Six years ago, a study from Michigan’s William Beaumont Hospital found that about 40 percent of runners suffer acute kidney injury after marathons.

That sounds bad. Is it? Should I never run another marathon? Never run more than a few miles? Never leave this chair?

The nephrologist Chirag Parikh, a professor of medicine at Yale, was unsure what to tell his patients. He knew that running marathons tends to be associated with at least temporary kidney damage, but he didn’t know how or why exactly that happened.

Today he’s a step closer to understanding, as his lab has published a new study elaborating on the relationship. In one of the country’s premier kidney-disease journals, the American Journal of Kidney Diseases, Parikh found that the rate of acute kidney injury was likely closer to 75 percent. And the effect was not subtle. “We demonstrated that there is the same amount of injury and inflammation after marathon running that we see in patients coming out of cardiac surgery or in the ICU,” he told me.

This could be worth taking seriously since the world is, in his clinical words, in the midst of “an epidemic of marathon running.”

But, he says, the effect on our kidneys is not necessarily a bad thing.

To understand why, he walked me back through some physiology of how the fist-sized beans in our backs filter our blood and spit waste into our bladders. Among that waste is a compound called creatinine, a metabolic byproduct that tends to be a fair proxy for how well our kidneys are working. Healthy kidneys filter creatinine from our blood easily, but when they’re struggling the amount increases.

Past marathon kidney studies have been based largely on measuring creatinine and urea in runners’ blood. But measuring these waste products is an imperfect thing. Creatinine increases when we break down muscle. Even when acute kidney injury is clearly present, it can be the result of dehydration, heat itself, stress, or any combination thereof. Parikh wanted to know which of these things was the primary culprit.

So on a cool day in central Connecticut, runners in the Hartford Marathon underwent a battery of tests. Signs encouraged them to drink from numerous hydration stands along the course. They surrendered their blood and urine before and after the race, and Parikh’s team took it. They hunted for damaged and dead cells in the urine that had washed out after acute injury to the kidney’s tubules. The team also measured proteins in the urine that signaled inflammation (NGAL and interleukins). The degree of physical stress on the kidneys amounted to a five- to 10-fold increase from normal.

What Parikh takes from the data is that marathon running does indeed induce structural damage in kidneys, and it’s likely due to physical stress.

So most news outlets will likely report this as a scary story about long-distance running—or any other type of extreme endurance training. The study definitely didn’t read to me like an endorsement. But Parikh isn’t sure that’s the right takeaway. I asked him if, hypothetically, he had a loved one who was running a lot of marathons, would he tell that person to stop? Hold an intervention?

“This is the million-dollar question,” he said. The day-after-the-race results showed that the kidneys in all people were quickly coming back to normal, and that’s reassuring. “The main question on our minds is what is the long-term consequence of small injuries? Chances are, the majority of people may be okay.”

At the same time, for people who are at-risk for kidney disease—people with diabetes, hypertension, of older age, for example—“we should consider whether marathon running is safe for them. That should be the focus of future studies.” This is a short-term study and can’t tell us if marathoning would be associated with any permanent renal damage. “For example, if someone runs marathons for 30 years, we don’t know if kidney disease would progress more quickly than in someone who didn’t run marathons.”

On that note, do we even know this inflammatory response is unhealthy?

“Usually we think about inflammatory states as ‘bad,’ right?” he said, giving the example of diabetes as an inflammatory state that damages organs in the long term. “It’s possible that the inflammatory milieu we saw in marathon runners may not be good for the body, but at the same time, science works in weird ways. Maybe small doses of inflammation could be good because it makes organs stronger.”

In ischemic conditioning, small doses of deprivation make organs better able to deal with larger insults later on. It amounts to exercising the body’s repair mechanisms. “Maybe if a marathon runner goes in for cardiac surgery later on,” he posits, “their kidneys will be better trained at recovering than the kidneys of someone who has never run a marathon.”

Vaccines also cause inflammation, and that’s ultimately good. Small, controlled doses of physical stress may be good in some people and not good in others. Knowing how much these findings mean in the long term would require further research. That comes down to funding, and marathons aren’t high on many priority lists that also include cancer and sudden infant-death syndrome. This study was funded by a grant from the National Institute of Diabetes and Digestive and Kidney Diseases, a part of the National Institutes of Health, the funding of which is slated to be cut substantially under President Trump’s budget proposal. I pointed that out to Parikh. It turns out he didn’t need that pointed out.

“So maybe this will happen through philanthropy or crowd funding,” he said, either feigning a laugh or laughing in a despondent way.

But the epidemic of marathon running is happening now. So what’s to be taken from this news? Is it at least a reasonable takeaway that a person can run too long? Staying well-hydrated and avoiding overheating are always paramount to kidney health, and those ends are achieved in different ways in different people. On the same note, Parikh thinks there’s probably an optimal running distance for each person, at each point in life.

“It’s possible that with good training you can gradually increase that distance,” he said. “We now have all these tools to measure urine proteins, and if someone is worried, they can work with experts to come up with safe distances that don't cause a lot of kidney injury. The way we use prescription eyeglasses isn’t to give everyone the same pair; we customize them to everyone’s vision. The same thing may go for running.”