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Are Marathons Bad for Your Kidneys?

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If you were to ask people at the finish line of a marathon why they just put themselves through 26.2 miles of sweat and pain, you'd likely hear things like "to accomplish a big goal," "to see if I could do it," and "to get healthier." But what if that last one isn't totally true? What if a marathon were actually damaging your body? That's the question Yale researchers addressed in a new study, finding that marathoners show evidence of kidney damage after the big race. (Related: The Real Risk of Heart Attack During a Big Race)

To look at the effect of long-distance running on kidney health, the scientists analyzed a small group of runners before and after the 2015 Hartford Marathon. They collected blood and urine samples, looking at a variety of markers of kidney injury, including serum creatinine levels, kidney cells on microscopy, and proteins in urine. The findings were startling: 82 percent of the marathoners showed "Stage 1 Acute Kidney Injury" soon after the race, meaning their kidneys weren't doing a good job of filtering waste from the blood.

"The kidney responds to the physical stress of marathon running as if it's injured, in a way that's similar to what happens in hospitalized patients when the kidney is affected by medical and surgical complications," said Chirag Parikh, M.D., lead researcher and a professor of medicine at Yale.

Before you freak out, the kidney damage only lasted a few days. Then the kidneys returned to normal.

Plus, you may want to take the findings with a grain of salt (yay electrolytes!). S. Adam Ramin, M.D., a urologic surgeon and medical director of Urology Cancer Specialists in Los Angeles, points out that the tests used in the study aren't 100 percent accurate at diagnosing kidney disease. For instance, a spike in the levels of creatinine in the urine can indicate kidney damage, but it can also indicate injury to the muscles. "I'd expect these levels to be high after a long race regardless," he says. And even if running a marathon does cause some real damage to your kidneys, if you're healthy then your body can recover just fine on its own, with no long-term issues, he says.

There's one thing to keep in mind, though: "This shows that you should be in good health to run a marathon, not run a marathon to improve your health," Ramin explains. "If you train properly and you're healthy, then a little damage to the kidney during the race isn't harmful or permanent." But people who have heart disease or diabetes, or who are smokers, shouldn't run a marathon since their kidneys may not be able to recover as well.

And as always, make sure to drink plenty of water. "The biggest risk to your kidneys during any exercise is dehydration," Ramin says.

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