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As exercise goes, running is the ur-workout — the one human beings have been doing since our species’ cave-dwelling days. There’s no lack of evidence that running is a wellspring of health and longevity. It’s among the most popular forms of aerobic exercise, and a mountain of research has linked aerobic training to health benefits ranging from reduced cancer risk to improved cognitive performance.

One 2015 study found that even a modest amount of running — five to 10 minutes a day at a slow pace — was associated with a 28 percent drop in all-cause mortality and an even greater reduction in risk of death from heart disease. “Runners, on average, lived three years longer compared to non-runners,” says study author Duck-Chul Lee, an associate professor of kinesiology at Iowa State University. Long-term runners — those who keep at it six years or more — seem to experience the greatest longevity benefits, Lee says.

Stronger and faster is not always the same as healthier.

But as running has become more popular, so too has running longer distances. More people than ever are participating in marathons (and even ultramarathons), leading experts to question whether more is really better.

“Stronger and faster is not always the same as healthier,” says James O’Keefe, MD, a cardiologist and associate clinical professor at the University of Missouri–Kansas City School of Medicine.

Engineering a car to win the Indy 500 is a very different goal than engineering a car to run smoothly for 500,000 miles, O’Keefe says. Likewise, designing a running plan to maximize speed and endurance is not the same goal as designing a running plan to maximize longevity. “Excessive strenuous exercise can actually erase some longevity benefits seen with moderate exercise,” he adds.

O’Keefe was once an avid runner. But he switched to other activities (including walking) after his research linked heavy running to health problems. One of O’Keefe’s studies found that while modest amounts of running are a bulwark against disease and death, too much actually erases those benefits. “Strenuous” runners — defined as people who ran faster than seven miles per hour and more than four hours per week — had similar mortality rates to sedentary adults, his study concluded.

Some researchers questioned these findings. In one response to the study, Martin Burtscher, MD, a professor at Austria’s Institute of Sports Science at the University of Innsbruck, pointed out that the study relied on self-reported pace, which could skew the findings. Burtscher says he hasn’t seen compelling evidence that people can “overdo it” with running and cites another recent study that found no drop-off in mortality benefits among people who did lots of vigorous exercise.

But the study Burtscher mentions looked at the links between “physical activity” and mortality — not at running. When you drill down into recent research on running specifically, a fair amount has found that running may increase a person’s risk for some health issues — especially heart-related ones.

The association between intense, long-term endurance exercise and atrial fibrillation—a heart irregularity that can increase a person’s risk for heart failure and stroke—is “well-accepted in the scientific community,” says Eduard Guasch, MD, a cardiac health researcher at the University of Barcelona Hospital Clinic.

While running is incredibly good for you — even in small doses — there may be some risks associated with heavy endurance training.

Guasch says it’s not yet clear how exercise may cause or contribute to the heart issue. One popular theory is that heavy amounts of endurance exercise “remodel” the heart in ways that cause dysfunction. But Guasch says this is still being debated. “There is not a clear threshold beyond which we can confidently state that a specific athlete is at an increased risk of [atrial fibrillation],” he says.

There’s also some evidence that a person’s genetic predisposition for atrial fibrillation or other heart problems could make heavy running especially risky. And for people with underlying heart conditions, marathon running is associated with a small but measurable risk of sudden cardiac death.

The takeaway (at least for now) is that while running is good for you — even in small doses — there may be some risks associated with heavy training. How much is too much? That question is hotly debated — and dependent on a person’s age, DNA, health status, and a bunch of other factors. “I think what the current available research demonstrates is that there is a range of durations and frequencies that provide longevity benefits for the average individual,” says Angelique Brellenthin, a postdoctoral researcher at Iowa State who, along with Lee, has studied the effects of running on lifespan.

Brellenthin’s research suggests a running limit of 4.5 hours a week (as often as six days per week). This dovetails with other recent research that found between 40 and 60 minutes a day of vigorous exercise is probably a safe upper limit for people who want to maximize their health.

O’Keefe offers more concrete figures: “Not more than 4.5 hours per week or 30 miles per week.”

There’s no question that some experts disagree. And if you’re one of the many people who run marathons for the personal challenge, sense of community, or sheer love of the sport, you can take heart in the inconclusive state of the evidence. But if you’re pounding pavement primarily to bolster your health, recent research suggests less may be more.