While catching up on last year’s Scientific American, I came across a fascinatingly counterintuitive article in the 2017 issue and couldn’t resist sharing it with you.
The article is called The Exercise Paradox: Does Exercise Have Nothing to Do with Weight Loss? Its author is the anthropologist Herman Pontzer of Hunter College, City University of New York.
Professor Pontzer first introduces a study he led himself. Together with his collaborators, he went deep into Africa to study an indigenous tribe—the Hadza. Using the doubly labeled water method (a way of precisely measuring a person’s daily energy expenditure with isotope-labeled drinking water), they measured the daily energy expenditure of men and women in the tribe. The Hadza still maintain a traditional hunter-gatherer lifestyle: men hunt, women gather. Compared with people in modern cities, they engage in remarkably intense physical activity every day.
By intuition, a hunter spending all day chasing giraffes across the savanna ought to burn far more energy than an urban clerk spending all day sitting in an office. Yet the results showed that the Hadza burn no more energy each day than those of us city dwellers who barely exercise.
Hadza men take in and burn about 2600 calories a day, while women average about 1900 calories—almost exactly the same as adults in Europe and the United States.
The researchers were initially baffled by this bizarre result. Their first thought was that something had gone wrong with their methods, so they analyzed every factor they could think of, including body weight, body-fat percentage, age, and sex. But they still found no difference in energy expenditure between people with high and low levels of physical activity.
In other words, unless you’re doing something extreme like running a marathon, the amount of energy your body burns each day is almost constant, whether you exercise or not.
In fact, this wasn’t an isolated finding. Two researchers reviewed energy expenditure across 28 studies involving different populations and found that people in developed countries enjoying the conveniences of modern life burned the same amount of energy as people in developing countries who engaged in more physical activity.
Professor Pontzer later conducted a more precise study, tracking the daily energy expenditure of 300 participants. The results showed only a very weak correlation between physical activity and metabolic rate. “People who spend the whole day lounging around at home burn only 200 fewer calories per day than people who do moderate exercise—say, working out a little each week and taking the stairs.” More importantly, as physical activity increases, energy expenditure rises only up to a certain point and then levels off: people doing high-intensity exercise burn no more energy than people doing moderate-intensity exercise.
So here’s the question:
The body can’t exactly violate the law of conservation of energy, can it? If total energy expenditure stays the same, then the extra energy burned during high-intensity exercise must be saved somewhere else. The question is: where does that saved energy come from?
Professor Pontzer says scientists still don’t know the answer, but they have two guesses.
Guess 1
People who are more active may save energy through subtle behavioral adjustments. For example, they may look for more chances to sit down, or sleep more deeply. In short, they reduce energy expenditure during the time they aren’t exercising.
Guess 2
Guess 2 is much more interesting—and, if confirmed, much more significant. To free up energy for exercise, the body may reduce many other invisible forms of energy expenditure. For instance, exercise often weakens the inflammatory response through which the immune system fights infection. One possible reason is that the energy normally used to produce inflammation has been “borrowed” by exercise. Professor Pontzer speculates that this may even be the real reason exercise improves health: “Energy is redirected away from activities such as inflammation, preventing inflammation from lasting too long and causing negative effects.”
All this evidence points to an obvious conclusion:
Obesity is caused by overeating and overdrinking; laziness is not the cause of obesity.
Exercise can’t make you lose weight. Eating less can! (In fact, many studies have found that physical exercise alone does not lead to weight loss.)
There’s a saying about weight loss: “Keep your mouth shut and move your legs.”
Now, that saying probably needs a little revision.
“If you want to lose weight, just watch what you eat. As for whether you move your legs, that’s entirely up to you.”
p.s.
Still, exercise remains extremely important. Exercise won’t make you lose weight, but it will make you healthier.
References:
Herman Pontzer. (2017). The Exercise Paradox: Does Exercise Have Nothing to Do with Weight Loss? Scientific American, (3), 22-27.
Note
- These short popular-science essays in the “New Psychology Knowledge Mini-Lessons” series were written to accompany the related content of Zhichao Wei: New Psychology Knowledge Mini-Lessons. They were published on the now-closed platform “Fantuan”; the original link no longer exists.
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