Inverse relationship between protein intake and plasma free amino acids inhealthy men at physical exercise

The effect of a "normal" (n = 8) and "high" (n = 6) protein intake (1 and 2 .5 g.kg(-1).day(-1), respectively) and of exercise on plasma amino acid (AA ) concentrations, insulin, and glucagon concentrations was followed through out a continuous 24-h period in adult male subjects at energy balance after six days on a standardized diet and exercise program. Subjects were fastin g from 2100 on day 6 to 1200 on day 7 and then fed 10 identical meals hourl y until 2100. Physical exercise was performed (46% maximal oxygen uptake) b etween 0830 and 1000 (fasting) and in a fed state (1600-1730) on each day. The normal-protein group showed fasting plasma AA concentrations that were higher (P < 0.05) than those for the high-protein group, except for leucine , methionine, and tyrosine. Glutamine, glycine, alanine, taurine, and threo nine concentrations were distinctly higher (similar to 30% or greater) thro ughout the 24-h period in subjects consuming the normal- vs, the high-prote in diets. Exercise appeared to increase, although not profoundly, the plasm a concentrations of amino acids except for glutamate, histidine, ornithine, and tryptophan. The profound diet-related differences in plasma AA concent rations are only partially explained by differences in the renal clearance of the amino acids. We speculate on the possible metabolic basis for these findings.