At one point or another, we've all wondered: What makes some of us gym rats and some of us couch potatoes? Fitness enthusiasts are more likely to stick to diet plans and exercise routines, while their lazy counterparts are more likely to watch Netflix and binge eat junk food. New research presented at an exercise research meeting of the American Physiological Society held in Phoenix, Ariz., suggests our genes may influence how much we enjoy exercise.

Working out can boost dopamine levels in the brain, improving mood and long-term memory. It stimulates pleasurable feelings in the brain, which contribute to the phenomenon known as "runner's high." Animal studies have shown an increase in dopamine levels from intense cardio exercise, suggesting humans could also experience increased dopamine release from exercise when done at a high intensity.

However, many people do not get this pleasurable sensation because their genes interfere with the release of dopamine.

"Variation in genes for dopamine receptors, as well as some other neural signaling genes, help explain why about 25 percent of the participants drop out of exercise or don't exercise at the recommended amount," said Rodney Dishman, study lead researcher, and a professor of kinesiology at the University of Georgia, in a statement.

Dishman and his colleagues began by studying lab rats that were selectively bred to be either fit and active, or unfit and inactive. The researchers found these two types of rats differed in genetics linked with dopamine activity. Then, they proceeded to a clinical trial of more than 3,000 adults, which replicated similar results.

Currently, only about half of adults in the U.S. get enough aerobic exercise; only 20 percent get the recommended amount of aerobic exercise combined with strength training, according to Dishman. Meanwhile, about a third spend no leisure time exercising.

The researchers suggest it’s a combination of genes and personality that help explain why people have a natural urge to be active, while others never do.

Previous research provides evidence there are some genetic traits that predispose people to being less motivated to exercise. In a 2013 animal study, researchers at the University of Missouri tested rats’ willingness to run on their wheels for six days as they spun around and around. Then, they bred rats who ran more often with one another, and proceeded to do the same with the “couch potato” rats. This process was repeated 10 times. The researchers found all ten generations of the “super fit runner” rats chose to run ten times more than the couch potato rats due to genetic differences between the two types of rats.

This doesn’t mean we’re doomed if we’re genetically predisposed to hate exercise. We can choose to be active; in other words, we can rewire our brain to make exercise a pleasurable and fulfilling experience. For example, we can find an exercise that we really enjoy, and even have other people work out with us. If the exercise is not enjoyable, then it’ll be harder to get motivated to do it.

"When people start viewing exercise as a duty or obligation, then that's not a formula for sustained activity. That just puts people in a constant state of dissatisfaction," said Dishman.

We can decide to exercise despite our DNA. After all, nothing is etched in stone, even our disdain for working out at the gym.

Source: Dishman R et al. Genetics of Exercise Avoidance. APS Intersociety Meeting: The Integrative Biology of Exercise VII. 2016.