Background: In the current literature, no agreement exists on estimates for aromatic amino acid (phenylalanine plus tyrosine) requirements as measured by stable-isotope techniques.

Objective: The goal of the present study was to determine the phenylalanine requirement in healthy men who were fed a diet without tyrosine by using the indicator amino acid oxidation method.

Design: Five healthy men were assigned to receive in random order diets devoid of tyrosine and with 8 graded intakes of phenylalanine (5, 10, 15, 25, 35, 45, 60, and 70 mg x kg(-1) x d(-1)). The phenylalanine requirement was measured by the rate of 13CO2 release (F13CO2) from L-[1-(13)C]lysine oxidation.

Results: The graded intakes of phenylalanine had no effect on lysine flux, as required for this method. The phenylalanine (ie, total aromatic amino acid) requirement, in the absence of tyrosine, was estimated to be 48 mg x kg(-1) x d(-1) by applying a two-phase linear regression crossover model to the F13CO2 data.

Conclusions: In the absence of tyrosine, the mean phenylalanine requirement is higher than the current FAO/WHO/UNU (1985) and Dietary Reference Intake (2002) recommendations.