Tyrosine requirement of healthy men receiving a fixed phenylalanine intake determined by using indicator amino acid oxidation

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Abstract
Background: The currently accepted total aromatic amino acid requirement for adults is based on nitrogen balance measurements in individuals who received their intake of aromatic amino acids solely as phenylalanine. Objective: The objective of this study was to determine the requirement for the amino acid tyrosine in healthy men receiving an adequate, but not excessive, intake of phenylalanine (9 mg•kg−1•d−1). Design: The effect of a graded intake of tyrosine was determined in 6 healthy men consuming energy-sufficient diets containing 1 g protein•kg−1•d−1. The tyrosine requirement was determined by using indicator amino acid oxidation methodology with l-[1-13C]lysine as the indicator. Subjects were studied at each of 7 tyrosine intakes. Results: A graded intake of tyrosine had no effect on lysine flux. The mean tyrosine requirement was determined from the response of the oxidation of l-[1-13C]lysine to breath 13CO2. A 2-phase linear regression crossover analysis of breath 13CO2 identified the breakpoint and upper 95% confidence limit, which represents the mean and safe intakes, to be 6.0 and 7.0 mg•kg−1•d−1, respectively. Conclusions: The safe intake of total aromatic amino acids calculated from the present results for tyrosine and our previous estimate for phenylalanine is estimated to be 21 mg•kg−1•d−1. This intake is 1.5 times the currently recommended total aromatic amino acid intake of the FAO/WHO/UNU (1985), 14 mg•kg−1•d−1. Furthermore, the absolute aromatic amino acid requirement may be dependent on the proportional balance of these amino acids in the diet.