Urinary creatine, which in the adult is found only in exceptional cases, for example in inanition, occasionally in women and in certain types of disease in which the catabolic processes are accelerated, is a constant and seemingly normal associate of creatinine in the urine of children up to the age of puberty. Of the various theories advanced in explanation of this, the most generally accepted is that the immature muscle, although producing no more creatine per unit of muscle mass than the adult, has a lower capacity for retaining it. Other theories, with the experimental evidence supporting them, have been thoroughly reviewed by Hunter.1
The variation in the amount of creatine excreted in normal children appears to be influenced by a number of factors, for example, the amounts of creatine and of protein ingested, and the periodicity of the food intake which may result in shorter or longer periods
DANIELS AL, HEJINIAN LM. GROWTH OF INFANTS FROM THE STANDPOINT OF PHYSICAL MEASUREMENTS AND OF NITROGEN METABOLISM: II. CREATINE. Am J Dis Child. 1929;38(3):499–506. doi:10.1001/archpedi.1929.01930090051006
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