The new results run counter to the idea that the obesity epidemic in the U.S. is caused by a lack of physical activity, said lead study author Lara Dugas, an assistant professor of public health sciences at Loyola University Chicago Stritch School of Medicine.
When it comes to figuring out the causes of obesity, "what we really need to look at is what people are eating," Dugas told Live Science. Previous research, for example, has linked a greater risk of obesity with the consumption of high-calorie food and sweetened beverages, she said. [How to Lose Weight in 2017 (and Keep It Off for Good)]
In the new study, the researchers found that the amount of time people spent exercising per week didn't seem to play a role in how well those people controlled their weight.
In fact, some of the people who exercised more than others in the study actually gained weight over the two-year study period, while some of those who exercised less than others lost weight over the same period, according to the study, which was published in January in the journal PeerJ.
The findings suggest that "physical activity was not enough to prevent weight gain," Dugas said.
The new study examined more than 1,900 people in the U.S., Ghana, South Africa, Jamaica and the Seychelles (an island nation in the Indian Ocean). At the beginning of the study, the researchers asked all of the study participants to wear tracking devices for one week to measure how much time they spent exercising. The researchers used the data to see whether the people in the study met the U.S. Surgeon General's physical activity guidelines, which recommend that people exercise at a moderate pace for at least 2 and a half hours per week.
The researchers also measured each participant's weight, height and body fat three times: at the start of the study, one year later and two years later. [Dieters, Beware: 9 Myths That Can Make You Fat]
When the study began, the participants in Ghana weighed the least, on average, and those from the U.S. weighed the most, according to the study. The average weight of both the men and women in Ghana was 139 lbs. (63 kilograms), while the average weight of American men was 206 lbs. (93 kg) and of American women, 202 lbs. (92 kg).
At the end of the two-year study period, the researchers found that some of the people who met the physical activity guidelines at the beginning of the study were more likely to gain weight than those who did not meet the guidelines.
For example, men in the U.S. who met the guidelines gained a half pound per year, on average, whereas those who did not meet the guidelines actually lost about the same amount per year. (This is not typical, as most people usually gain weight over time.)
But the pattern of exercisers either not losing weight or actually gaining weight over time wasn't only true for the Americans in the study — the researchers observed the same pattern in people in each of the five countries. Because of this, these results likely apply to other populations as well, Dugas said.
It’s not entirely clear why exercise may not help people lose weight, or may sometimes even be linked to weight gain. One possible explanation is that because exercise tends to increase appetite, it may simply cause people to eat more than they otherwise would, Dugas said.
Still, the results of the study certainly don't mean that people should stop exercising, Dugas said. Exercise has a lot of other health benefits, she said.
For example, previous research has shown that people who exercise regularly have a reduced risk of heart disease, diabetes and cancer, compared with people who don't exercise, the researchers said. Moreover, exercise has been linked with a better mood and mental health. And other research shows that people who exercise tend to live longer than those who don’t, according to the study.
Originally published on Live Science.