Deuterium is one of the two hydrogen isotopes. The nucleus of most hydrogen atoms in nature contains one proton (chemical symbol: 1H), but the nucleus of some hydrogen atoms can contain one proton and one neutron or one proton and two neurons. In the first case, it is called deuterium (chemical symbol: 2H or D), and the second - tritium (chemical symbol: 3H or T).

The abundance of deuterium in the water on Earth is approximately one deuterium atom to 6400 hydrogen atoms (156,25 parts per million-ppm), or 0.0156%. This deuterium concentration changes very little from a natural water source to another.

The deuterium concentration in the adult body is of approximately 120 to 140 ppm. Although it doesn't seem much, if we compare this quantity to the mass of other vital elements in the blood, we can see that deuterium is present in an amount six times greater than calcium and ten times greater than magnesium.