Living organisms can react with the isotope molecules in different ways. As dis­cussed in previous chapters, the cause of the physical and chemical isotope effects can be easily understood, but the biological effects are much more complicated. The most important isotope effects occur in the case of the isotope of hydrogen since the hydrogen bond plays a very important role in the secondary and tertiary structures of the proteins and nucleic acids. When substituting deuterium for hydro­gen, the strength of the hydrogen bond increases, i. e., the cleavage of a deuterium bond requires more energy. This increase is similar to the differences in the partial pressure of water under the effect of hydrogen—deuterium substitution. Heavy water (D2O) inhibits or can stop the proliferation of cells. The experience shows that living organism may die when the deuterium—hydrogen substitution happens quickly. However, when the deuterium—hydrogen substitution is slow, the living organisms can adapt to the heavy water. During the adaptation phase, cell destruc­tion or cell proliferation may be observed. After the adaptation, the cells develop as usual.

Recently, there have been some reports claiming that drinking deuterium-free water has desirable physiological effects, such as reducing the risk of cancer. This effect may have been observed in vitro. However, because of the fast isotope exchange of deuterium and hydrogen in the environment (air, nutrients, etc.), deute­rium concentration of the human body cannot be lowered in this manner.