This article considers theoretical questions regarding the role of intramolecular isotopes ratios in living organism functioning, human body in particular. Special attention is given to isotopes fractioning of carbon in human body as the most studied. The aim of this investigation is to identify and to study strategic connection between features of metabolic transformations in human body, its functional status and intramolecular isotopes ratios, peculiar to this organism. The relation between intramolecular isotopes ratios values of human body separate tissues and human organism functional status is observed.