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Minimum methionine requirement and cysteine sparing of methionine in healthy school-age children^1,2,3

¹ From the Research Institute, The Hospital for Sick Children, Toronto, Canada (MAH, JMT, RE, MR, VL, and PBP); the Department of Nutritional Sciences, University of Toronto, Toronto, Canada (ROB and PBP); and the Department of Agricultural, Food and Nutritional Science, University of Alberta, Edmonton, Alberta, Canada (ROB and PBP)

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 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 · 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.

Key Words: Sulfur amino acid • minimum methionine • indicator amino acid oxidation • amino acid requirement • cysteine sparing • children

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