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Amino Acid Balance and Nitrogen Retention in Man as Related to Prior Protein Nutriture¹

Nutrition Laboratories, Departments of Animal Sciences and Home Economics, Rutgers-The State University, New Brunswick, New Jersey

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.

Manuscript received 21 May 1965.