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This study examined the effect of a yearlong exercise intervention on F2-isoprostane, a specific marker of lipid peroxidation and a general marker of oxidative stress.In a randomized, controlled trial, 173 overweight or obese, postmenopausal, sedentary women were randomized either to an aerobic exercise intervention (60%-75% observed maximal HR) for ≥45 min·d−1, 5 d·wk−1 (n = 87), or to a stretching control group (n = 86), on an intent-to-treat basis. Baseline and 12-month measures included urinary F2-isoprostane, maximal O2 uptake, body weight, body fat percentage, waist circumference, and intra-abdominal fat surface area. Urine samples were available from 172 and 168 women at baseline and 12 months, respectively.During the 12-month study, controls minimally changed maximal O2 uptake (+0.2%) and body weight (+0.1 kg), whereas exercisers increased maximal O2 uptake (+13.6%; P < 0.0001 vs controls) and decreased body weight (−1.3 kg; P = 0.007 vs controls). F2-isoprostane increased slightly among controls (+3.3%) and decreased in exercisers (−6.2%), although the effect was not statistically significant (P = 0.26). In planned subgroup analyses, F2-isoprostane decreased linearly with gain in maximal O2 uptake (Ptrend = 0.005) relative to controls; exercisers who increased maximal O2 uptake by >15% decreased F2-isoprostane by 14.1% (P = 0.005 vs controls). A borderline statistically significant trend was observed between decreased waist circumference and F2-isoprostane (P = 0.06). Similar subgroup analyses by 12-month changes in body fat percentage, weight, and intra-abdominal fat were not statistically significant.These findings suggest that aerobic exercise, when accompanied by relatively marked gains in aerobic fitness, decreases oxidative stress among previously sedentary older women and that these effects occur with minimal change in mass or body composition.