New US research suggests that the physical stress caused by running a marathon can cause short-term kidney damage.
Led by Yale University, the study looked a small group of 22 runners who took part in the 2015 Hartford Marathon.
Blood and urine samples were collected 24 hours before the marathon (day 0), immediately after the marathon (day 1), and the day after (day 2).
A variety of markers of kidney injury, including serum creatinine levels, kidney cells on microscopy, and proteins in urine, were then analyzed.
From the results the team found that 82% of the runners showed Stage 1 Acute Kidney Injury (AKI) soon after the race, a condition in which the kidneys fail to filter waste from the blood.
The researchers explained that the marathon-related kidney damage could be caused by the sustained rise in core body temperature, dehydration, or decreased blood flow to the kidneys that occur during the event.
Lead researcher and Professor of Medicine, Chirag Parikh, M.D., also explained that, "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."
Previous research has already shown that taking part in other unusually vigorous activities in warm climates --such as harvesting crops and military training -- can also damage the kidneys, however little is known about what effects marathon running might have on kidney health.
Although the kidneys had fully recovered within two days after the marathon race and the sample size was small, the research still raises questions on the possible long-term effects caused by the activity, especially when carried out in warmer weather.
The study also comes at a time when marathons are becoming increasingly popular, with more than a half million runners participating in marathons in the United States in 2015.
"We need to investigate this further," added Parikh, "Research has shown there are also changes in heart function associated with marathon running. Our study adds to the story-even the kidney responds to marathon-related stress."
The results of the study can be found published online in the American Journal of Kidney Diseases.