Amino Acid Balance and Nitrogen Retention in Man as Related to Prior Protein Nutriture

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Abstract
A series of experiments was carried out with male college students to determine the influence of prior protein nutriture on the utilization of high N-low tryptophan diets. There was an inverse relationship between prior protein nutriture and the subsequent utilization of the high N-low tryptophan diet. When subjects consumed a balanced, high protein diet they were in strong positive N balance, which, when followed by the high N-low tryptophan diet, led to little N retention. When the initial protein intake consisted of a low level of hydrolyzed casein, supplemented with adequate tryptophan, the initial N balance was essentally zero; during the intake of the high N-low tryptophan diet, the subjects were in greater positive N balance than previously noted following intake of the balanced, high protein diet. When, during a third experiment, the initial period consisted of a low protein intake coupled with an inadequate intake of tryptophan, the subjects were in strong positive balance during the subsequent ingestion of the high N-low tryptophan diet. When, during a fourth experiment, the subjects were immediately given the high N-low tryptophan diet, they were in severe negative N balance. Significant differences in urinary creatinine excretion were observed in some experiments, with no consistent pattern relative to N intake. The variations in urinary creatinine as well as the changes from normal observed in plasma urea N and amino acid N suggest that in young adults there are marked differences in the ultilization of high N diets inadequate in one or more amino acid which, in immature animals, would bring about depressed growth. Changing the N source used to supplement hydrolyzed casein from gelatin to an isonitrogenous mixture of nonessential amino acids did not change the results obtained with the imbalanced diet.