Contrary to our hypothesis, fat mass change after 12 weeks of exercise training in premenopausal women was not related to initial body weight or fat mass. A number of exercise and caloric restriction studies reported that loss of body weight or fat was positively correlated with baseline body fat (9,14,17–19,40). Our results show that no baseline anthropometric measure was a significant predictor of fat loss. One potentially important finding was that the changes in body weight and fat mass at 4 weeks were moderately correlated (r = 0.52 and 0.48, respectively) with change in body fat mass at 12 weeks. During energy-restricted weight loss programs, early weight loss has been reported to be a significant predictor of subsequent weight loss (32). Our results suggest that this may also be true for exercise training. If so, monitoring weight or fat changes during the initial weeks of an exercise program might enable identification of potential compensators. Additional research may be able to demonstrate feasibility of using data on changes in body weight and body fat to address compensation and perhaps reduce overcompensation (i.e., unintended gain in body fat) with exercise training. In our study, compensators comprised approximately two-thirds of our study sample and, as a group, actually gained fat mass during exercise training (Table 2). As suggested in a recent review on resistance to exercise-induced weight loss (28), there is a need to identify factors that could be used to address the overcompensation phenomenon.
A higher exercise dose is likely to increase the chances of fat loss in response to training (2,6–8,26). However, regardless of the exercise dose, there is considerable individual variability in weight change after exercise training. Compensation is routinely observed in exercise training studies, but the mechanisms are not well understood. Downregulation of spontaneous physical activity has been reported in some (4,13,24,27,29) but not all studies (15,25). Increases in energy intake may also account for compensation (18,21,43,44). Woo et al. (44) reported that lean women may be more likely than obese women to increase energy intake in response to exercise training. If so, it could be expected that women with a greater fat mass or body fat percentage would be more likely to lose body weight or fat. Our results do not support this scenario because there were no statistically significant differences between compensators and noncompensators in baseline fat mass or body fat percentage. Furthermore, baseline body fat mass and body fat percentage were not correlated with changes in fat mass in our study. Additionally, both groups had the same amount of lean body mass, which suggests that lean body mass is not an important factor in determining compensatory responses to exercise training.
Net energy expenditure during exercise training sessions did not correlate with changes in fat mass, despite a more than 2.5-fold difference between low and high ends of the range of net ExEE among women (3,754–9,903 kcal). Similarly, a 13-week exercise program in overweight sedentary men showed that a cumulative exercise dose of approximately 27,000 kcal resulted in similar fat loss (−4.0 kg) as a dose of approximately 52,000 kcal (−3.8 kg) (31). This suggests that compensatory factors may be so dominant that ExEE cannot reliably be used to predict loss of fat mass with exercise training. In the DREW study, the highest exercise dose (12 kcal·kg−1·wk−1) resulted in energy compensation, whereas the lower exercise dosages (4 and 8 kcal·kg−1·wk−1) produced weight loss equal to expectations (3). However, expected weight loss was based on the assumption that the composition of lost body weight consisted of 70% fat tissue and 30% lean tissue, providing an energy value of 7,700 kcal·kg−1 (10,42). Our data illustrate that an assumed composition and energy value of weight loss (e.g., 7,700 kcal·kg−1) does not reflect the considerable individual variability in changes in both fat mass and lean body mass after exercise training (Figure 1). Although higher levels of net ExEE can produce greater weight loss and reduce the number of individuals who actually gain body weight or body fat (2,7), considerable individual variability remains.
The strengths of our study include the fact that exercise training sessions were supervised with intensity monitored continuously by heart rate monitors, aerobic capacity was assessed every 4 weeks to adjust training HR accordingly, and there was 100% adherence to exercise sessions. Additionally, energy compensation was determined by using actual changes in fat and lean mass, rather than estimated on the basis of assumed composition of weight loss (10,42).
Our study also has limitations. We did not assess energy intake, although we instructed our subjects throughout the study not to intentionally change their diet. Compensators may have increased energy intake in response to exercise training, as has been reported by King et al. (19). However, most studies show that marked individual variability in body weight or body fat changes in response to exercise training is not predicted by changes in energy intake (2,3,6,7,22,24). Even studies using very detailed and rigorous 7-day food records (24) and multiple dietary assessments throughout the intervention (7,22) did not show any difference in dietary intake between compensators and noncompensators. These findings may reflect the fact that self-report dietary instruments have been shown to be unreliable for accurate quantitative assessment of energy intake (23). We also did not monitor free-living physical activity outside our formal exercise training. The exercise dose in our study was below the recommended level for weight loss (5), resulting in a relatively low net ExEE. However, our exercise dose of 90 minutes per week of vigorous-intensity exercise is higher than the minimum recommendation for adults of 75 minutes per week of vigorous-intensity activity (12). Furthermore, because our exercise dose produced considerable heterogeneity in weight change in response to exercise training, our study design was sufficient to test our hypotheses.
In conclusion, our results demonstrate considerable individual variability in body weight and composition changes in response to aerobic exercise training in premenopausal women and that changes in fat mass were not predicted by baseline body weight or body fat, aerobic fitness level, or net ExEE. Changes in fat mass after 12 weeks of exercise training were, however, moderately correlated with changes in body weight and fat mass at 4 weeks and less so with change in body fat percentage at 4 weeks and with baseline submaximal exercise VE and energy expenditure. Further research is necessary to determine whether knowledge of these predictors can be used to enhance effectiveness of aerobic exercise for weight control. The fact that two-thirds of the women demonstrated compensation, and as a group gained fat mass after an exercise program consistent with current public health recommendations (12), suggests that this should be a high priority.
Gain in body weight or body fat with aerobic exercise training is not considered a desirable outcome, yet is a consistent observation in controlled research settings and thus may be a common occurrence in clinical and professional practice. Since the change in body weight at 4 weeks was the strongest single correlate of body fat change after 12 weeks of exercise training, monitoring of body weight changes during the initial weeks of exercise training could prove to be useful in helping prevent unintended weight/fat gain in women who initiate an aerobic exercise program. Body weight is easy to measure and has the advantage over other predictors that may not be feasible outside a research setting (e.g., body composition testing, indirect calorimetry for assessment of energy expenditure) and therefore could potentially be used by coaches, personal trainers, and other professionals to assist individuals in achieving their weight loss goals and avoid unintended fat gain. Early identification of compensators through monitoring of body weight changes could lead to individualization of exercise programs to specifically fit the needs of each individual and prevent exercise-induced fat gain.
There are no conflicts of interest to report. No funding was received for this research. J.R.R. is supported by a training grant from the National Institute of Diabetes and Digestive and Kidney Diseases (T32-DK083250). The results of the present study do not constitute endorsement of the product by the authors or the National Strength and Conditioning Association.
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