Sitting in front of a TV or computer screen for more than two hours each day makes you twice as likely to get heart disease or die prematurely, according to a study in the Journal of the American College of Cardiology.

Researchers at England’s University College London found that people who spent more than two hours a day seated in front of an electronic screen were 125 percent more likely to have a cardiovascular event and 48 percent more likely to die from any cause. And exercising didn’t change that.

Why? Because being seated reduces the activity of lipoprotein lipase, an enzyme that breaks down blood fats (triglycerides) to provide muscle energy. In other words, it lowers your metabolic rate—or how many calories your body uses to perform basic functions.

If you can, take a two-minute walk every half hour you’re staring at a computer, and avoid TV—an activity linked to “negative mental states...and unhealthy snack consumption,” says Emmanuel Stamatakis, the study’s lead author. His advice: “Avoid sitting unless it’s necessary, and replace TV with more active forms of entertainment.”