A small study including 22 people who took part in the Hartford, Connecticut Marathon suggested that the long test of endurance takes a toll on the kidneys.
Researchers believe that this may be a result of sustained high core body temperature, dehydration or decreased blood flow to the kidneys. While running, blood is pumped to the skin and muscles—and as a result, the kidneys may not get as much blood as they typically would. Regardless of the cause, 82 percent of the participants showed kidneys that failed to filter waste from the blood.
In order to draw the conclusions above, the team of researchers looked at blood and urine samples collected before and after the marathon, recording creatinine (a waste product excreted by the kidneys) levels in the runners’ blood and proteins their in urine. Kidney cells were also observed under a microscope.