Human taste buds can tell the difference between normal water and “ heavy ” water

It may be a little shocking, 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 type of water with the chemical formula 2H2O or simply D2O.

Here is how they differ. A typical hydrogen atom contains a proton in its nucleus. The isotope of deuterium, however, has a neutron in addition to the proton, which gives the hydrogen atom greater mass. Therefore, the water formed with this type of heavy hydrogen is generally referred to as … heavy water.

Apart from this key difference between H2O and D2O – which gives heavy water about 10% more specific gravity than ordinary water – these two types of water are chemically identical, although deuterium exhibits slightly different binding behavior than hydrogen ordinary (also known as protium, by the way).

Due to this altered binding behavior – which can affect body chemistry if you ingest deuterium into D2O – Scientists generally say that it is not a good idea to drink heavy water, at least not in large doses.

However, small amounts are considered harmless to humans and are in fact often given to participants in science experiments.

Due to this accidental consumption, which now dates back almost a century, the question has long been raised as to whether heavy water tastes the same as regular drinking water – or whether its subtle isotopic variation gives off a taste. different that people can perceive.

“There is anecdotal evidence from the 1930s that the taste of pure D2O is distinct from the neutral of pure H2O, being primarily described as ‘gentle’, ”says an international team of researchers led by early authors and biochemists Natalie Ben Abu and Philip E. Mason in a new study.

“However, Urey and Failla [the former being Harold Urey, the scientist who discovered deuterium] addressed this question in 1935, concluding with authority that on tasting “none of us could detect the slightest difference between the taste of ordinary distilled water and the taste of pure heavy water”. “

But was this conclusion a little premature? Ben Abu and Mason say Urey and Failla’s unequivocal opinion on the subject effectively stifled further research in this area for much of the next century, at least in terms of human taste testing.

Tests on rats have shown that excessive water consumption can be fatal for animals, but the evidence as to whether rats can taste the difference remains uncertain.

Over the past two decades or so, advances in our understanding of human taste receptors have caused old cases like this to reopen – and in their new research, Ben Abu, Mason and their team can finally confirm that it there really is something a little bit. different on 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 detection) between H2O and D2O, with the latter having a distinct sweet taste, ”explains lead author and physical chemist Pavel Jungwirth of the Czech Academy of Sciences.

In a tasting experience with 28 participants, most people were able to distinguish between H2O and D2O, and tests with mixed amounts of water revealed that greater proportions of heavy water were perceived to taste sweeter.

In tests on mice, however, the animals did not appear to prefer drinking heavy water over plain water, although they did show a preference for sugar water – suggesting that in mouse, D2O does not elicit the same sweetness that people might experience.

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 the sweetness of natural sugars and artificial sweeteners.

Laboratory experiments with HEK 293 cells confirmed the same, showing robust responses in cells expressing TAS1R2 / TAS1R3 when exposed to D2O.

In addition, computer modeling with molecular dynamics simulations revealed slight differences in the interactions between proteins and H2O against D2O, which the team says requires further study to be fully explained, but ties in with previous research and provides another example of nuclear quantum effects in chemical systems, including that of water.

“Our results indicate that the human sweet taste receptor TAS1R2 / TAS1R3 is essential for the sweet taste of D2O ”, the authors conclude.

“At the molecular level, this general behavior can be attributed to the slightly stronger hydrogen bond in D2O against H2O, which is due to a quantum nuclear effect, namely the difference in energy from the zero point… While clearly not a practical sweetener, heavy water gives a glimpse of the widely open chemical space of sugar molecules. . “

The results are reported in Communications biology.