Why did Bob stop losing weight? We need to talk about energy expenditure compensation.

Repeatedly, intervention studies report that participants undertaking a new exercise regimen lose weight initially, but then their weight loss quickly plateaus. A careful look at the literature shows that plateauing happens even when participants' food intake does not increase and their exercise intensity is maintained. So how can we explain this, given that if the body is consistently in energy debt, surely weight loss must result. I argue that energy expenditure compensation-reductions in energy expended on some biological processes to counteract increases in energy expended on activity levels-is an under-recognized compensatory response to heightened exercise. We observe energy expenditure compensation 'in the field', for example, people in pre-industrialized nations expend a lot of energy each day on physical activity but nonetheless have a daily energy expenditure commensurate with that of relatively sedentary Westerners. But most researchers and practitioners have not connected the aforementioned laboratory and field observations-that is, if our activity levels are consistently heightened for long enough, our bodies adaptively compensate in terms of overall energy expenditure, such that if we undertake an exercise regimen, in the long run we only lose a fraction of the weight we aspire to. We need to raise awareness about energy expenditure compensation, how it can limit weight loss and how in light of this knowledge we might better prescribe 'weight loss regimens' to encourage additional weight reduction in those who aspire to it.