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Acute exercise results in transient change in redox balance. High concentrations of reactive oxygen species (ROS) can lead to oxidative damage to macromolecules. However, moderate periodic increases in ROS, such as experienced with habitual exercise, may activate signal transduction pathways which stimulate increases in endogenous antioxidant systems. This study tested the hypothesis that physically fit older adults would have less oxidative stress than unfit age-matched controls, due to greater circulating concentrations of non-enzymatic antioxidants and greater capacity to upregulate antioxidant enzymes. We compared 37 fit (mean age 65.2 ± 5 years) and 35 unfit (mean age 67.7 ± 4 years) men and women. Fitness status was classified by VO(2 max) and maximal leg power. Basal levels of oxidative stress were assessed by measuring urinary markers of nucleic acid damage and lipid peroxidation. Antioxidant status was assessed by measuring total antioxidant power and ratios of reduced to oxidized glutathione in plasma, at rest. The capacity to counteract an oxidative insult was assessed by measuring changes in plasma F(2)-isoprostanes in response to forearm ischemia-reperfusion. The fit individuals had significantly lower levels of urinary markers of oxidative damage (all P <0.05) and lower F(2)-isoprostane response to the oxidative challenge (P < 0.05), but there were no group differences in antioxidant status. The lower levels of oxidative stress in the fit individuals were not mediated by known effects of exercise training such as adiposity, HDL concentrations, or small molecular weight antioxidants. These data suggest that reduced oxidative stress associated with physical fitness results from differences in activity of antioxidant enzymes.
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