Increasing physical activity to burn more calories works up to a point, but then the body adjusts to keep energy use stable, according to a new study.
As a result, people who are extremely active burn a similar number of calories as only moderately active people, researchers say. And for people trying to modify their weight, increasing exercise will not translate to endless increases in calories expended.
In a previous study, lead author Herman Pontzer of Hunter College at the City University of New York found that people in subsistence farming or hunter-gatherer societies, who are moving around and walking great distances for much of the day, have similar daily calorie burn to people in developed countries who are more sedentary.
"When I first got into this area with hunter gatherers in Tanzania, we measured daily energy expenditures and they were very physically active every day," but weren't burning more calories than adults in the US or Europe, Pontzer told Reuters Health by phone.
For the new study, Pontzer and colleagues studied energy expenditure in 332 adults, aged 25 to 45 years, drawn from populations in Ghana, South Africa, Seychelles, Jamaica and United States.
They measured total energy expenditure using specially "labelled" water, and measuring how the water molecules are eliminated over time in saliva, urine or blood samples. The researchers also tested exhaled carbon dioxide to measure resting metabolic rate and had the subjects wear accelerometers to record daily activity levels.
The researchers found that for less active people, energy expenditure increased alongside increases in physical activity. But at higher levels of activity, calorie burn plateaued.
Resting metabolic rate tended to be constant at 1,540 calories per day, and activity increased calorie burn up to 2,600 calories per day, at which point additional activity didn't appear to burn any more calories, according to the results published in Current Biology.
"The body works pretty hard to keep energy expenditure in check," Pontzer said.
Although the "calories in" side of the equation - how much you eat - can change, the "calories out" side of the equation doesn't want to budge, he said.
For weight loss, he said, "it makes more sense to focus on calorie intake." Though this study compared calorie burn in different populations, metabolism from person to person can vary a lot, he noted.
"I think this paper adds to what we've known for a while now, (that) diet is a more effective tool for weight loss than exercise," Pontzer said. "You still need to exercise, I'm not saying it can't help with weight loss, exercise is super important for your health."
Body weight is one measure of health, but so is fitness, and exercise is essential for fitness, said Diana Thomas, director of the Center for Quantitative Obesity Research and professor at Montclair State University in New Jersey, who was not part of the new study.
When people exercise very hard sometimes they lose very little weight and often they lose no weight at all, Thomas told Reuters Health.
People who exercise at a high level and haven't lost weight, if that is their goal, should see a dietitian, she said.
For people who have lost weight, however, exercise is key for weight maintenance, Thomas said.