Working out has numerous health benefits, but if you are trying to lose weight, exercise alone may not be enough: The body may adapt to higher levels of physical activity, so you may not burn more calories even if you exercise a lot, a new study suggests.
The researchers found that the people in the study who engaged in moderate levels of physical activity burned about 200 more calories per day, on average, than those who had the lowest levels of physical activity. However, the people who were the most physically active burned the same number of calories, on average, as those who were moderately active, the researchers found.
It is not clear why, exactly, higher levels of physical activity may not lead to burning more calories, the researchers said.
In any case, "You still have to exercise," because exercise is important for health, said study author Herman Pontzer, an associate professor of anthropology at the City University of New York. [How Many Calories Am I Burning? (Inforgraphic)]
However, high levels of exercise may not result in more calories being burned. "It seems like the body is adapting to higher levels of activity and working, adapting to keep energy expenditure the same," Pontzer said.
"It is just that it is not going to be the easiest way to lose weight — instead, you will want to focus on your diet for that," Pontzer told Live Science.
In the new study, the researchers looked at the levels of physical activity and the numbers of calories burned among 332 people ages 25 to 45 over the course of a week. The people in the study lived in Ghana, South Africa, the Seychelles (an island nation in the Indian Ocean), Jamaica and the United States.
There was a small but measurable link between people's physical activity level and the total number of calories they burned per day, the researchers found. But this link held only when the researchers compared the people with moderate activity levels to the people who had the most sedentary lifestyles. People who had moderate levels of activity burned about 200 more calories per day, on average, than those who were mainly sedentary, the researchers found.
In contrast, "The most physically active people expended the same amount of calories each day as people who were only moderately active," Pontzer said in a statement.
The researchers had also previously investigated the relationship between activity levels and energy expenditure among a group of people called the Hadza, who are traditional hunter-gatherers in northern Tanzania.
"The Hadza are incredibly active, walking long distances each day and doing a lot of hard physical work as part of their everyday life," Pontzer said. "Despite these high activity levels, we found that they had similar daily energy expenditures to people living more sedentary, modernized lifestyles in the United States and Europe."
The vast majority of calories that people burn every day are not spent on physical activity, but on "the basic work that your cells do to keep you alive," Pontzer said. This work involves moving nutrients and waste products in and out of cells and keeping electrolytes balanced, for example.
"Sowhat we think is happening, as we get more and more active, our bodies adjust by spending a little bit less on those activities and making room for the increased energy expenditure on [physical] activity," he said.
The findings may mean that there is a "sweet spot" for physical activity: While exercising too little is unhealthy, exercising a lot may prompt the body to make adjustments to adapt, the researchers said.
The new study was published Thursday (Jan. 28) in the journal Current Biology.