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-and, if so, to what extent-is 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 (.) kg(-1) (.) d(-1)) along with 21 mg cysteine (.) kg(-1) (.) d(-1) 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 (CO2)-C-13 in the breath ((FCO2)-C-13), Which identified a breakpoint at the minimal (FCO2)-C-13 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 (.) kg(-1) (.) d(-1), 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 (.) kg(-1) (.) d(-1), 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.