Abstract
Aging is associated with declines in skeletal muscle function, mitochondrial capacity, and changes in redox balance, which collectively contribute to frailty and chronic disease risk. This study investigated the effects of a 12-week resistance training (RT) program combined with a small dose of high-intensity interval training (HIIT), with or without polyphenol supplementation, on mitochondrial respiratory capacity (MRC) and oxidative stress in middle-aged and older adults (55-70 years). Forty-one participants were randomized to receive either a polyphenol supplement or a placebo for 30 days before the training intervention. Following the training intervention, aerobic capacity, lean mass, and strength improved significantly in both groups. Training also increased MRC in the placebo group but not in the polyphenol group, which displayed higher MRC following the supplementation phase, possibly reflecting either a supplement effect or baseline variation. The training resulted in a 20 % decrease in skeletal muscle H(2)O(2) emission across both groups, suggesting enhanced mitochondrial efficiency or antioxidant defenses. However, gene expression of selected antioxidants was unchanged, and plasma oxidative stress markers malondialdehyde (MDA) increased, and 3-nitrotyrosine (3-NT) remained unchanged. Circulating antioxidants showed distinct changes with training, as ascorbic acid increased with training in both groups, while α-tocopherol increased only in the placebo group and β-cryptoxanthin and retinol declined in the polyphenol group, suggesting potential supplement-nutrient interactions. Uric acid increased in both groups, likely reflecting exercise-induced purine turnover. In conclusion, combined RT and HIIT improved mitochondrial bioenergetics and muscle redox balance in middle-aged and older adults, whereas polyphenol supplementation did not augment these adaptations and may have blunted some vitamin-related responses. These findings underscore resistance-based exercise as a potent intervention for maintaining physical and mitochondrial health with age.