Does the time of day that exercise is performed matter for weight loss? The objective of this proposal is to examine the impact of an equivalent dose of morning vs. evening aerobic exercise on change in body weight, energy intake (EI) and components of energy expenditure (EE) in adults with overweight or obesity. Nearly two-thirds of US adults who attempt to lose weight report engaging in exercise as a primary strategy for weight loss. However, weight loss from exercise alone is often substantially less than predicted based on calories burned in exercise. This is due to compensatory changes that occur in response to exercise initiation (e.g. increases in EI and decreases in non-exercise EE) that limit the energy deficit produced by exercise. Thus, strategies that reduce the compensatory response to exercise could enhance the weight loss efficacy of exercise. It is possible that exercise time of day could impact compensatory behaviors and weight loss, however, there have been no adequately powered, prospective, randomized studies comparing weight loss induced by morning vs. evening exercise. The study design is a 7-month supervised trial in which adults with overweight or obesity will be randomized to supervised aerobic exercise (2000 kcal/wk) performed either in the morning (AM-Ex, 6-10 AM) or the evening (PM-Ex, 3-7 PM). The supervised exercise phase will be followed by a 6-month maintenance phase during which participants continue to exercise at the target of 2000 kcal/week during the randomized AM or PM exercise windows, but exercise is no longer supervised. Aim 1 will compare the effects of AM-Ex vs. PM-Ex on changes in body weight and body composition with the primary study outcome of weight change at 7-months. Aim 2 will compare the effects of AM-Ex vs PM-Ex on changes in EI and appetite. Aim 3 will compare the effects of AM-Ex vs PM-Ex on changes in EE, non-exercise physical activity and sedentary time. Exploratory Aim 4 will compare the effects of AM-Ex vs PM-Ex on changes in meal and sleep timing. This approach is rigorous and innovative as the exercise energy deficit will be matched between groups, exercise will be prescribed based on EE, and free-living total daily energy expenditure (TDEE) and EI will be assessed objectively (using doubly-labeled water). This study is significant as it could provide important insight on how the timing of exercise impacts weight loss and compensatory behaviors.
The investigators are doing this study to learn more about how exercising at different times of the day (morning versus evening) affects body weight, sleep, eating patterns, and other factors.