Methionine and cysteine kinetics at different intakes of cystine in healthy adult men

We investigated plasma methionine and cysteine kinetics in eight healthy adult men receiving for 6 d each of five L-amino acid diets supplying 13 mg methionine · kg^-1 · d^-1 without cystine or 6.5 mg methionine · kg^-1 · d^-1 plus 0, 5.2, 10.5, or 20.9 mg cystine · kg^-1 · d^-1. On the morning of day 7, primed, constant intravenous infusions of L-[²H₃-methyl, 1-¹³C]methionine and L-[3,3-²H]cysteine were given for 8 h (for the first 3 h subjects remained in a fasted state and for the next 5 h received small, equal meals at hourly intervals to achieve a fed state). Methionine and cysteine fluxes and rate of methionine oxidation were estimated from plasma methionine and cysteine labeling and ¹³C in expired air. Methionine oxidation declined (P < 0.05) with lowered methionine intake. Cysteine flux was similar across diets and dietary cystine did not affect tracer methionine oxidation. If there is a sparing effect of dietary cystine on the methionine requirement in adults, it probably takes place during the 'first-pass' removal of these amino acids within the splanchnic region.