Effects of dietary histidine and arginine on plasma amino acid and urea concentrations of men fed a low nitrogen diet.

The effects of dietary histidine and arginine on fasting and 1 and 2 hour postprandial plasma free amino acid and urea concentrations were studied in six young men. For 1 week each, they were fed six different diets containing 6.3 g of nitrogen daily. Each diet contained eight indispensable amino acids, cystine and tyrosine proportioned as in casein and a different mixture of dispensable nitrogen: A) six dispensable amino acids plus argine (diet 1) or plus histidine and arginine (diet 2) in the casein pattern, or B) an isonitrogenous amount of glycine and diammonium citrate alone (diet 3), with histidine (diet 4), with arginine (diet 5) or with histidine and arginine (diet 6). The fasting plasma concentrations of the seven indispensable amino acids assayed and their similar postprandial patterns were unaffected by the dietary treatments. Both fasting and postprandial plasma histidine concentrations were significantly lower when the histidine-low diets were fed than when the histidine-supplemented diets were fed. Histidine supplementation promoted a reduction in fasting plasma urea nitrogens. Proline concentrations were lowered significantly when proline was removed from the dietary amino acid mixtures, but plasma arginine concentrations were unaffected by arginine removal. Plasma histidine was maintained at lower concentrations in dietary histidine deficiency than when histidine was added to the low nitrogen diets.