Why Physical Activity Does Little to Control Weight

There are things we know we don’t know and things we know we know; but sometimes the things we know we know aren’t so. An article by Herman Pontzer in the February 2017 issue of Scientific American, “The Exercise Paradox,” describes new research findings that challenge our conventional wisdom about diet, exercise, and weight loss.

We think we know these things:

  1. You can’t lose weight by dieting alone; you have to exercise too.
  2. People who exercise burn more calories; if you burn more calories without taking in more calories, simple physics shows you will lose weight.
  3. One of the causes of the obesity epidemic is that people are less physically active today than they were a few decades ago.

Why Physical Activity Does Little to Control Weight

Surprises from the Hazda

Before examining these ideas, let’s look at the new research. Pontzer studied the Hadza, a hunter-gatherer tribe in Africa. They have to do hard physical work to hunt and gather their food. It would seem obvious that they would expend more energy than the average American. But they don’t. Hazda men eat and burn about 2600 calories a day; Hazda women, about 1900 calories. That’s the same as adults in Europe and America. Tests showed that Hazda adults burn the same number of calories to walk a mile as Westerners do. So what’s going on?

Not just the Hazda

There is plenty of corroborating evidence. Studies of farmers in Guatemala, the Gambia, and Bolivia showed their energy expenditures were similar to those of city dwellers. Rural Nigerian women were far more active than African-American women in Chicago, but their energy expenditure was the same. An analysis of ninety-eight published studies confirmed that people in third world countries with physically demanding lives had similar energy expenditures to more sedentary people in the developed world, who had the advantage of labor-saving conveniences.

And animal studies show similar results. Captive primates expend the same number of calories a day as primates in the wild. In Australia, kangaroos and sheep kept in pens expended the same number of calories as those allowed to roam free. In China, pandas expended the same amount of energy in zoos and in the wild.

Couch potatoes do expend about 200 fewer calories a day than people who are moderately active. But the most active people burn no more calories than moderately active people.

How Can this Be?

Apparently we have some kind of built-in energy expenditure control that adjusts to maintain a given level. The Hazda’s bodies adjust to higher activity levels and manage to keep total energy expenditure the same. They may do this by diverting energy from the body’s maintenance and housekeeping tasks. Exercise is known to reduce inflammatory response by the immune system and to decrease hormone levels. Lab animals do not increase their energy expenditure with exercise; instead, they ovulate less frequently and repair tissue damage more slowly.

Primates burn only half as many calories as other mammals, corresponding to lower rates of growth and reproduction. Humans use about 400 more calories a day than chimpanzees, and chimpanzees use more calories than gorillas and orangutans. Apparently, humans evolved to have a metabolism that burned more calories than other primates in order to support the extra activity of our brains. There is an energy cost to having to find more food, but we compensate by cooking to make more calories available, by using our brains to obtain food more efficiently (by farming, trading, transportation, etc.), and by sharing food in times of scarcity. Our tendency to store fat helps us survive famines but it also makes us obese.

Back to the Three Things We “Know”

  1. “You can’t lose weight by dieting alone; you have to exercise too.” I knew this couldn’t be true, because concentration camp inmates lost weight without exercising. And I watched my own husband lose sixty pounds in five months by eating sensibly and not exercising at all.
  2. “People who exercise burn more calories; if you burn more calories without taking in more calories, simple physics shows you will lose weight.” The physics of calories in/calories out is undeniable, but our metabolism adjusts to compensate for calories out. So effective weight loss requires finding a way to reduce the “calories in” side of the equation. No matter how hard your metabolism is working to sabotage your diet, physics insures that reducing calorie intake from your current level will result in weight loss. The principle is simple; the implementation is not.
  3. “One of the causes of the obesity epidemic is that people are less physically active today than they were a few decades ago.” I think we can now say with confidence that this is not true. The obesity epidemic was caused by increased calorie consumption and not by decreased activity.

This Doesn’t Mean You Can Stop Exercising

Exercise is still vitally important to health for other reasons. It helps prevent a variety of diseases. It makes sense to exercise to stay healthy, but it doesn’t make sense to exercise to lose weight.

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