Most marathon runners worry more about pulling a hamstring than injuring a kidney. But a new Yale-led study shows that running a marathon (26.2 miles) can cause short-term kidney damage.\n Published in the September issue of CJSAN (Clinical Journal of American Society of Nephrology), it was a follow-up to a previous study from Yale that showed similar results.\n The purpose of the research is not to show that running is bad for your health, but to better understand how the physical stress of a marathon affects the kidneys, explains lead author Sherry Mansour, DO, MS, an instructor of nephrology at Yale School of Medicine.\n Approximately half a million Americans participate in marathons each year, yet the effect on the kidneys has not been studied in detail before, Dr. Mansour says. “It was a curiosity of ours, and we didn’t anticipate our initial findings,” she says.\n In the first study, conducted at the 2015 Eversource Hartford Marathon, researchers collected blood and urine samples from 22 runners 24 hours before the race, 30 minutes after, and 24 hours following the event. The samples were used to measure sodium levels and certain proteins that could indicate kidney injury.\n Runners who had AKI lost about 4 liters in sweat compared to only 2 liters in those without AKI. Imagine filling two 2-liter bottles of soda with your own sweat.\n Sherry Mansour, DO, MS, instructor of nephrology at Yale School of Medicine Eighty-two percent of the runners showed acute kidney injury (AKI), a reversible condition in which kidneys can’t filter waste from the blood like they normally do. This makes it difficult for the kidneys to properly balance body fluids and electrolytes, including sodium, calcium, and potassium.\n “I wasn’t surprised by the rise in creatinine, which is a blood marker used to detect kidney injury. It could have simply been due to dehydration, which is just a change in the concentration of creatinine without there being actual structural damage to the kidney. Usually, we call this a pre-renal injury, which is easily fixed by giving some fluids,” says Dr. Mansour, who was also lead author of the first study, published in 2017. “But I was surprised that when we looked at their urine, it had what we call a muddy brown cast, which signifies actual structural damage to the organ, and the urine also had high levels of proteins that capture true injury in the kidney.”\n Although the damage went away within 48 hours, the findings left the researchers wanting to understand why this damage occurs in certain runners and not in others.\n “Going into the second study, our hypothesis was that those who get AKI probably sweat a lot differently than those who don’t,” Dr. Mansour says. “When you run, your core body temperature rises and you tend to sweat to try to cool down. When you sweat, you lose both water and salt, which we hypothesized might trigger both a hormonal and an inflammatory response that may injure the kidneys.”\n Runners who had AKI lost about 4 liters in sweat compared to only 2 liters in those without AKI. Imagine filling two 2-liter bottles of soda with your own sweat.Sherry Mansour, DO, MSFor the second study, 23 runners were recruited from the 2017 Eversource Hartford Marathon. Blood and urine samples, along with other measurements, were taken at the same time points as the first study. Runners also wore sweat-collection patches and a bio-harness that continuously recorded body temperature.\n Fifty-five percent of the runners developed AKI after the marathon, and 74 percent tested positive for indicators of some injury to the renal tubes, which are portals in the kidneys that regulate water and electrolytes (solutes) in the blood by reabsorbing only what is needed. The findings from the runners’ samples, which allowed for microscopic examination of their urine, were similar in severity to those of patients in a hospital’s intensive care unit, the study says.\n The runners with AKI had distinct sodium and sweat volume loss (in other words, the amount of sodium lost through sweat and the total sweat loss). Sweat primarily consists of water and electrolytes. Of the electrolytes, sodium is key in balancing the amount of water in the body.\n “Runners who had AKI lost about 4 liters in sweat compared to only 2 liters in those without AKI. Imagine filling two 2-liter bottles of soda with your own sweat,” Dr. Mansour says. “And those with AKI lost about 3 grams of sweat sodium compared to 1 gram in those with no AKI. There was also biological evidence that runners with AKI lost more sodium and water in their sweat as they had substantially higher levels of a hormone known as copeptin, which is produced when there is a decrease in circulating volume in our bodies.”\n However, there was no major difference in body temperature between runners with and without AKI. “The lack of difference in core body temperature could have been because the race was in Connecticut during the fall, where ambient temperatures do not reach high enough levels,” Dr. Mansour adds.\n Future research, she says, will examine how adequate sodium and fluid replenishment during a marathon may optimize blood flow to the kidneys and reduce the incidence of AKI in runners.\n “I’m hoping that in the future we can help runners find personalized drinking protocols to keep them hydrated and avoid kidney injury based on how they sweat,” Dr. Mansour says. “We owe it to the running community to answer these questions.”\n Given that the kidney damage among runners in the studies was temporary, Dr. Mansour says many runners may ask whether this is anything to worry about. “My answer is that we don’t yet know. But we do know that in the sick population, AKI is linked to bad outcomes, including a higher chance of developing chronic kidney disease, which is permanent damage to the kidneys,” she says. “There is no long-term study of kidney disease in runners. And we aren’t saying running is bad for you but urging runners to keep up with their fluid and salt losses and to talk with their doctors about running.”\n To learn more, visit Yalemedicine.org.