Repeated studies have shown that many people who begin an exercise program lose little or no weight. Some gain.
To better understand why, anthropologists leading one of the new studies began with a research trip to Tanzania. There, they recruited volunteers from the Hadza tribe, whose members still live by hunting and gathering.
Providing these tribespeople with a crash course in modern field-study technology, the researchers fitted them with GPS units, to scrupulously measure how many miles each walked daily while searching for food. They also asked them to swallow so-called doubly labeled water, a liquid in which the normal hydrogen and oxygen molecules have been replaced with versions containing tracers. By studying these elements later in a person’s urine, researchers can precisely determine someone’s energy expenditure and metabolic rate.
The researchers gathered data for 11 days, then calculated the participants’ typical daily physical activity, energy expenditure and resting metabolic rates. They then compared those numbers with the same measures for an average male and female Westerner.
It’s long been believed that a hunter-gatherer lifestyle involves considerable physical activity and therefore burns many calories, far more than are incinerated by your average American office worker each day. And it was true, the scientists determined, that the Hadza people in general moved more than many Americans do, with the men walking about seven miles a day and the women about three.
But it was not true that they were burning far more calories. In fact, the scientists calculated, the Hadza’s average metabolic rate, or the number of calories that they were burning over the course of a day, was about the same as the average metabolic rate for Westerners.
The implication, the scientists concluded, is that “active, ‘traditional’ lifestyles may not protect against obesity if diets change to promote increased caloric consumption.” That is, even active people will pack on pounds if they eat like most of us in the West.
The underlying and rather disheartening message of that finding, of course, is that physical activity by itself is not going to make and keep you thin. (It’s worth noting that the Hadza people were almost uniformly slight.)
The overarching conclusion of that study is not really new or surprising, says Dr. Timothy Church, who holds the John S. McIlhenny Endowed Chair in Health Wisdom at the Pennington Biomedical Research Center in Louisiana and who has long studied exercise and weight control. “It’s been known for some time that, calorie for calorie, it’s easier to lose weight by dieting than by exercise,” he says.Here’s why:human metabolism appears to be less revved by activity than was once believed.