Creatine Metabolism in Men: Creatine Pool Size and Turnover in Relation to Creatine Intake

Creatine metabolism was studied in relation to creatine intake and creatinine excretion. Young men were fed 0.23 g creatine/day for 9 days and then 10 g/day for 10 days consecutively. Thereafter, the diet fed was creatine-free. From day 81 through 90, isonitrogenous amounts (4 g N/day) of either an equimolar mixture of the creatine precursors arginine and glycine or of alanine were added to the diet. As reported in a previous paper, creatinine excretion increased during creatine feeding, continued to remain elevated immediately afterwards and then decreased gradually during the period of feeding the creatine-free diet, whereas two subjects not fed creatine showed no significant changes in creatinine output throughout the experiment. The present paper describes studies in which di-15N-creatine was injected into the same subjects on two occasions during the creatine-free period, near the beginning of this period and 43 days later. By isotope dilution, the creatine pool sizes were calculated and the rate of conversion of this pool to creatinine was computed. The pool of body creatine diminished during the creatine-free period in parallel with the daily output of creatinine, that is, the fractional rate of conversion of creatine to creatinine was very similar for all subjects (0.0169 ± 0.0006 day-1, n = 13). In contrast to the marked constancy of the rate of conversion of creatine to creatinine, apparent fractional creatine synthesis rates were much more variable between subjects (0.011 to 0.016 day-1). Administration of the creatine precursors arginine and glycine significantly increased apparent creatine synthesis, whereas administration of alanine depressed synthesis. From these data on di-15N-creatine metabolism, it can be concluded that (a) the size of the body pool of creatine can be influenced by dietary creatine, (b) administration of precursor amino acids can increase the rate of synthesis of creatine, (c) creatinine output is a constant fraction of the body creatine pool and can change independently of lean body mass.