Effect of Protein and of Free L-Methionine Intake on Amino Acid Excretion by Human Subjects

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Abstract
The ability of the adult human being to cope with a broad range of amino acid intake and even an excessive intake of one of the essential amino acids, methionine, without undue metabolic stress as judged by N balance, urinary S partition studies, and urinary amino acid excretion is illustrated in the study. Human subjects were fed at 2 levels of the same protein; in addition each level of protein intake was supplemented with 9 g of free L-methionine daily for 4 days. Between 78 and 93% of the methionine-S was excreted as inorganic sulfate S; the subjects were always in N equilibrium or positive balance. Methionine, cystathionine, and histidine content of the urine increased when the methionine supplement was fed. Glycine content decreased. Little effect on the excretion of 3-methylhistidine, 1-methylhistidine, and anserine was noted. Doubling the intake of protein caused a twofold increase in excretion of α-aminoadipic acid and cystathionine; no such relationship was noted in the other 25 ninhydrin-reacting substances determined in the urine by ion-exchange chromatography. The fasting plasma aminogram was altered when the protein intake was increased.