Therefore, for healthy children of the Russian Federation, exogenous factors such as time of the year, place of residence, and preventive administration of cholecalciferol, as well as intrinsic factors such as age and sex, play a determining role in the development of vitamin D deficiency; while genetic factors—polymorphic variants of the genes of phase 1 enzymes of xenobiotic metabolism (CYP2C9, CYP2C19, CYP2D6, CYP3A4) and the VDR gene—do not. This evidence concerns the gene CYP2C9 and vitamin D deficiency.