum effect, namely difference in zero-point energy… While clearly not a practical sweetener, heavy water provides a glimpse into the wide-open chemical space of sweet molecules."
The findings are reported in Communications Biology.