Phenylalanine flux, oxidation, and conversion to tyrosine in humans studied with L-[1-13C]phenylalanine.

Phenylalanine metabolism was determined in 41 studies of adult males (n = 10) consuming an energy-sufficient diet and receiving graded levels of dietary phenylalanine and excess tyrosine (40 mg.kg-1.day-1). After a dietary adaptation period to either 4.2 or 14.0 mg.kg-1.day-1 of phenylalanine; flux, plasma concentration, oxidation, and conversion to tyrosine were measured at test phenylalanine intakes of 5, 7, 10, 14, 21, 28, or 60 mg.kg-1.day-1. Oxidation was low and constant (1.3 mumol.kg-1.h-1) at intakes at or below 10 mg.kg-1.day-1 and increased linearly above this level. Conversion to tyrosine was minimal (2.1%) at these intakes. Breakpoint analysis showed the phenylalanine requirement with excess tyrosine to be 9.1 mg.kg-1.day-1. Plasma phenylalanine concentrations confirmed this estimate of requirement. Prior adaptation did not significantly affect overall flux, plasma concentration, or oxidation nor did it affect the requirement estimate. With the assumption that tyrosine can supply two-thirds of the aromatic amino acid requirement, these data suggest that the aromatic amino acid requirement should be 30 mg.kg-1.day-1 and the World Health Organization recommendation of 14 mg.kg-1.day-1 is an underestimate.