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.