Phenylalanine and tyrosine kinetics in relation to altered protein and phenylalanine and tyrosine intakes in healthy young men

Plasma phenylalanine (Phe) and tyrosine (Tyr) turnover and the rate of conversion of phenylalanine to tyrosine (Phehyd) and of phenylalanine oxidation (Pheox) after reduced intakes of Phe and Tyr were determined in a metabolic study involving five healthy young adult men. In a pilot study, six postabsorptive young men received either 12- or 4-h infusions of [²H₂]Phe and [1-¹³C]Tyr or [1-¹³C]Phe and [²H₂]Tyro. From these results a primed 8-h constant infusion of [1-¹³C]Phe and [²H₂]Tyr and [²H₃]leucine was used in the metabolic study (first 3 h fasted, the 5 h fed) at the end of 1-wk periods during which subjects received an adequate nitrogen L-amino acid based-diet followed by a restricted intake of Phe and Tyr. This procedure was again repeated after 1 and 3 wk when subjects were given a diet low in both nitrogen and Phe and Tyr. Phe and Tyr fluxes were not significantly affected by diet during the fasted metabolic state but Tyr fluxes were lower when the restricted intakes were given. Compared with the rate during the fasting state, Pheox was significantly higher (P < 0.01) when the adequate diet was consumed; Pheox and Phehyd for fed and fasted states were similar when Phe and Tyr were restricted.