Researchers at the Yale School of Medicine have found that the physical stress of running a marathon may cause short-term acute kidney injury.

Led by Yale School of Medicine nephrology professor Chirag Parikh, the researchers examined 22 runners in the 2015 Hartford Marathon before and after the race and found potential structural damage to renal tubules in the kidney afterward. The study, which was performed in collaboration with scientists from Quinnipiac University, was published in the American Journal of Kidney Disease on March 28.

“Our results revealed that 82 percent of the [marathon] runners developed kidney damage, similar in severity to kidney damage in sick patients in intensive care units and post-cardiac surgery,” Parikh said. “All runners, especially those with comorbidities such as hypertension, diabetes and heart disease need to be in close communication with their physician when deciding to run a marathon.”

Parikh’s research group became interested in understanding the effect of heat stress and physical activity on kidneys, due to the recent epidemic of kidney disease among young males working in sugarcane farms in Central America. According to Parikh, the researchers decided to use marathon running for their study, as it offers a controlled setting in which to better elucidate the effects of intense physical activity.

The study involved 22 marathon participants, with blood and urine samples collected one day before the competition, the day of the race and one day after the marathon. The researchers investigated several markers of kidney injury, including the levels of various proteins in the urine and of serum creatinine, a waste product in the blood that comes from muscle activity.

Eighty-two percent of the runners developed Stage 1 acute kidney injury, in which the organ suddenly fails to filter waste from the blood, said Sherry Mansour GRD ’17, a clinical fellow at the medical school and a co-author of the study. However, in contrast to sick patients with kidney damage, the kidney damage incurred by runners was temporary and findings in blood and urine returned to normal levels within 48 hours after running.

“It has been shown that the blood supply to the kidneys decreases by about 25 percent during running, as this blood is shunted to the muscle and the skin,” Parikh said. “This decrease in blood supply causes cell death in the kidney, particularly in the tubules.”

He added that these dead cells and debris, or granular casts, were evident in the runners’ urine samples when examined under a microscope.

Additionally, the rise in core body temperature while running causes heat stress, which also can injure the kidney, Mansour said. These two factors, as well as potential inadequate hydration and use of nonsteroidal anti-inflammatory drugs, medications that relieve muscle soreness, may have caused the observed damage.

There was no evidence of chronic kidney disease — long-term progressive loss in kidney function — in the study participants. In contrast, CKD was found in agricultural workers in Central America, who experience similar levels of physical stress as the runners. According to the paper, this variance may have been due to greater heat exposure and limited access to hydration and health resources.

“Given the results of our study, we advise runners to stay well hydrated during the race, to avoid NSAID use and if possible, to avoid running in hotter temperatures,” Parikh said. “We also recommend adequate recovery time after the race to allow for injured organs to recover.”

Parikh said that the researchers will continue to investigate the unanswered questions arising from their findings. These include how long runners should wait in between marathons, if there any long-term effects on the kidneys from frequent running and if there are any interventions that could prevent this short-term kidney damage.

Approximately 50 percent of patients admitted to intensive care units in U.S. hospitals will develop some stage of acute kidney injury, according to medical technology company Astute Medical.