Continuous 24-h L-[1-C-13]phenylalanine and L-[3,3-H-2(2)]tyrosine oral-tracer studies at an intermediate phenylalanine intake to estimate requirements in adults

The daily rates of whole-body phenylalanine oxidation (phe-ox) and hydroxylation (phe-OH) were determined in young men (n = 10) receiving [C-13]phenylalanine and [H-2(2)]tyrosine via primed constant oral infusion (four also received simultaneously [H-2(4)]tyrosine and [H-2(3)]leucine via primed constant intravenous infusions) continuously for 24 h (first 12 h fast and then 12 h fed). The subjects were given a diet supplying a proposed requirement phenylalanine intake (six subjects: 39 mg phenylalanine . kg(-1). d(-1) without tyrosine; four subjects: 36 mg phenylalanine plus 6.8 mg tyrosine), based on an otherwise adequate L-amino acid mixture for 6 d before the tracer study. Our hypothesis was that the subjects would be in approximate body phenylalanine equilibrium at these intakes. Estimates of the daily rate of phe-ox were 26.9 +/- 7.5 mg . kg(-1). d(-1) (17.2 +/- 5.2 and 9.7 +/- 3.2 mg . kg(-1). d(-1) during the 12-h fast and fed periods, respectively), and for phe-OH they were 32.1 +/- 11.9 mg . kg(-1). d(-1) (21.7 +/- 10.5 and 10.4 +/- 2.5 mg . k(-1). d(-1) during the 12-h fast and fed periods, respectively). The daily phenylalanine balance was approximately neutral (P > 0.05) when based on phe-ox or phe-OH (+4.73 +/- 7.34 and -0.41 +/- 12.6 mg . kg(-1). d(-1), respectively). In comparison with recent, comparable 24-h tracer studies at deficient (22 mg . kg(-1). d(-1)) and generous (100 mg . kg(-1). d(-1)) phenylalanine intakes, these results support the hypothesis that a phenylalanine intake of 39 mg . kg(-1). d(-1) (without significant tyrosine) approximates the mean requirement in healthy adults. This contrasts with the upper requirement value of 14 mg . kg(-1). d(-1) for the total of the aromatic amino acids proposed in 1985 by FAO/WHO/UNU.