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?