But it's not necessarily true, a research team says. They found that people who exercise a lot don’t burn extra calories for their efforts beyond a certain point. Their new study is published in Current Biology.
Don’t drop that gym membership just yet, though. WebMD asked two experts to discuss these findings and the role of exercise.
The experts include the study’s lead researcher, Herman Pontzer, PhD, a professor of anthropology at City University of New York, and Edward L. Melanson, PhD, an associate professor in the division of endocrinology, metabolism, and diabetes at the University of Colorado Anschutz Medical Campus, Aurora.
On one point both agree: The new research is not discouraging exercise, which is crucial to keeping your body and mind healthy. But it does provide more evidence that diet, not exercise, is the key to losing weight.
What did the study find?
Pontzer and his team measured the daily activity levels of more than 300 men and women -- along with how many calories they burned -- over the course of a week. They came from five different countries across Africa and North America: the U.S., Ghana, Jamaica, the Seychelles, and South Africa. People in some of those nations tend to be more physically active than many Americans.
The researchers had everyone's body mass index (BMI). They measured activity and calorie burning for a week, but didn't track whether people gained or lost weight.
Exercise did have an effect on how many calories people used, called energy expenditure. But the amount of calories burned didn’t increase dramatically as people got more exercise. Those who had a moderate activity level burned a few more calories daily, on average around 200, compared with the most inactive people. But those who exercised beyond the moderate activity level saw no effect of their extra effort as far as how many calories they burned.
Although the study didn't define ''moderate'' in hours of activity, Pontzer describes moderate exercisers as those who are active ''but not serious athletes” -- someone who walks a couple miles a day or bikes to work and back, for example.
Pontzer's team did find that those with higher body fat percentages actually burned more calories with exercise -- presumably, he says, because there is more fat to burn.
What do the findings say about the role of exercise in weight loss?
The study didn’t focus on this specifically, but Pontzer says exercise ''can be part of a successful weight loss strategy. We need to think about exercise and diet as two different tools."
“Exercise is good at lots of things, such as maintaining heart health,” he says. “Diet is going to be the better tool for managing your weight."
The findings may show that the inactive lifestyle of many Americans hasn't contributed as much to the nation’s obesity epidemic as public health officials believe, Melanson says. He stresses that the study does not prove that, but simply adds information to the ongoing debate about why Americans are so overweight.
The study finding, in Melanson's view, seems to tip the scale a bit more toward overeating, not under-exercising, to explain the current U.S. obesity epidemic.
Why is it that more exercise is not better? Do we hit a kind of plateau?
Yes, Pontzer says. "If you are more active, your body might be adapting,'' he says. "We hit an energy expenditure plateau. It's part of the reason your body adapts to your new exercise routine." That may be why many find it so hard to lose weight, he says.
But, he believes, there is such a thing as an exercise “sweet spot” -- the point at which workout benefits, including calories burned, peak. That spot is probably different for everyone, Pontzer says, although the new study didn’t look into that.
How do you find your ''sweet spot"?
Pay attention to your body, Pontzer says. You know you're out of the sweet spot and overdoing it when you feel worn out constantly and you need more time to recover from exercise, he says. At that point, it's time to work out less.
What's the best take-home advice from this research?
Even if researchers eventually decide that diet plays a bigger role than exercise in weight control, Melanson says, ''the public health message is not going to change one bit. Exercise does matter [for overall health], whether you lose weight or not.''
Working out has lots of health perks, including diabetes prevention, blood pressure control, stress reduction, and boosting your mood to help with depression. It can also contribute to brain and immune system health, experts say.
"This [study] says the first thing to do is diet and it won't hurt to throw in exercise," Pontzer says.