Phenylalanine and tyrosine kinetics for different patterns and indispensable amino acid intakes in adult humans

In a previous paper (Am J Clin Nutr 1993;58:670-83) we described results for plasma amino acid changes, leucine kinetics, and body leucine and nitrogen balance in 20 young men receiving diets for 3 wk. The diets were based on the 1985 FAO/WHO/UNU amino acid requirement pattern (modified FAO diet; n = 7), the Massachusetts Institute of Technology (MIT) requirement pattern (MIT diet; n = 7), or the egg-protein pattern (Egg diet; n = 6). It was concluded, in comparison with the MIT and Egg diets, that the modified FAO diet was not capable of maintaining body amino acid homeostasis or balance. Here we report results from tracer studies with L-[O- ²H₅]phenylalanine and [²H₂]tyrosine that were carried out within the same experiment. The modified FAO diet failed to maintain a mean body phenylalanine balance as determined from rates of phenylalanine hydroxylation (corrected for deuterium-isotope-kinetic effects); balance was achieved with the MIT and Egg diets. These results further underscore the inadequacy of the internationally proposed amino acid requirement pattern for healthy adults. We recommend interim use of the MIT, tentative amino acid requirement values in all considerations of adult human amino acid requirements and nutrition.