Why is medicine not sweet?

Heavy water tastes sweet

Astounding difference: Although heavy water differs from normal water only in its hydrogen isotope, deuterium, it also behaves differently biochemically, as a study has revealed. Unlike normal H2O it triggers a reaction in our taste receptors. As a result, heavy water does not taste neutral, but slightly sweet. Evidence of this reaction clears up a controversy that has existed for almost 100 years.

In 1931, the chemist Harold Urey discovered that the element hydrogen has a heavy isotope. About one in 6,400 naturally occurring hydrogen atoms carries an additional neutron in the nucleus and thus becomes deuterium. If this deuterium reacts with oxygen, heavy water is formed (D.2O). Although this has about ten percent higher density and slightly different freezing and boiling points, heavy water reacts chemically in the same way as its normal counterpart.

Does heavy water taste different?

But at one point, opinions have been divided since the time of Urey: “There were anecdotal reports from the 1930s that the taste of heavy water differs from that of normal, neutral-tasting H.2O is different, ”explain Natalie Ben Abu from the Hebrew University of Jerusalem and her colleagues. Accordingly, the D2O taste slightly sweet.

Urey followed up these rumors together with a colleague and stated in 1935: "None of us could notice the slightest difference in the taste of ordinary distilled water and the taste of pure heavy water." That put an end to the assumptions and hardly anyone left the question further - until now.

Clear sweetness

Ben Abu and her team have now re-examined the question of taste using modern methods. To do this, they carried out taste tests with human subjects and mice, analyzed the binding behavior of heavy water to cell cultures of taste receptors and carried out biophysical modeling.

The surprising result: Urey was wrong. “Despite the fact that the two isotopes are actually chemically identical and should taste the same, we have clearly shown that humans H2O and D2O can distinguish by taste, ”reports co-author Pavel Jungwirth from the Czech Academy of Sciences in Prague. "Heavy water therefore has a distinctly sweet taste."

Human sweet receptor reacts

Strangely enough, mice do not seem to notice this sweetness: In the tests they showed no preference for heavy water, although they are usually quite susceptible to sweets. Apparently, their taste receptors did not respond to the deuterium-enriched water. The scientists therefore suspected that D2O activates a sweet receptor that only humans have, but not mice.

In fact, a supplementary experiment with human volunteers showed that if an inhibitor for this TAS1R2 / TAS1R3 receptor was added to the heavy water, they too tasted good2O no more sweetness. In cell cultures, the researchers were also able to observe how this receptor responded electrophysiologically in the presence of heavy water. A proportion of ten percent heavy water in the added water sample was sufficient for this reaction.

Mechanism still puzzling

"This result confirms that the heavy water gets its sweetness through the taste sensor TAS1R2 / TAS1R3", states the research team. “Our study clears up an old controversy about the taste of heavy water. People can therefore very well distinguish between heavy and normal water by taste. "

Why this is so and why this sweet receptor reacts to the heavy water at all is still open. As the researchers explain, the deuterium in the water molecule causes a somewhat stronger bond to the oxygen and thus slightly changes the quantum states in the molecule. This also changes the molecular vibrations and the proteins of the receptor appear a bit more solid and compact than in pure water.

But why this is enough to influence the receptor protein, the models could not clarify either. Here the scientists hope for the results of future analyzes.

Reaction also in other body tissues?

The newly discovered sweet effect is not only interesting as a curiosity: heavy water is also used in medicine as a contrast marker, for example in magnetic resonance imaging. "Because the sweet receptor is not only found on the tongue but also in other tissues in the human body, our discovery that heavy water causes reactions in it is also relevant for doctors and their patients," write Ban Abu and her colleagues. (Communications Biology, 2021, doi: 10.1038 / s42003-021-01964-y)

Source: Czech Academy of Sciences (IOCB Prague)

April 14, 2021

- Nadja Podbregar