Marathons can be bad for your health, scientists have warned after a study found 80 percent of competitors suffer kidney injury because of dehydration.
Researchers said that although the kidneys of the participants in the 26.2 mile race fully recovered within two days, their findings raise questions concerning the potential long-term impact at a time when marathons are increasing in popularity.
The findings were published by the American Journal of Kidney Diseases, as thousands of people prepare for next month's London Marathon.
Previous research has shown that engaging in unusually vigorous activities - such as military training - in warm climates can damage the kidneys, but little is known about the effects of marathon running.
A team of researchers led by Professor Chirag Parikh, of Yale University in the US, studied a small group of participants in the 2015 Hartford Marathon.
They collected blood and urine samples before and after the event. They analysed a variety of markers of kidney injury, including serum creatinine levels, kidney cells on microscopy, and proteins in urine.
The researchers found that 82 percent of the runners that were studied showed Stage 1 Acute Kidney Injury (AKI) soon after the race. AKI is a condition in which the kidneys fail to filter waste from the blood.
Prof Parikh said: "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 hospitalised patients when the kidney is affected by medical and surgical complications."
The researchers stated that potential causes of the marathon-related kidney damage could be the sustained rise in core body temperature, dehydration, or decreased blood flow to the kidneys that occur during a marathon.
While the measured kidney injury resolved within two days of running the marathon, the researchers said the study still raises questions about the effects of repeated strenuous activity over time, especially in warm climates.