For millions of masochists—er, "people"—running a marathon is a major life goal, one that requires months of rolling out of bed at inhumane hours of the morning to put normal people's step goals to shame.
Even if you've never been tempted to run 26.2, odds are you know someone who has: In 2015 (the most recent year data is available), 509,000 people went the full distance, up from 353,000 in 2000, according to industry research firm Running USA. Last year, a record 52,000 runners crossed the finish line in New York City, which hosts the largest marathon in the United States.
From the sidelines, at least, most of these people probably look like the epitome of health—lean and fit, with resting heart rates that only Michael Phelps would scoff at. But new evidence suggests that the stress long-distance endurance running puts on a runner's body may go well beyond what can be treated with a foam roller and a bag of ice.
Researchers at Yale, for instance, announced earlier this week that 82 percent of marathoners suffer from a condition known as stage 1 acute kidney injury—when the kidneys fail to filter waste products from the blood.
With AKI, kidney cells get damaged and die—potentially due to a rise in core body temperature, dehydration, and a lack of blood flow to the kidneys. Fortunately for runners, the kidneys usually fully recover within 48 hours.
But what if you're the sort of runner who makes a habit of marathoning? "Could multiple episodes of kidney injury combine over time to lead to permanent damage? That's what we need to find out," says Chirag Parikh, a professor of medicine at Yale and the study's lead researcher.
Research already suggests that, when taken to extremes, running could pay diminishing returns. For instance, in a 2015 study of 1,098 healthy runners and 3,950 healthy, albeit sedentary, people, scientists found that the light and moderate runners were less likely to die. More serious runners, however, were just as likely as the sedentary types to kick the bucket during the study. People who jogged between one and two and a half hours per week, meanwhile, had the lowest risk of death. Studies have also shown that competitive male endurance athletes—like serious marathoners—may have lower levels of testosterone, and even abnormal sperm.
Correlation does not equal causation, of course, so if marathoning is your thing, studies like these aren't reason to nix the long runs. "When you think about physical activity, the biggest benefit is going to be in someone who goes from doing nothing to doing something," explains James Borchers, director of the sports medicine program at The Ohio State University Wexner Medical Center. While Borchers believes the "sweet spot" to be running 15 to 25 miles per week—slightly more than the study above suggests—he warns, "after that, a lot of problems can happen."
Exercise—all exercise, not just running—puts stress on your system, he notes. But when performed for hours and hours on end, like during a marathon, triathlon, or ultra-marathon, those stress levels can get too high—especially if there's not enough time for recovery. (According to Borchers, most experts estimate that it takes at least a couple of weeks for your body's systems to fully recover, if they actually do, after a marathon.)
That stress, called "oxidative stress," may explain why, according to a 2012 study, male marathon runners showed increased levels of arterial plaque—the same plaque that can cause heart attacks and strokes. Even in people with no symptoms of heart disease, the biomarkers observed in these runners have been linked to higher mortality rates.
"Some folks say that everyone can run," Borchers says. "While that's technically true, some people may be better suited than others to handle the stress."
Anatomical differences in joint structure, muscle lengths, and muscle fiber types, as well as disparities in immune function and genetics, influence what your body can endure—as well as how well it recovers from exercise. In the end, these factors may play into who can run back-to-back marathons without injury, while others may risk blowing out their knee at mile five.
The key, while it may seem obvious, comes down to listening to your body. "Injury history seems to be the biggest predictor of future issues," says Andrew Gerken, a physician at the Hoag Orthopedic Institute in California who works with endurance runners. It makes sense: If you end your runs thinking, "Everything hurts and I'm dying," you're likely pushing too hard, too fast, or too far," Gerken says.
It's also fine to admit you're just not built for this—there are plenty of other fitness milestones you can achieve, along with the medals to prove it. "A marathon is more than anybody needs for good health," Gerken says. "It's a nice goal, but you need to ask yourself why that's your goal before blindly flying into it."
UPDATE 3/29/17: A previous version of this story quoted Borchers as suggesting people should run "15 to 25 hours a week." It's since been corrected to "15 to 25 miles per week."
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