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Contributions of Individual Body Tissues to Nitrogen Excretion in Adult Rats Fed Protein-Deficient Diets

Department of Nutrition, School of Medicine, Tokushima University, Tokushima 770, Japan

An attempt was made to estimate the contributions of various tissues to nitrogen (N) excretion in rats fed protein-free diet or 2 and 10% lactalbumin diets for 14 to 31 days. The contributions were calculated as differences between the amounts of dietary N utilized for tissue protein and tissue growth. The amount of dietary N utilized for tissue protein was calculated from the recovery of ¹⁵N of orally administered [¹⁵N]leucine and N intake. The following results were obtained. 1) With a 10% protein diet, the contribution of tissues to N excretion was affected more by the turnover rates of individual tissues than by the size of the tissues, but with a protein-free diet the contribution was greatly affected by the size of the tissues. 2) With 2% lactalbumin diet, the contributions were not intermediate between those with protein-free diet and 10% lactalbumin diet: urinary N excretion was similar to that with protein-free diet in spite of the difference in N intake, probably mainly because the contribution of skeletal muscle was lower. 3) Differences in the dietary protein level affected the utilization of dietary protein only a little, but influenced body protein metabolism very much.

KEY WORDS: • tissue contributions to N excretion • whole-body protein metabolism • ¹⁵N-labeled amino acid • N balance

Manuscript received 26 July 1982.