Heavy water is deuterium oxide, or D2O or 2H2O. Its physical and chemical properties are similar to those of light water, H2O, but the hydrogen atoms are of the heavy isotope deuterium, in which the nucleus contains a neutron in addition to the proton found in the nucleus of the hydrogen atom. This isotopic substitution alters the bond energy of the hydrogen-oxygen bond in water, altering the physical and chemical properties of the substance. Gilbert Newton Lewis isolated the first sample of pure heavy water in 1933.
Now D20 tastes almost exactly like H20. However, seeds will not sprout in heavy water, and when rats are given heavy water exclusively, they die of thirst although glutted with water.
Heavy water present in ordinary water has been found to be extremely harmful to living organisms in many ways. For example, if a chemical reaction in a cell produces as much energy as it consumes, then the cells will become self-sustaining, neither growing nor dying. This level is referred to as critical mass and is analogous to the approximate age of 20 in human terms. This is the age when the body is most stable and physically perfected.
However, deuterium oxide causes cellular metabolism to operate at reduced rates because its higher atomic weight slows down chemical reactions. In our cells, this dampening effect is the equivalent of aging (and eventual death) and begins naturally after the age of about 20. Deuterium accelerates the effect, leading to cellular putrefaction.
Given that we absorb heavy water by drinking and bathing (two thirds of our bodily water is absorbed through the skin), in the ratio of 1 drop for every 6000 drops of ordinary water, we see that such an accumulation would cause definite negative effects in the body. And unlike short-lived laboratory test animals, our longevity makes us increasingly vulnerable to cellular aging.
In addition to dampening the chemistry of cell metabolism, heavy water also impacts cell division, or mitosis.