Minimum methionine requirement and cysteine sparing of methionine in healthy school-age children

Background: Cysteine can provide a portion of the sulfur amino acid requirement in adults. Whether this is true in children\u97and, if so, to what extent\u97is not known. Objectives: The objectives were to determine minimum methionine requirements in healthy, school-age children when excess cysteine is provided and to subsequently determine the cysteine-sparing effect by comparing these methionine requirements with those determined previously in the same children when no cysteine was provided. Design: Six healthy, school-age children randomly received graded intakes of methionine (0, 2.5, 5, 7.5, 10, and 15 mg \ub7 kg\u961 \ub7 d\u961) along with 21 mg cysteine \ub7 kg\u961 \ub7 d\u961 in the diet. The mean methionine requirement was determined by using a biphasic linear regression crossover analysis of measurements of the rate of appearance of 13CO2 in the breath (F13CO2), which identified a breakpoint at the minimal F13CO2 in response to graded levels of methionine intake. Results: The mean and population-safe minimum methionine requirements, in the presence of excess dietary cysteine, were found to be 5.8 and 7.3 mg \ub7 kg\u961 \ub7 d\u961, respectively. The mean and population-safe (upper 95% CI) methionine requirements, in the absence of dietary cysteine, were previously determined to be 12.9 and 17.2 mg \ub7 kg\u961 \ub7 d\u961, respectively. These values represent a cysteine-sparing effect of 55% and 58% in comparison with mean and population-safe methionine requirements, respectively. Conclusion: Excess intake of dietary cysteine results in the reduction in the requirements for methionine to a minimum obligatory requirement level