Oxidative stress occurs when the production of reactive species, derived largely from oxygen and nitrogen, exceeds degradation by the antioxidant defense system. The ensuing damage to DNA, protein, and lipid has been implicated in cardiovascular and pulmonary diseases, diabetes, neurodegenerative disorders, and some cancers (3,19). Thus, intervention strategies to reduce oxidative stress, especially among overweight/obese persons who generally have high oxidative stress levels (19), have widespread appeal. Such efforts with human study subjects have been hindered by difficulties in the measurement of oxidative damage. Recently, however, sensitive and stable methods have become available to measure F2-isoprostanes (7). F2-isoprostanes are a family of isomeric F2-prostaglandin-like compounds, derived from free radical-catalyzed peroxidation of arachidonic acid, independent of the cyclooxygenase enzyme. A recent multi-institutional study concluded that F2-isoprostane was the most accurate method to assess oxidative stress in vivo from urine or plasma samples (9).
Reduced oxidative stress, probably achieved through improved antioxidant defenses and/or reduced reactive species formation, may be one mechanism that links physical activity to reduced risk of chronic disease (3,19). Noncontrolled exercise intervention studies among premenopausal women have noted 25% (6) and 34% (18) decreased F2-isoprostane concentrations after 12-15 wk of exercise training; however, no intervention effects were observed in an 8-wk controlled exercise trial among older patients with type 2 diabetes (15). Given the lack of data on this topic, we investigated the effect of a yearlong aerobic exercise intervention compared with a stretching control program on F2-isoprostane concentrations in postmenopausal women. We hypothesized that the 12-month exercise intervention would decrease F2-isoprostane and that this effect would be mediated by changes in maximal O2 uptake and by measures of body size/fat distribution.
This work was conducted with ancillary funding, using previously collected urine specimens and data from the Physical Activity for Total Health study (ClinicalTrials.gov Identifier: NCT00668174). The main objective of the parent trial was to examine the effects of exercise on steroid hormones among 173 postmenopausal, previously sedentary, overweight/obese women, as described previously (11-13). A subgroup of participants (n = 115) from the parent trial, who met additional inclusion criteria, were included in separately funded studies of the effects of exercise on immune function (5) and inflammation (4).
Setting and participants.
Participants were 173 women who resided in the area of Seattle, WA. Eligibility criteria included the following: age = 50-75 yr; body mass index (BMI) between 25 and 40 kg·m−2 (or between 24.0 and 24.9 kg·m−2 if body fat percentage > 33%); postmenopausal, not taking hormones in the previous 6 months; nonsmoker; sedentary at baseline (moderate- or vigorous-intensity activity < 60 min·wk−1 and maximal O2 uptake < 25 mL·kg−1·min−1); alcohol consumption of fewer than two drinks per day; fasting glucose < 140 mg·dL−1; and no history of cancer, diabetes, or cardiovascular disease. All women provided written informed consent, and all procedures were approved by the Fred Hutchinson Cancer Research Center Institutional Review Board.
Randomization and exercise intervention.
Randomization was stratified by BMI (<27.5 or ≥27.5 kg·m−2) to ensure similar numbers of heavier and lighter women in each group. The exercise intervention progressed to ≥45 min·d−1 of moderate-intensity aerobic exercise (60%-75% of observed maximal HR), 5 d·wk−1, by the eighth week of the trial, where it was maintained to the end of study. For months 1-3, the intervention participants attended three mandatory exercise sessions at a study facility (University of Washington or a commercial gym) and exercised twice per week at home. For months 4-12, the intervention group attended at least one session per week at a study facility and conducted the remaining sessions at home or at a study facility.
Exercisers wore Polar HR monitors during all exercise sessions and maintained exercise logs that included information on the duration, mode, relative perceived exertion, and peak HR of all sport and recreational activities estimated at ≥3 METs (1). Exercise logs were reviewed weekly by study staff to monitor adherence with the study protocol and to intervene when needed. Women in the control group attended once-weekly 45-min stretching and relaxation sessions and were asked to not otherwise change exercise habits for the duration of the trial.
All women were asked not to change their dietary habits for the duration of the trial. At baseline (immediately before randomization), at 3 months, and at 12 months, participants completed self-reported questionnaires on diet (120-item food frequency questionnaire), alcohol consumption, medications, and dietary supplement usage, as described previously (17).
