24-HOUR L-[1-C-13]TYROSINE AND L-[3,3-H-2(2)]PHENYLALANINE ORAL TRACER STUDIES AT GENEROUS, INTERMEDIATE, AND LOW PHENYLALANINE INTAKES TO ESTIMATE AROMATIC AMINO-ACID-REQUIREMENTS IN ADULTS

Daily pattern and rates of whole-body tyrosine oxidation and phenylala nine hydroxylation were determined in young adults (15 men, 1 woman) r eceiving [C-13]tyrosine and [H-2(2)]phenylalanine via primed, constant oral infusion and [H-2(4)]tyrosine by vein (five subjects also receiv ed [H-2(3)]leucine simultaneously by vein) continuously for 24 h (12 h fast then 12 h fed). Subjects were given a diet supplying 96.6 (n = 5 ), 35.6 (the proposed requirement; n = 5), and 18.5 mg phenylalanine.k g(-1).d(-1) (n = 6) based on an otherwise adequate L-amino acid mixtur e for 6 d before the 24-h tracer study began. (Each diet was low in ty rosine: 6.79 mg.kg(-1).d(-1).) Our hypothesis was that subjects would be in tyrosine equilibrium, positive balance, or both, at the 96.6- an d 35.6-mg intakes and in distinctly negative balance at the 18.5-mg in take. The diurnal pattern in phenylalanine and tyrosine kinetics was d ependent on the intake and, presumably, on the adequacy of dietary phe nylalanine. Whole-body tyrosine balances, determined from rates of phe nylalanine hydroxylation and tyrosine input and oxidation were negativ e (0.05 < P < 0.1 from zero balance) with the low (18.5 mg) phenylalan ine intake [total aromatic amino acid (AAA) intake: 25.3 mg.kg(-1).d(- 1)] but at equilibrium (P > 0.05 from zero balance) with the two highe r phenylalanine intakes. Whole-body AAA balance (AAA intake - tyrosine oxidation) was negative (P < 0.05 from zero balance) with the low int ake, at equilibrium with the intermediate intake, and apparently disti nctly positive (P < 0.05) with the generous intake. Despite model limi tations, as discussed, these findings lend further support for a propo sed, tentative value for a total mean requirement of 39 mg AAA.kg(-1). d(-1).