Dietary cysteine reduces the methionine requirement in men

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 x kg(-1) x 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 x kg(-1) x 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 x kg(-1) x 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 x kg(-1) x 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.