Could exercise be—gasp!—bad for you? Well, not exactly, but there is a downside to too much of it, a new study from the British Heart Foundation has found. In mice, endurance-based exercises were linked with a drop in production for a key heart-regulating protein, resulting in lowered heart rates of the subjects, the BBC reports. The finding may explain why human fitness devotees often have lower resting heart rates—sometimes with dangerous lapses between beats—making them more likely to require a pacemaker as they age.
In the past, people have believed that it's the nervous system's increased activity that tricks athlete's hearts into slowing down. "But our research shows this is not the case," says the lead author, according to the Times of India. "Actually the heart's pacemaker changes in response to training." But sorry, couch potatoes: The researchers said that overall, the benefits of working out exceed the drawbacks. (Click to read about a previous study suggesting that moderate runners live longer than those who log mega-miles.)