So if more Americans train, how can it get fatter?
Some experts believe that the results could reflect two groups of people – the ones and not the physical ones, so to speak.
"It's possible that the people who become more active may already have normal weight," said John Jakicic, director of the Physical Activity and Weight Management Research Center at the University of Pittsburgh.
The figures come from an annual national survey that over 60 years have been an important indicator of health trends in the US. About 35,000 adults answer the survey each year, including questions about how often, how long and how hard they work out in their free time.
The survey gives a good idea of trends, but it's not perfect. People generally exaggerate how much they exercise, how they overstate their height, and reduce their weight, said Jakicic.
Ten-year-old federal recommendations state that adults should do weightlifting or other muscle-strengthening exercises at least twice a week. They also advise adults to do high-intensity aerobic activity, such as walking or 1
In a report On Thursday, the Centers for Disease Control and Prevention released the survey responses from 2010 to 2015 and found that the level of recreational activity was more common in some states than in others.
Almost a third of non-older adults in Colorado, Idaho, and New Hampshire met guidelines for exercise. Only about a seventh in Mississippi, Kentucky and South Carolina did so.
A higher level of exercise was more common in people who worked than those who did not work, the study found as well.
National, In the years covered by the CDC report, the stress levels were flat. But recent data shows that more adults said they would be training at recommended levels in 2016 and 2017.
It's not clear why, said Jena Shaw Tronieri, a weight loss expert at the University of Pennsylvania.
One possibility: Exercising many adults to cope with stress, and the last two years have seen increasing political and social turmoil.
"I do not know if that will explain the increase lately, but we know that these situational factors are part of the context," she said.
Of course, unhealthy eating has a lot to do with obesity. Research shows that "a change in diet is needed to see dent or reduction in obesity," said Tainya Clarke, CDC, one of the authors of the report.