A new study links being genetically lean to more-efficient calorie burning during exercise.

Why is it that one person can stay slim while eating a lot of calories, while another tends to gain weight despite eating fewer? According to new research from Kent State University, the muscles of naturally thin people may just work differently.

A research team, led by Chaitanya K. Gavini of the university’s school of biomedical sciences, compared female rats that had a genetic tendency to be lean with those that had a genetic predisposition toward obesity. The team wanted to see how the physiology of each group’s muscles affected their metabolism. The researchers had already discovered that an animal’s intrinsic aerobic capacity (its ability to transport oxygen and use it during exercise) is a good indicator of its daily physical activity—those with a higher aerobic capacity are usually leaner than those with a lower one.

The rats in each group had similar amounts of lean body mass and weight, but those with high aerobic capacity were more active than those with low aerobic capacity. While resting, all of the rats used about the same amount of energy, but when they were subjected to mild exercise, the “lean” rats’ genes showed “poor fuel economy,” meaning they burned more calories than the “fat” rats—those with the genetic predisposition toward weight gain.

The study was published in the journalEndocrinology and Metabolism.

Read More: Stress and Excess Weight Could Make Conceiving More Difficult »

What caused the difference in the number of calories burned by the lean rats and the fat rats?

The researchers think it could be that the lean rats had higher levels of proteins that support energy expenditure and lower levels of proteins that encourage energy conservation. Calories can’t just disappear—they have to be either used as energy or stored, usually as fat, explained Colleen M. Novak, Ph.D., one of the researchers.

“The muscles will use energy to move us around. But in some cases muscles can be less efficient at this,” said Novak. “The energy can’t disappear, though. In the case of muscles, it is dissipated as heat. That is why you get hot when you exercise.” She went on to note that when Gavini measured the temperature of the rats’ muscles during physical activity, he found that the lean rats’ muscles got warmer than those of the obesity-prone rats.

It may also be that the lean rats’ sympathetic nervous system, which helps regulate the body’s reaction to stress, played a role by stimulating the muscles to use more energy. “The thin rats have higher drive from the sympathetic nervous system to their muscles, but we are in the process of figuring out whether this is what might be causing the differences we see between the overweight rat and thin rat muscles,” said Novak.

It’s in Your Genes: Fried Foods Cause More Weight Gain in Some People »

Novak said that whatever made the lean rats more likely to burn calories, they were born with it. That doesn’t mean that the fat rats’ natural tendency toward obesity can’t be changed; the researchers just don’t know how to do that yet.

What does all this mean for human metabolism? Novak said that active people tend to be thin, and those with a high aerobic capacity are generally more active. But just because these people are inherently lean doesn’t mean they will stay that way—they could still gain weight if they regularly choose to eat unhealthy foods and skip workouts.

“In the bigger picture, [the study] would also imply that any advantage these intrinsically lean people have in staying thin may be canceled out if they are not physically active,” she said.

You can’t get a low-capacity rat—one with low endurance—to be like the thin rats without some work, she said. But exercise resulted in health benefits and weight loss in the overweight rats, so even those with obesity-promoting genes aren’t doomed to be overweight.

“Genetics can be overcome,” Novak said.

It’s Science: How Yoga Improves Overall Athletic Performance »