At baseline and at 12 months, all women had a clinic visit for spot urine collection after a 12-h fast. All samples were processed within 1 h of collection, aliquoted into 1.8-mL tubes, and stored at −70°C. Date and time of collection and time since last meal were recorded, as well as medications used, vigorous activities in the past 8 h, and consumption of alcoholic beverages in the previous 48 h. Participants were instructed to refrain from alcohol (48 h) and vigorous exercise (8-12 h) before their clinic appointments. Participants who did not comply had their clinic appointments rescheduled. All participants were contacted 1 wk after their clinic visit to track recent illness; participants who reported being ill during this interval returned for a second clinic visit when symptom free.
F2-isoprostane was measured by its major urinary metabolite, 15-F2t-isoprostane, 2,3-dinor-8-iso-prostaglandin F2α via gas chromatography-mass spectrometry-based methodology (14). The assay was done on a Hewlett-Packard (Palo Alto, CA) 5971 MSD quadrupole instrument, using selective ion monitoring, after solid-phase chromatography, thin-layer separations, and derivatization.
Baseline and 12-month samples from each participant were assayed in the same batch. Each batch contained an approximately equal number of exerciser and control samples. Pooled quality control samples were included in each batch. Laboratory personnel were blinded to intervention/quality control status. Intra-assay and inter-assay coefficients of variation were 5.8% and 9.8%, respectively. Assay results were measured as nanograms per milligram (ng·mg−1) of creatinine to account for hydration status. F2-isoprostane values were non-normally distributed and were log-transformed and presented as geometric means with 95% confidence intervals.
Maximal O2 uptake (aerobic fitness) was assessed at baseline and at 12 months in all participants by a maximal-graded treadmill test using a Medgraphics automated metabolic cart (Medgraphics, St. Paul, MN), using a previously described protocol (5). Also measured at baseline and 12 months were the following: body weight to the nearest 0.1 kg and height to the nearest 0.1 cm, both in duplicate, to compute BMI; waist circumference to the nearest 0.1 cm; total body fat and body fat percentage by dual-energy x-ray absorptiometry (DEXA, QDR 1500; Hologic, Waltham, MA); and intra-abdominal and subcutaneous fat images by a one-slice computed tomographic scan at L4-L5 (model CT 9800 scanner; General Electric, Waukesha, WI).
Spearman correlation coefficients were calculated for the associations between baseline F2-isoprostane values and baseline values for maximal O2 uptake, percentage body fat, body weight, waist circumference, and intra-abdominal body fat, and between 12-month changes in F2-isoprostane values and 12-month changes in maximal O2 uptake, percentage body fat, body weight, waist circumference, and intra-abdominal body fat. Correlation coefficients were computed among all women, with no consideration of intervention group assignment. Age (continuous) was included in the correlation models.
The main intervention effect was assessed on the basis of a comparison between exercisers and controls as defined at randomization, regardless of exercise adherence (i.e., intent-to-treat). The difference between groups was assessed with a generalized estimating equation (GEE), which accounts for repeated observations on the same subjects over time (20). In the primary trial analysis, the GEE model was specified, with follow-up (i.e., 12 months) F2-isoprostane values as the outcome variable and group assignment (exercise or control) as the main predictor variable. Baseline F2-isoprostane values and age (continuous) were included as covariates in the primary GEE model.
In planned secondary analyses, we examined subgroup effects with controls (referent group) compared with exercisers stratified by exercise adherence and changes in maximal O2 uptake and body composition/distribution. For exercisers, these subgroups were meant to create approximately equal, yet easily interpretable tertiles. Because the control group was expected to have minimal changes in any of these parameters, we decided a priori not to stratify further the control group. The subgroups for exercisers were defined as follows: average exercise adherences for 12 months of <136, 136-195, or >195 min·wk−1; 12-month increases in maximal O2 uptake of <5%, 5%-15%, or >15%; body fat percentage decrease of <0.1%, decrease of 0.1%-2%, or decrease of >2%; body weight decreases of <0.5, 0.5-3, or >3 kg; waist circumference decreases of <0.5, 0.5-3, or >3 cm; and intra-abdominal fat surface area decreases of <1, 1-8, or >8·cm−2).
In the subgroup analyses, each GEE model included follow-up F2-isoprostane values as the outcome variable, with group assignment as the main predictors. The group assignment variable had four mutually exclusive values: controls (denoted by a "0" indicator variable) and three indicator values to denote the exercise group ("1", "2", and "3"), depending on their exercise adherence or category of 12-month change in maximal O2 uptake or body composition/distribution, as defined above. Tests for linear trends were obtained by including the four-level group assignment variable (i.e., 0, 1, 2, and 3) as a continuous value in the GEE regression model. Baseline F2-isoprostane values and age (continuous) were again included as covariates in these models. Missing outcome data were omitted from GEE analyses. All statistical tests were two-sided.
