It was proved in two independent experimental studies with the same group of 11 young sportsmen that creatine (CR) given as food supplement interfered with the metabolism of one- and two-carbon units. CR, given in one dose of 5 g daily during one month, induced reciprocal changes of muscle mass and body fat. These changes were characteristic of each of the four proband groups, into which the probands were divided on basis of the increased or decreased level of CR in urine, and the increased or decreased levels of folates and vitamin B-12 in blood. The comparison of results of short- and long-term CR supplementations showed that individual deviations found within the rank of each group are characteristic for each person under study. The changes of the level of thiodiglycolic acid in urine proved to be good markers for deviations from homeostasis.