Why does a hunter-gatherer who walks all day burn no more calories than a Westerner?

Because the body's energy use is surprisingly stubborn. Most of the energy you burn is spent just staying alive. Basal metabolic rate, the energy needed to keep the body functioning at rest for breathing, circulation, body temperature, cell growth and brain and nerve function, accounts for about 70 percent of the calories an individual burns in a day. It is natural to expect that a person who walks and forages all day burns far more than a Westerner.

Herman Pontzer and colleagues tested that expectation among the Hadza, hunter-gatherers living in a savannah-woodland environment in Northern Tanzania. They measured total daily energy expenditure with doubly labeled water, a method in which the subject drinks water carrying heavy forms of hydrogen and oxygen. Heavy oxygen leaves in exhaled carbon dioxide as well as in body water, heavy hydrogen only in body water, so the gap between them reveals how much carbon dioxide the person produced. The Hadza were more physically active than Westerners, as expected. But once body size was taken into account, their average daily energy expenditure was no different from that of Westerners.

A 2016 study of 332 adults in five populations, also led by Pontzer, found that total energy expenditure rose with physical activity mostly over the lower range, and plateaued in the upper range. The authors read this as support for a Constrained model, in which the body adapts to keep total expenditure within a narrow range. Pontzer, an evolutionary anthropologist at Duke, developed it partly to explain the exercise paradox, where more physical activity does not always lead to weight loss.

This is not settled. The same authors note that the older Additive view, where more activity means a matching rise in energy burned, is supported by exercise intervention and accelerometry studies, while ecological studies of active populations challenge it.