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Nothing about metabolism, however, is simple, and one last and important complication regarding efforts to walk off weight is a still poorly understood phenomenon known as metabolic compensation. Once again, studies of the Hadza play a role in how we understand this phenomenon. When Herman Pontzer and his colleagues measured daily energy expenditures in the Hadza, they were surprised to find that the highly active Hadza spend about the same total number of calories per day as sedentary industrialized people with the same lean body weight.44 In addition, when Pontzer and colleagues collected energetic data from adults in many countries including the United States, Ghana, Jamaica, and South Africa, they observed that more active people spent only slightly more calories per day than more sedentary people who weighed the same. In addition, individuals who were more physically active didn’t have total energy budgets as high as their exertions would predict.45 How could someone who spent five hundred extra calories a day exercising not have a total energy budget that is five hundred calories higher? The proposed explanation is that people’s total energy budgets are constrained: if I use five hundred extra calories walking, I’ll spend less energy on my resting metabolism to help pay for my exertions.46 This controversial idea (termed the constrained energy expenditure hypothesis) is still being tested, as is its relevance to weight loss. If correct, then contrary to many people’s expectations, exercisers might spend almost the same number of total calories per day as similar-sized but more sedentary individuals despite devoting more energy to being active. To appreciate the implications of this phenomenon, consider that the Hadza spend about 15 percent more of their total energy budget on activities like walking, digging, and carrying.47 In addition, as we will see later, exercise can stimulate repair and maintenance mechanisms that elevate people’s resting metabolic rates—an “afterburn”—for a few hours to as much as two days afterward.48 Yet if very active Hadza hunter-gatherers and exercising industrial people have total energy budgets that are about the same as similar-sized but physically inactive industrials, then they must spend less energy on other things like maintenance or reproduction. This may seem implausible, but we have already seen this phenomenon in people who lose a lot of weight, like the extreme dieters in the Minnesota Starvation Experiment whose resting metabolic rates plummeted.
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Daniel E. Lieberman (Exercised: Why Something We Never Evolved to Do Is Healthy and Rewarding)