The effect of dietary protein intake on glutamine and glutamate nitrogen metabolism in humans.

The response of glutamine and glutamate kinetics were studied in five healthy young adult men on diets containing deficient (0.1), adequate (0.8), or surfeit (2.2 g.kg-1.d-1) amounts of protein. Glutamate, glutamine, and phenylalanine kinetics were measured in the postabsorptive state at the end of each diet period. Urinary urea and ammonia excretion correlated with protein intake (the sum of the two was 2.1 +/- 0.2, 5.7 +/- 0.3, and 11.9 +/- 1.2 g N g.kg-1.d-1 for the respective 0.1, 0.8, and 2.2 g.kg-1.d-1 protein intakes). Glutamate and glutamine concentrations varied inversely with protein intake. Phenylalanine concentrations and phenylalanine flux did not change significantly with the changing protein intake. Both glutamate and glutamine fluxes varied inversely with protein intake (glutamate flux was 177 +/- 15, 120 +/- 10, and 125 +/- 11 mumol.kg-1.h-1 and glutamine flux was 373 +/- 29, 343 +/- 26, and 318 +/- 15 mumol.kg-1.h-1 at the respective 0.1, 0.8, and 2.2 g.kg-1.d-1 protein intakes). These changes in glutamine or glutamate flux in response to alterations in dietary protein intake were attributable to changes in de novo production.