Traditionally, governments and public health authorities have placed a high emphasis on recommending physical activity as a means of combating weight gain and obesity. Yet, randomized controlled trials have found exercise alone to have a relatively modest effect on body weight, yielding weight loss averages in the order of 4 to 11 pounds (2 to 5 kilograms) in people with overweight and obesity. Some researchers have posited that metabolic adaptation causes the body to compensate for energy burned during exercise by reducing the baseline metabolic rate and ultimately “canceling out” some of the exercise-induced calorie deficit. Findings from a recent study indicate that much of the effort of exercise is lost to energy compensation, especially among people with excess bodyweight.

The researchers’ analysis revolved around energy expenditure data collected in a cohort of more than 1,700 healthy adults, excluding those involved in competitive sports training or were pregnant or breastfeeding. They focused on three measures: baseline energy expenditure (i.e., the energy required to fuel basic functions such as respiration, tissue repair, and immune defense, minus any physical activity), total energy expenditure (i.e., total calories burned over the same period of time), and physical activity expenditure (inferred by subtracting baseline from total energy expenditure).

Looking at the relationships between the three types of energy expenditure, the researchers corroborated what many smaller studies have suggested. That is, the more energy a person used during physical activity, the more their baseline metabolism slowed down to avoid a calorie deficit. The result of this adaptation is that total energy expenditure is approximately 28 percent lower on active days than might be expected given the number of calories burned during physical activity.

This metabolic compensation was independent of the participants' sex or age. However, it was significantly affected by fat mass and body mass index (BMI), as individuals in the 90th percentile of BMI recouped approximately 49 percent of calories burned through physical activity by lowering their baseline metabolism.

These findings suggest that exercise may be a considerably less effective weight loss tool for people with overweight or obesity. They also raise interesting questions about causality, such as whether people who have overweight are better metabolic “compensators” because of their high level of adiposity (which may have hormonal and signaling properties conducive to this form of adaptation) or whether they become overweight because they start out life by being better “compensators” and are more likely exceed their baseline metabolic needs with food. The possibility of innate differences in compensation receives some support from evidence for significant ethnic disparities in baseline metabolism. It is worth mentioning that exercise benefits human health independently of potential weight loss, through effects such as improved endothelial cell function, neurogenesis, glucose regulation, and inflammation.

• Link to full publication.