title: Continuous 24-h L-[1-13C]phenylalanine and L-[3,3-2H2]tyrosine oral-tracer studies at an intermediate phenylalanine intake to estimate requirements in adults.
abstract: The daily rates of whole-body phenylalanine oxidation (phe-ox) and hydroxylation (phe-OH) were determined in young men (n = 10) receiving [13C]phenylalanine and [2H2]tyrosine via primed constant oral infusion (four also received simultaneously [2H4]tyrosine and [2H3]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.kg-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.
authors.affiliation: Laboratory of Human Nutrition, Massachusetts Institute of Technology, Cambridge, USA.
authors.forename: A
authors.initials: A
authors.lastname: Basile-Filho
authors.forename: A E
authors.initials: AE
authors.lastname: el-Khoury
authors.forename: L
authors.initials: L
authors.lastname: Beaumier
authors.forename: S Y
authors.initials: SY
authors.lastname: Wang
authors.forename: V R
authors.initials: VR
authors.lastname: Young
mesh_terms: D000284:Administration, Oral; D000328:Adult; D001944:Breath Tests; D002153:Calorimetry, Indirect; D002245:Carbon Dioxide / Q000032:analysis; D002940:Circadian Rhythm; D005215:Fasting / Q000097:blood; D006801:Humans; D006900:Hydroxylation; D007262:Infusions, Intravenous; D007700:Kinetics; D007930:Leucine / Q000097:blood; D008297:Male; D009751:Nutritional Requirements; D010649:Phenylalanine / Q000008:administration & dosage / Q000097:blood / Q000378:metabolism*; D014443:Tyrosine / Q000008:administration & dosage / Q000097:blood*
chemical_list: D002245:Carbon Dioxide; D014443:Tyrosine; D010649:Phenylalanine; D007930:Leucine