Some factors affecting the nitrogen sparing action of methionine and threonine in rats fed a protein free diet.

It was previously reported that methionine and threonine supplementation of a protein free diet had a greater nitrogen sparing action than methionine supplementation alone. Investigated in this study were (1) effects of depletion of labile body protein on the nitrogen sparing action of methionine and threonine supplementation of a protein free diet, (2) the effects of graded levels of methionine and threonine on urinary nitrogen excretion, (3) the effect of supplementation of other amino acids to the protein free diet plus methionine and threonine on the urinary excretion of nitrogen and (4) sex differences in the nitrogen sparing action of methionine and threonine supplementation of a protein free diet. After 10 days of feeding a protein free diet to deplete the body labile proteins, nitrogen excretion in urine of female rats was significantly reduced within the first 2 days of feeding a protein free diet supplemented with methionine and threonine. After 7 days of feeding a protein free diet supplemented with methionine and threonine, nitrogen excretion was reduced when even as little as 0.0188% of each amino acid was added. The supplementation of all essential amino acids to the protein free diet did not reduce the urinary excretion of nitrogen further than the supplementation of methionine and threonine, but improved the nitrogen balance slightly. The excretion of urinary nitrogen by female rats fed the protein free diet supplemented with small amounts (0.0188%) of methionine and threonine was significantly reduced after 7 to 14 days of feeding. In males, small amounts of methionine and threonine had no significant effect at either 7 or 14 days.