Methionine and cysteine kinetics at different intakes of methionine and cystine in elderly men and women

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Abstract
Earlier nitrogen balance studies led to the conclusion that requirements for methionine in older individuals are much higher than those in younger adults. Hence, we examined the kinetics of whole-body methionine, cysteine, and leucine metabolism postabsorptively using a continuous intravenous infusion of L-[C2H3, 1-(13)C]methionine, L-[2H3]leucine, and [3,3-2H2]cysteine in 12 elderly men (n = 5) and women (n = 7) given as a 3-h infusion after a 12-h fast (study 1) and in 8 elderly men (n = 4) and women (n = 4) as an 8-h infusion according to a 3-h fasted, 5-h fed protocol (study 2) for 6 d. Before tracer infusion, each of 3 L-amino acid diets supplying the following nominal, but known, amounts (mg x kg(-1) x d(-1)) of methionine and cysteine, respectively, were used in study 2: diet 1: 13 and 0; diet 3: 6.5 and 5.2; and diet 5: 6.5 and 21. Studies 1 and 2 gave values for plasma methionine flux that agreed with the leucine flux data, which, in turn, also appeared to be comparable with findings in healthy younger adults. In study 2, methionine oxidation rates were the same across all diets in the fasted state and the same with diets 1 and 3 in the fed state but lower with diet 5, suggesting a modest sparing effect of dietary cystine on methionine oxidation. Estimated daily methionine balance was at equilibrium for diet 1 and negative (significantly different from zero, P<0.05) with diets 3 and 5. The results were evaluated against our previous findings in younger adults.