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Original Research Communication |
1 From the Departments of Nutritional Sciences and Paediatrics, the University of Toronto; The Research Institute, The Hospital for Sick Children, Toronto; the School of Dietetics & Human Nutrition, McGill University, Ste-Anne-de-Bellevue, Canada; and the Department of Agricultural, Food and Nutritional Science, University of Alberta, Edmonton, Canada.
Background: Despite early evidence suggesting that dietary cysteine has a sparing effect on methionine requirements, some recent reports question the existence of a measurable sparing capacity.
Objective: The goal of the present study was to determine whether dietary cysteine could reduce the requirement for methionine in men consuming diets with and without cysteine.
Design: Six men were randomly assigned to receive graded intakes of methionine while fed a diet containing either no exogenous cysteine or an excess of cysteine (21 mg·kg-1·d-1). The methionine requirement was determined by measuring the oxidation of L-[1-13C]phenylalanine to 13CO2 and estimated by using a linear regression crossover analysis.
Results: The mean and population-safe (upper limit of the 95% CI) methionine requirements in the absence of exogenous cysteine were found to be 12.6 and 21 mg·kg-1·d-1, respectively. The mean and population-safe methionine requirements in the presence of excess dietary cysteine were found to be 4.5 and 10.1 mg·kg-1·d-1, respectively, representing a cysteine sparing effect of 64% in a comparison of mean methionine requirements and of 52% in a comparison of population-safe methionine intakes. Furthermore, the difference between population-safe intakes with and without dietary cysteine establishes a safe cysteine intake of 10.9 mg·kg-1·d-1 in the presence of adequate methionine intakes.
Conclusion: Our data suggest that dietary cysteine can reduce the exogenous requirement for methionine in men. These results strongly support the existence of a cysteine sparing effect in humans.
Key Words: Sulfur amino acid • indicator amino acid oxidation • amino acid requirement • stable isotopes • methionine • phenylalanine • cysteine sparing • men
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