From the 173 women, 87 women were randomized to the 12-month exercise intervention and 86 were randomized to the stretching control group. One exerciser did not have a baseline urine sample; one control was excluded from the study for hormone use initiated after randomization and three exercisers did not complete the study (>97% retention; Fig. 1). Women were, on average, aged 61 yr (range = 50-75 yr). At baseline, participants had a mean BMI of 30 kg·m−2, 47% body fat, and relatively low maximal O2 uptake (mean = 20 mL·kg−1·min−1). There were no statistically significant differences between exercise and control groups at baseline for any of the descriptive demographic, smoking, dietary, phenotypic, or physiologic parameters (Table 1). For 12 months, exercisers averaged 171.3 min·wk−1 (SD = 87.9 min·wk−1, median = 170 min·wk−1) of moderate- to vigorous-intensity activity. Exercisers increased maximal O2 uptake by 13.6%, whereas controls increased maximal O2 uptake by 0.2% (P < 0.0001). Exercisers decreased body weight and body fat percentage compared with controls (exercisers: −1.3 kg and −1.4%; controls: +0.1 kg and −0.1%, P = 0.007 and P = 0.001, respectively).
There were moderate correlations between baseline F2-isoprostane values and baseline values for maximal O2 uptake (r = −0.37, P < 0.0001), percentage body fat (r = 0.45, P < 0.0001), body weight (r = 0.30, P = 0.0002), waist circumference (r = 0.40, P < 0.0001), and intra-abdominal body fat (r = 0.31, P < 0.0001). There was a moderate association between 12-month changes in F2-isoprostanes and 12-month changes in maximal O2 uptake (r = −0.27, P = 0.02), whereas correlations between 12-month changes in F2-isoprostanes and 12-month changes in body fat (r = −0.08, P = 0.52), body weight (r = 0.12, P = 0.32), waist circumference (r = 0.20, P = 0.10), and intra-abdominal body fat (r = −0.14, P = 0.24) were weaker and not statistically significant.
There were no statistically significant between-group differences for controls versus exercisers, including analyses where exercisers were stratified by exercise adherence, from baseline to 12 months for dietary intake of total energy, fat, protein, carbohydrate, cholesterol, fiber, sucrose, calcium, and alcohol or for the number of servings per week of vegetables and fruits, as described in detail previously (17). There were also no statistically significant between-group differences for use of nonsteroidal anti-inflammatory drugs, any dietary supplements, and multivitamins during the yearlong trial (17). In addition, there were no statistically significant between-group differences for any of the above dietary/lifestyle/pharmacologic factors when compared across subgroups of gain in maximal O2 uptake, with the singular exception of a dietary fat intake decrease (P = 0.05) among exercisers who gained >15% maximal O2 uptake relative to controls. Inclusion of change in dietary fat intake into the GEE model for gain in maximal O2 uptake had no material effect on the study results (data not shown).
From baseline to 12 months, F2-isoprostane increased slightly among controls (+3.3%) and decreased modestly in exercisers (−6.2%), although the exercise intervention effect was not statistically significant (P = 0.26), as shown in Table 2. In planned subgroup analyses, however, with controls again as the referent group, F2-isoprostane decreased linearly among exercisers with gain in maximal O2 uptake (Ptrend = 0.005); exercisers who increased aerobic fitness by >15% decreased F2-isoprostane by 14.1% (P = 0.005 vs controls). Similar linear trends, although not statistically significant, were observed for exercise adherence (Ptrend = 0.11) and waist circumference (Ptrend = 0.06). When 12-month changes in maximal O2 uptake and waist circumference were included in the same GEE model, the P value for waist circumference was attenuated further toward the null (Ptrend = 0.25), whereas the P value for gain in maximal O2 uptake was only marginally attenuated by the inclusion of change in waist circumference (Ptrend = 0.008).
We examined the effect of a yearlong aerobic exercise intervention compared with a stretching control program on urinary F2-isoprostane, a marker of systemic oxidative stress, among previously sedentary, overweight/obese, postmenopausal women. Overall, exercisers decreased F2-isoprostane concentrations, as predicted by our primary hypothesis, but the effect was not statistically significant, except in planned analyses by strata of gain in aerobic fitness.
