Skeletal muscle and erythrocyte redox status is associated with dietary cysteine intake and physical fitness in healthy young physically active men

Purpose To investigate the association between redox status in erythrocytes and skeletal muscle with dietary nutrient intake and markers of physical fitness and habitual physical activity (PA). Methods Forty-five young physically active men were assessed for body composition, dietary nutrient intake, muscle strength, cardiorespiratory capacity and habitual PA. Blood and muscle samples were collected to estimate selected redox biomarkers. Partial correlation analysis was used to evaluate the independent relationship of each factor with redox biomarkers. Results Dietary cysteine intake was positively correlated ( p < 0.001) with both erythrocyte ( r = 0.697) and muscle GSH (0.654, p < 0.001), erythrocyte reduced/oxidized glutathione ratio (GSH/GSSG) ( r = 0.530, p = 0.001) and glutathione reductase (GR) activity ( r = 0.352, p = 0.030) and inversely correlated with erythrocyte protein carbonyls (PC) levels ( r = − 0.325; p = 0.046). Knee extensors eccentric peak torque was positively correlated with GR activity ( r = 0.355; p = 0.031) while, one-repetition maximum in back squat exercise was positively correlated with erythrocyte GSH/GSSG ratio ( r = 0.401; p = 0.014) and inversely correlated with erythrocyte GSSG and PC ( r = − 0.441, p = 0.006; r = − 0.413, p = 0.011 respectively). Glutathione peroxidase (GPx) activity was positively correlated with step count ( r = 0.520; p < 0.001), light ( r = 0.406; p = 0.008), moderate ( r = 0.417; p = 0.006), moderate-to-vigorous ( r = 0.475; p = 0.001), vigorous ( r = 0.352; p = 0.022) and very vigorous ( r = 0.326; p = 0.035) PA. Muscle GSSG inversely correlated with light PA ( r = − 0.353; p = 0.022). Conclusion These results indicate that dietary cysteine intake may be a critical element for the regulation of glutathione metabolism and redox status in two different tissues pinpointing the independent significance of cysteine for optimal redox regulation. Musculoskeletal fitness and PA levels may be predictors of skeletal muscle, but not erythrocyte, antioxidant capacity. Trial registration Registry: ClinicalTrials.gov, identifier: NCT03711838, date of registration: October 19, 2018.