“Heavy Water” is the name given to water where the hydrogen atoms have neutrons making deuterium. Chemically, it is considered almost identical to ordinary water, but some early researchers described it as having a sweet taste, a claim received with great skepticism. Now, however, biologists and biochemists have vindicated those early physicists, and explained how humans can taste extra neutrons.

Besides being the most common element in the universe, hydrogen is usually as simple as an element can be – just one proton and one electron, sometimes known as protium. However, a small proportion of hydrogen atoms formed in the Big Bang also have a neutron, making it deuterium. The water in our oceans reflects this.

The neutron roughly doubles the weight of the hydrogen atom, so when a molecule of water includes a deuterium atom it weighs about 5 percent more than ordinary water, double that when both protium atoms are replaced. Heavy water, produced by isolating the deuterium-containing molecules in seawater, slows down neutrons, affecting nuclear reactions and making fission and fusion easier to achieve. Our taste buds, however, are influenced by chemical, not physical, reactions and the chemistry is hard to distinguish. Consequently, reports in the 1930s of a sweetness to heavy water were considered unlikely, particularly since deuterium's discoverer insisted it is tasteless.

However, Dr Pavel Jungwirth of the Czech Academy of Science thought there might be something to the claim. He gave people unidentified samples of purified heavy and light water and asked if they could tell the difference.

"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," Jungwirth said in a statement.

Mice, on the other hand, showed no preference between bowls of each.

Jungwirth explored this further and in a paper in Communications Biology has announced that lactisole, a sweetness inhibitor, removes people's capacity to tell the two waters apart, providing a clue to how we do it.

Lactisole is known to suppress the TAS1R2/TAS1R3 receptor, one of the pathways by which we detect sweetness. TAS1R2/TAS1R3 is known to vary between rodents and humans. Modeling of proteins in TAS1R3 receptors show small but significant differences in the way they respond to the presence of heavy water, becoming more rigid. Apparently our brains, never having encountered high concentrations of heavy water in our evolutionary history, interpret this as sweetness, a remarkable case of our senses detecting a small quantum effect.

Heavy water can be damaging to cells, but only at such high concentrations it's never likely to pose a threat. Indeed, it is used as a tracer for medical diagnosis.

Much effort is devoted to the quest for sweeteners that tickle our tastebuds, but don't add to our waistlines or risk of diabetes. Theoretically, drinking heavy water instead of soft drinks could be a way to become slimmer, although probably not lighter since your density would rise. However, for non-billionaires, the money would run out long before any benefits became obvious.

That doesn't mean Jungwirth's discovery has no practical application. By showing our tastebuds use more complex mechanisms than previously recognized, the work could help scientists discover more realistic ways to give the sensation of sweetness without the calories.