Methionine kinetics and balance at the 1985 FAO/WHO/UNU intake requirement in adult men studied with L-[2H3-methyl-1-13C]methionine as a tracer

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Abstract
The upper range of the requirement for methionine plus cystine in healthy adults was proposed in 1985 by FAO/WHO/UNU to be 13 mg·kg body wt−1 · d−1. To explore the validity of this estimate, five healthy, young adult men were given for 7 d a diet based on an l-amino acid mixture supplying 13 mg methionine · kg−1 · d−1 (87 µmol · kg−1 · d−1) without cystine. Constant intravenous infusions of l-[2H3-methyl-1-13C]methionine were given on days 5 and 7 while subjects were in the fed and postabsorptive states, respectively. Estimates were made of methionine oxidation, and daily methionine balance was derived from the intake-oxidation data. For the five subjects, methionine balances were −0.9, +0.7, +3.5, −3.1, and −3.8 mg kg−1 · d−1, or −6, +5, +23, −21, and −26 µmol · kg−1 · d−1. These findings lead to the conclusion that the upper range of the requirement for methionine plus cystine probably exceeds 13 mg · kg−1 · d−1 in healthy young adults. The implications of this conclusion for establishing an appropriate amount of sulfur amino acids in an amino acid requirement pattern for adults is discussed.