Creatine metabolism in men: urinary creatine and creatinine excretions with creatine feeding

Abstract

Changes in dietary creatine intake often result in changes in the daily urinary creatinine excretion of humans. The metabolic effects of the presence or absence of creatine in the diet were studied. Young men were fed creatine, 0.23 g/day for 9 days and 10 g/days, consecutively. The subjects were physically trained for vigorous exercise during the 9 day low creatine dietary period. The diet was creatine free for 71 days after creatine feeding. During the last 10 days of the creatine free period, isonitrogenous amounts (4 g nitrogen/day) of either an equimolar mixture of arginine and glycine (precursors to creatine synthesis) or alanine were added to the diet. Creatinine excretion increased during creatine feeding and continued to be elevated afterwards (+10 to 30%). Creatinine excretion decreased gradually during the period of feeding the creatine free diet. The creatinine excretions of two subjects not fed creatine were unchanged during the same period. Creatinine excretion increased in two of four subjects fed precursor amino acids and was unchanged in the others and in the alanine fed subjects. Balances of nitrogen, potassium, and phosphorus were all positive during creatine feeding, and nitrogen balance remained positive throughout the study. Potassium accumulation (1.3 g/day) during the creatine feeding period was in excess of that predicted from nitrogen balances. Positive potassium balance did not continue during the creatine free period and did not occur in the subjects not fed creatine. Creatine deposition itself and/or posttraining glycogen accumulation are possible explanations for the potassium retention. Maintenance of nitrogen and potassium balances coincident with declining creatinine excretion indicated a degree of independence between lean body mass and urinary creatinine excretion.