Hydrogen to deuterium isotopic substitution has only a minor effect on physical and chemical properties of water and, as such, is not
supposed to influence its neutral taste. Here we conclusively demonstrate that humans are, nevertheless, able to distinguish D 2 O from
H 2 O by taste. Indeed, highly purified heavy water has a distinctly sweeter taste than same-purity normal water and adds to perceived
sweetness of sweeteners. In contrast, mice do not prefer D 2 O over H 2 O, indicating that they are not likely to perceive heavy water
as sweet. HEK 293T cells transfected with the TAS1R2/TAS1R3 heterodimer and chimeric G-proteins are activated by D 2 O but not by H 2 O.
Lactisole, which is a known sweetness inhibitor acting via the TAS1R3 monomer of the TAS1R2/TAS1R3, suppresses the sweetness of D 2 O in
human sensory tests, as well as the calcium release elicited by D 2 O in sweet taste receptor-expressing cells. The present multifaceted
experimental study, complemented by homology modelling and molecular dynamics simulations, resolves a long-standing controversy about the
taste of heavy water, shows that its sweet taste is mediated by the human TAS1R2/TAS1R3 taste receptor, and opens way to future studies of
the detailed mechanism of action.