Dietary cysteine reduces the methionine requirement in men

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Abstract
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.