Running may be aging you more than you know.

Running has forever been touted as one of the most effective forms of exercise—it sheds, shapes and tones. You go for a run and come back still reeling from a runner’s high, feeling energized and healthy. But what if running wasn’t actually good for you? What if you were actually negatively affecting your body and decreasing your lifespan?

Runners tend to push themselves to the limit, always trying for a faster speed and longer duration. But just maybe, less is more. Decades worth of studies on the effects of endurance athletics were recently reviewed in the journal Heart. The reviewers found that people who exercise too much and people who don’t exercise at all have shorter lifespans than moderate exercisers. The health benefits of exercise seem to diminish for those who run more than 20 miles a week, over 8 miles in an hour, or 6 to 7 days a week. Why? It seems that all the hard work required of the heart during strenuous exercise can cause stretching and tearing of the arteries. Combined with the free radicals produced by the body’s burning of fat and sugar for fuel, plaque buildup and stiffening of arteries can occur, creating a risk factor for heart disease.

Running may also negatively affect other body parts as well. Constant pounding can cause stretching of the breast ligaments, leading to droopy breasts. A 2008 study in Free Radical Biology and Medicine also found that strenuous exercise—about 90 minutes of running—can lead to cellular damage of the skin’s collagen and elastin.

There has been a longstanding belief that running causes damages to the joints. However, the good news here is that a recent study found that runners do not have a higher incidence of osteoarthritis than walkers, according to Medicine and Science in Sports and Exercise.

As for all of the health benefits? From lowering blood pressure to reducing the risk for many diseases, obviously running is doing something good for our bodies. “In similar fashion to the heart, the muscles and tendons and joints in the body can receive great benefits from high—intensity exercises, like running,” says Scott Goldsmith, MD, an orthopedic surgeon in Tampa, FL. “However, the potential risks of these frequent workouts must be considered and appropriate form, nutrition, and recovery remain essential components for overall success.” 

More research still needs to be done to determine just how harmful high endurance running is. In the meantime, keep it moderate. Life is a marathon and running one might not be the best option for your long—term health.