This yearlong aerobic exercise intervention among postmenopausal women resulted in statistically significant reductions in overall and abdominal adiposity without any intervention to change dietary intake. Furthermore, women in the exercise group, who achieved a higher duration of physical activity, experienced greater average decreases in adiposity. The combination of facility- and home-based program was achievable and resulted in statistically significant increases in physical activity.
Our study differed from previous randomized trials1, 15, 16, 18, 19, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56
of exercise interventions among postmenopausal women that were at least 6 months duration and examined adiposity outcomes. It had a large sample size, used quantitative imaging techniques that are preferable for measuring adiposity, had a low drop-out rate, excellent adherence and used a supervised, high volume of exercise. Of these trials, only two studies carried out by Irwin et al.
in the United States15, 56
and Velthuis et al.
in the Netherlands47
were sufficiently comparable with ours with respect to study design, exercise volume and outcome measures. Irwin et al.15
found statistically significant decreases in adiposity of a smaller magnitude than that in our trial and a dose–response with increasing exercise adherence, whereas the trial by Velthuis and colleagues56
only found statistically significant decreases for total body fat. The Dose–Response to Exercise in postmenopausal Women trial carried out by Church et al.44, 46
was designed to test the effects of three different doses of exercise on cardiorespiratory fitness over a period of 6 months in sedentary postmenopausal, overweight and obese women with elevated blood pressure. These investigators found statistically significant decreases in waist circumference among exercisers, but not in weight or percent body fat, and no dose–response trend was observed.44, 46
A statistically significant effect might have been observed given a longer duration; alternatively, the absence of a dose–response trend may be explained by compensatory increases in energy intake that increase as the weekly volume of exercise increases.46
In our study, energy intake decreased among controls relative to exercisers, suggesting that at least some compensation by energy intake was made.
In this study and most other exercise studies, weight change is largely accounted for by a loss of fat mass with the preservation of lean mass.15, 16, 43, 49
Although the consensus is that overweight or obese subjects will experience greater changes in adiposity than those of normal weight,1, 15, 16, 57, 58
in our study, we did not find that baseline BMI modified the effect of the intervention on changes in adiposity, possibly because most women were overweight or obese.
A recent comprehensive review concluded that without caloric restriction, aerobic exercise in the range of 13–26 MET-hours per week results in decreases in abdominal adiposity.1
This conclusion is supported by our study in which 17 MET-hours per week on average over the intervention year were expended by the exercisers relative to controls. With this volume of exercise, average reductions in intra-abdominal fat and subcutaneous abdominal fat of 17 and 10%, respectively, were achieved. Other studies including postmenopausal women have also observed reductions in abdominal adiposity with aerobic exercise.15, 16, 17, 18, 44, 59
In addition, we found larger reductions with longer weekly duration of exercise; women who exercised >225
min per week had nearly 25% reductions in intra-abdominal fat observed. This dose–response relationship is supported by previous randomized and nonrandomized trials (reviewed in Ohkawara et al.60
), but was not observed in the Dose–Response to Exercise in postmenopausal Women trial.44
Our study and others also suggest that intra-abdominal fat may be lost in a higher proportion than overall fat in response to increased energy expenditure, and that intra-abdominal fat may be decreased even in the absence of significant weight loss or after controlling for change in weight.15, 16, 46, 58, 61
Even after controlling for percent change in total fat mass, exercisers in the current study still achieved a statistically significant change in intra-abdominal fat relative to controls (data not shown).
Lower adiposity, intertwined with other related mechanisms such as levels of endogenous sex hormones, metabolic hormones, growth factors and immune factors, likely has a role in the association between physical activity and breast cancer.62, 63, 64
Abdominal adiposity is etiologically relevant to breast cancer risk;65
positive associations with waist–hip ratio, waist circumference or other measures of central adiposity have been found in most studies of postmenopausal breast cancer risk.3, 66, 67, 68, 69
It remains to be seen whether adding a dietary intervention component has additive effects on reductions in adiposity, particularly on intra-abdominal adiposity.70
The limitations of this study include the lack of compliance amongst controls, as 23% reported >150
min per week of physical activity during the intervention, and the limited generalizability of the sample because of the exclusion criteria, including the presence of comorbid conditions such as diabetes and hypercholesterolemia. A per protocol analysis in which participants in either the exercise or control group achieving <150
min per week were compared with participants in either group achieving
min per week indicated that the difference in the weight loss between these groups was almost exactly similar to that between the intervention groups (data not shown). Furthermore, despite instructions not to change dietary energy, 20% of exercisers and 31% of controls decreased their caloric intake >300
kcal per day and 14% of exercisers and 8% of controls increased their caloric intake >300 kcal per day. Although food frequency questionnaires are limited as means of measuring dietary intake, because responses are self-reported and subject to measurement error, we repeated the same measurement at the beginning and end of the trial in both exercisers and controls.
It is of importance to note that this study was designed as an efficacy trial, not an effectiveness study, and future research will evaluate how likely uptake of this level of exercise in postmenopausal women would be and how to develop strategies to enhance adoption and maintenance. Furthermore, the level of exercise that was prescribed for our study population is comparable with that found amongst the highest activity groups in observational studies20, 21
and is also within the range of public health recommendations of exercise for chronic disease reduction.1
Although this study has provided some preliminary evidence on how aerobic exercise can be used to decrease adiposity levels that may be associated with cancer risk, it has not addressed the question of the exact dose and type of activity needed for the optimal reduction in adiposity levels. Our preliminary findings from analyses conducted within the exercise arm of the trial need to be substantiated in a future randomized controlled trial that compares different doses of activity in separate arms of the trial and the consequent effect on adiposity and other metabolic hormones. Finally, we recognize that in clinical practice, a multipronged approach should be used to decrease chronic disease risk including intervening on other modifiable lifestyle risk factors. Results from one observational study suggest that postmenopausal breast cancer risk may be reduced by 10% for every 5
kg of weight lost from a woman's highest body weight attained earlier in life,27
which translates into a risk reduction of 4–5% in our exercise group. Higher risk reductions for breast cancer (and other chronic disease associated with obesity) might be attained using a weight loss strategy that incorporates both physical activity and dietary changes.
In conclusion, this trial has addressed a gap identified in the scientific literature for rigorous and well-designed randomized controlled trials of higher volume, longer duration and exercise interventions that quantify using direct imaging methods the impact of exercise on body composition.31
It provides direct empirical evidence that previously sedentary, mostly overweight, postmenopausal women can achieve and sustain high levels of aerobic exercise that result in statistically significant reductions in all measures of adiposity. These levels of change in adiposity through exercise could be beneficial for chronic disease risk reduction.