Serum Levels and Urinary Excretion of Amino Acids in Preadolescent Girls on Two Levels of Protein Intake

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Abstract
Concentrations of amino acids were measured in the serum and urine of preadolescent girls fed two levels, 22.1 and 40.3 gm. per day, of protein for thirty-six days. Average fasting serum levels were lower and less variable at the end of the study for all but five of the amino acids measured; aspartic acid, arginine and histidine remained essentially unchanged and changes were equivocal in concentrations of proline and ornithine. Urinary losses of threonine, valine, tyrosine, phenylalanine, aspartic acid and glutamic acid were extremely low at the end of the experiment, ranging from 2.11 to less than 1 mg. per 23 hours. Amino acid levels in serum and urine did not differ much in subjects fed the two different levels of protein. Values for the urinary output of amino acids by these subjects were much lower than those reported in other studies. It is suggested that the higher level of protein may not have been sufficient to provide for more than minimum serum levels of most of the amino acids measured, after needs for growth and maintenance had been met.