Dietary Sulfur Amino Acid Effects on Fasting Plasma Cysteine/Cystine Redox Potential in Humans

Abstract

Objective: Oxidation of plasma cysteine/cystine (Cys/CySS) redox potential (EhCySS) has been associated with risk factors for cardiovascular disease in humans. Cys and CySS are derived from dietary sulfur amino acids (SAA), but the specific effects of SAA depletion and repletion on Cys/CySS redox indices are unknown. The present study examined the effect of dietary SAA intake level on free Cys, free CySS, and EhCySS in human plasma under fasting conditions. Methods: Healthy individuals aged 18–36 y (n = 13) were equilibrated to foods providing the RDA for SAA and then fed chemically defined diets without SAA (0 mg·kg–1·d–1; n = 13) followed by SAA at levels approximating the mean (56 mg·kg–1·d–1; n = 8) or 99th percentile (117 mg·kg–1·d–1; n = 5) intake levels of Americans. Fasting plasma samples were collected daily during 4-d study periods and analyzed for free Cys, free CySS, and the EhCySS. Results: The SAA-free diet significantly (P < 0.05) decreased plasma-free Cys concentrations and oxidized EhCySS values after 4 d of SAA depletion. With SAA repletion at 56 mg·kg–1·d–1, plasma-free Cys increased significantly and values for EhCySS became more reduced. Administration of a diet providing a higher dose of SAA (117 mg·kg–1·d–1) resulted in a significantly higher level of free Cys and a more reduced EhCySS. Conclusions:These results show that free Cys and Cys/CySS redox potential (EhCySS) in fasting plasma are affected by dietary SAA intake level in humans. Significant changes occur slowly over 4 d with insufficient SAA intake, but rapidly (after 1 d) with repletion.