Few previous studies have examined the effects of exercise alone on F2-isoprostane concentrations (6,15,18), and no earlier studies, to our knowledge, were conducted exclusively among postmenopausal women. These studies suggest that exercise may reduce quite strongly F2-isoprostanes among relatively younger women (6,18), whereas this beneficial effect may be attenuated among older study subjects, as observed currently and previously (15). The three earlier studies and the current study noted similarly decreased body mass and increased aerobic fitness after exercise intervention, despite the wide range of intervention periods and age groups studied. Thus, these data suggest a potential age effect for the capacity of exercise to reduce oxidative stress. The mechanisms to explain such an effect are discussed in detail elsewhere (2) and are largely on the basis of experiments with laboratory animals: essentially, older animals have been observed to experience damage to cardiac and skeletal muscle after short-term exercise training if it is not initiated before a certain age, indicative of a physiologic age threshold. Alternatively, if exercise is initiated before this age, older animals experience the same health benefits as younger exercising animals. However, it is important to note that the older adults in the current study, and the participants in the previous study (15), marginally decreased oxidative stress levels, although the effects were not statistically significant, suggesting that the laboratory animal data are not entirely generalizable to humans.
We observed an inverse linear association between aerobic fitness and oxidative stress, suggesting a hormetic effect of exercise training (10). This finding for aerobic fitness and oxidative stress complements recent results from a subset of participants in this trial (n = 115) where we reported that the effect of exercise on inflammation (C-reactive protein and serum amyloid A) was restricted to participants who were obese (BMI ≥ 30 kg·m−2 or waist circumference > 88 cm) at baseline (4). In contrast, baseline obesity status did not modify the influence of exercise on F2-isoprostane in the current study (data not shown). For our study participants, in particular, these results suggest that gains in aerobic fitness improve oxidative stress levels, probably because of exercise-induced adaptations of the antioxidant defense system (3,8,16), and this effect occurs independent of general or abdominal obesity status. Although our subgroup analyses among exercisers by strata of weight/fat loss were largely not statistically significant, it is worth noting that this trial was not designed to elicit much weight loss, and our exercise participants only reduced body weight by approximately 1.3 kg, on average. Therefore, we could not assess the effects of much weight/fat loss on oxidative stress, a limitation that future studies should address.
Particular strengths of this study include the following: the selection of a sensitive and stable biomarker for oxidative stress (9); the long period of exercise intervention, with high participant retention; the good participant adherence to the exercise protocol; the bona fide effects of the aerobic exercise intervention, quantified by direct measures of maximal O2 uptake; and the gold standard randomized, controlled trial design. Limitations of this study include the highly homogeneous study sample that limits comparisons to other groups and the fact that we studied only one biomarker of oxidative stress, F2-isoprostane; future studies should consider other measures of lipid peroxidation, such as TBARS, and biomarkers that additionally reflect oxidative damage to protein and DNA.
In conclusion, in a randomized, controlled trial with excellent retention and good adherence to the aerobic exercise intervention, conducted among 173 previously sedentary, overweight/obese, postmenopausal women, we observed a modest reduction in F2-isoprostane overall that was not statistically significant. When exercisers were stratified by gain in aerobic fitness, as measured by maximal O2 uptake, an inverse and linear association was observed between aerobic fitness and F2-isoprostane, suggesting a training effect on oxidative stress.
The F2-isoprostane component of this study was supported by a research grant from the National Institutes of Health (NIH; CA RO3-130043). The parent trial, the Physical Activity for Total Health study, from which the urine samples and other data were acquired, was supported by a research grant from the NIH (CA RO1-69334). P.T.C. was a Research Fellow of the Canadian Cancer Society through an award from the National Cancer Institute of Canada and was also supported by an NIH Transdisciplinary Research on Energetics and Cancer Postdoctoral Fellowship (NCI U54 CA116847). The sponsors had no role on the design or conduct of the study, the collection, the management, the analyses, the interpretation of the data, or on the preparation, review, or approval of the manuscript.
The authors have no conflicts of interest to disclose.
A.M. and C.M.U. are senior authors.
The results of this study do not constitute endorsement by the American College of Sports Medicine.
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Keywords:©2010The American College of Sports Medicine
INTERVENTION; WOMEN; AEROBIC; MAXIMAL OXYGEN UPTAKE; F2-ISOPROSTANE; OVERWEIGHT/OBESE