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 re****ted
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.
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