Alteration in the urinary excretion rate of amino acids and nitrogen by dietary means in obese and normal human subjects.

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Abstract The effect of altered nutritional states in man on the urinary excretion of total nitrogen, urea, taurine, and 13 individual amino acids was examined. With the exception of leucine and β-aminoisobutyric acid (AIB), the excretion of these urinary constituents was decreased during starvation, as well as during maintenance on an isocaloric protein-free diet. Although there was no significant difference in the urinary excretion of nitrogen, urea, taurine, and amino acids by nonobese and obese subjects during starvation, there was a marked difference in the excretion of these constituents by fasted and protein-deprived normal subjects. A greater excretion of nitrogen and urea, and a lesser excretion of taurine, occurred during starvation than during the isocaloric protein-free dieting. Among the examined amino acids there was a significant difference in the excretion rate of 5 amino acids in these 2 groups of subjects, while their renal clearance of amino acids remained the same. The subjects excreted a greater amount of 3 branched-chain amino acids, β-AIB, and a smaller amount of alanine during starvation than during the isocaloric protein-free dieting. These observations together with our previous data on plasma amino acid concentrations suggest that the difference in urinary excretion pattern of free amino acids exhibited during starvation and during protein-free dieting is principally a reflection of differences in plasma amino acid composition during these altered nutritional states. Moreover, excess stored lipids (obesity) does not provide any metabolic advantage in reducing the amino acid catabolism during the short-term (6 days) starvation period.