More might be less when it comes to exercise, according to a study released on Thursday that suggests people's bodies stop burning extra calories beyond a certain level.
"It's a surprise," said lead author Herman Pontzer of the department of anthropology at City University of New York.
He knows his findings won't be popular with people watching their waistlines.
"We're taught that there's a simple one-to-one relationship between activity levels [exercise] and energy expenditure [burning calories], that the more active you are, the more calories your body burns every day," he said.
But Pontzer said his research is part of a growing body of evidence that suggests that just isn't the case.
"Our bodies adapt to higher activity levels so that people don't necessarily burn extra calories even if they exercise more."
The research was published Thursday in the journal Current Biology.
Pontzer and his team measured the daily energy expenditure and activity levels of more than 300 men and women in five countries, including the U.S., Jamaica and South Africa, over the course of one week.
They found that the number of calories burned during exercise reaches a plateau.
"As we move from moderate activity levels up to more and more activity, our bodies adapt, so that energy expenditure per day stays basically the same, even as we're more and more active," Pontzer said.
It's as if the body chooses the amount of energy it burns, he said, reaching a "sweet spot" for physical activity.
"Too little, and we're unhealthy, but too much and the body makes big adjustments in order to adapt," he said.
That's why people might see weight loss in the short term with exercise but then stop seeing progress.
Value of exercise
Dr. Mark Tarnopolsky of McMaster University in Hamilton said being physically active provides many other benefits beyond just the way your body looks.
"What this shows is that being a slave to the number of calories that you are burning with exercise is probably not a good investment of your time and efforts," said Tarnopolsky, who was not involved in the study.
"Those are things that people need to talk about instead of being obsessed with per cent body fat," said Tarnopolsky, whose research focuses on the potential for exercise as therapy for people with genetic neuromuscular diseases.
Pontzer agrees the study is not an excuse to skip your workout.
"We know that exercise is important for our hearts, our mental health and immune system health," he said.
"This study doesn't change any of that. We still need to exercise."
But the better tool for weight loss, he said, is a well-balanced diet.