We know running as an excellent form of cardio — and therefore a great way to burn fat [1] and lose weight [2]! — but its role in building vs. burning muscle is a little less clear-cut. The hope, of course, is that when you run, you're getting energy from either your body's supply of glycogen (aka glucose), which you typically source from carbohydrates, or from your fat stores. It's often desirable to burn glycogen and fat. Muscle? Not so much.

Does Running Burn Muscle?

Losing muscle mass from running is a possibility, but good news: with the right diet and strength training regimen, it's avoidable. "If you're only running and not doing strength work, then it is possible that you're going to definitely lose some muscle mass," said Michael Fredericson, MD, professor and director of physical medicine and sports medicine at Stanford University in California. The trick, he told POPSUGAR, is to mix up your workout schedule so that you're running on some days and doing strength training, whether that's bodyweight or weightlifting work, on the other days. That ensures that you're continuing to strengthen and build your muscles on the days you're not doing your straight cardio. (Try this weekly workout plan [3] that incorporates both strength training and running.)

Diet also comes into play. Basically, "Make sure you're getting in adequate calories," Dr. Fredericson said, because while creating a slight calorie deficit can help you lose weight (if that's a goal you're after), dipping too far into that deficit can lead to muscle loss. Make sure you're eating enough protein, which repairs and rebuilds your muscles [4] to prevent them from wasting. You also want to eat healthy amounts of good fats, like avocado and nuts, which are good sources of calories to keep you full and give you a longer-lasting form of energy than carbs.

Far from losing muscle through running, said Sander Rubin, MD, sports medicine specialist at Northwestern Medicine, running can actually help you work and build your muscles, as long as you're incorporating strength training and eating enough. "Your core muscles, your leg muscles, your shoulder muscles are all going to be aspirated during running," he told POPSUGAR. If you're trying to build muscle, running can "absolutely" be part of your routine, he said, and you definitely shouldn't avoid it just because you're worried about losing muscle. You can take steps to avoid that potential downside, and the benefits of running, like improving endurance and overall body strength, are valuable for any fitness routine.