It has long been assumed that a lack of physical activity is one reason for the alarming rates of overweight and obesity seen in the United States and other developed countries. According to this theory, our bodies have evolved to be far more active than they are with today’s typical sedentary lifestyle, and therefore the body burns fewer calories than it would with a substantially higher level of physical activity. While it seems reasonable, this explanation has not held up well to scientific scrutiny — including a new study involving people whose lifestyle could not be more different from what is typical in Western countries.
As one of the researchers noted last month in an essay in The New York Times, he and his colleagues decided to examine whether a consistently higher level of physical activity corresponds to a consistently higher number of calories burned by the body. Their subjects were members of the Hadza tribe in Tanzania, a group that lives much the way all humans are believed to have lived before the agricultural revolution began some 12,000 years ago. The Hadza live in grass huts and spend the day gathering food, with the women digging up roots and picking berries while the men hunt with traditional poison-tipped arrows and gather honey. As a separate article in the Times notes, men walk an average of seven and women an average of three miles each day. While this represents significantly more physical activity than the typical American gets, and the Hadza burn just as many calories through exercise as would be expected, their overall energy expenditure — the number of calories burned — is no greater than what is typically seen in more sedentary people. (Overall energy expenditure was measured through a sophisticated type of urine analysis.)
As the second Times article notes, this finding is actually consistent with weight-loss studies that have been carried out in the United States. Contrary to popular belief — or at least popular hope — exercise doesn’t raise the metabolic rate of most people when they’re not exercising and can, in fact, actually slow it down. According to the Hadza researcher, this may be because our bodies have evolved to keep energy expenditure as consistent as possible. Since lack of food rather than overabundance of it has been a greater problem throughout most of human history, it makes sense that the body would not want to burn extra calories in the face of increased physical activity — especially since that activity may be necessary to get the food in the first place, as in cases of hunting and foraging. This stasis in energy expenditure supports the idea that the Western diet is more responsible for overweight and obesity than is a sedentary lifestyle.
And yet most weight-loss experts, even when they consider that overall energy expenditure does not necessarily increase with more physical activity, recommend exercise as part of a weight-loss program. This invites speculation that physical activity may have benefits that are distinct from its direct effect on calories burned, such as helping to regulate hunger. And, of course, exercise does burn a greater number of calories while it is being performed, so getting a very large amount of it will almost certainly lead to more calories being burned overall.
What do you think — in your experience, does your diet or level of physical activity seem to have a greater effect on your weight? Is it reasonable to expect people in modern societies to have the same activity level, or to follow the same diet, as hunter-gatherers? Do you feel like your body wants more physical activity, or is it a struggle to motivate yourself to exercise? Do you feel it would be easier for you to get more exercise, or to change your diet? Leave a comment below!