Americans spend $27 billion a year on gym memberships. They spend that money because they believe exercise is the key to losing weight — and fitness companies happily promote this narrative.

But a growing body of research suggests that Americans trying to lose weight won’t get the results they desire by slogging through extra miles on the treadmill: They need to cut calories to do it.

What people eat — and in what quantity — appears to influence body weight more than how active they are. Exercise remains critical to other aspects of health, such as preventing heart disease, but some scientists say the role it plays in weight loss has been misunderstood for years.

The latest evidence comes from a study published last week in the journal Current Biology that suggests people who are highly active don’t burn more calories than those who are moderately active.

Researchers tracked 332 adults in five countries, including the United States, and found that “total energy expenditure increases with physical activity at low activity levels but plateaus at higher activity levels,” according to the paper.

In other words, moving around increased calories burned only up to a point. That contradicts the previous understanding of how activity and energy expenditure are directly linked. We’re used to thinking that more activity burns more calories, with no limit to that relationship. If there is a limit, exercising beyond it won’t help people lose weight.

How could that be? Researchers don’t fully understand what’s going on, but they have some theories.

The body doesn’t burn calories just when moving muscles. Cells and organ systems use energy to keep us functioning — to digest food and run the immune system, for example. The bodies of highly active people may do those jobs more efficiently, thereby using less energy, said Herman Pontzer, an anthropology professor at Hunter College in New York and the lead author of the study.

“It would only take small changes about how the body spent energy in those systems” to make a difference in total energy usage, he said.

Another possibility is that people who are more physically active expend less energy fidgeting or making other small movements during the day, an accumulation of tiny changes that add up to significant energy savings.

“Those movements have very low energy expenditures and they’re hard to capture,” said Diana Thomas, director of the Center for Quantitative Obesity Research at Montclair State University in New Jersey.

Thomas wasn’t involved in the research but wrote an editorial accompanying the article. She said more research is needed to understand why people who are more active don’t always burn more calories.

But scientists have known for years that the body can adjust how much energy it uses for normal maintenance. In extreme circumstances, such as starvation and very-low-calorie diets, metabolism slows down to conserve energy. Something similar might be at work in the metabolism of highly active people.

Amy Luke, a public health researcher at Loyola University in Chicago and a co-author of the new study, first wondered in the 2000s about the accepted understanding that more activity means more calories burned. She was comparing overweight and obese women in the United States with lean women in rural Nigeria, expecting to find that the Nigerians expended more energy.

“This was, at the time, I thought, a no-brainer,” she said. In a paper published in 2009, she reported a surprise: Both groups of women were burning the same number of calories. And how much exercise they got didn’t predict weight changes.

Luke’s takeaway from years of research: If you want to lose weight, eat fewer calories.

The new findings aren’t an excuse to lie on the couch and cancel your gym membership. “We’re not saying that exercise isn’t important,” Pontzer said. “Of course it is.” But for losing or managing weight, “diet is going to be your best tool.”

Exercise can actually make it harder for people to moderate how much they eat. “It increases appetite, and therefore you’re hungrier and you feel the need to replenish yourself,” said Kristin Kirkpatrick, manager of wellness nutrition services at the Cleveland Clinic Wellness Institute. She said that many of her patients “start eating above and beyond actual hunger once they start exercising.”

The need to cut calories, though, is often drowned out by the marketing of the fitness industry.

“The message is always ‘Join the gym, get this piece of execise equipment.’ Everything’s about toning and making your body better,” Kirkpartick said. “It kind of sets you up for this unrealistic expectation of what your gym membership will do for you.”

— Bloomberg News