The study, published in the American Journal of Kidney Diseases, found that around 80% of runners who complete marathons have kidney function that resembles that of patients who just went through heart surgery or are hanging on for dear life in the ICU. Blood and urine samples collected from marathoners post-run showed high levels of creatine and inflammatory proteins, which looks similar to patients who have an acute kidney injury. This doesn’t mean the marathoner’s kidney’s were permanently damaged, just that the activity affected their function in the same way.
Dr. Chirag Parikh, Yale nephrologist and lead author of the study, suggests the cause could be a number of reasons. For starters, you’re likely to become dehydrated during a marathon, which puts a huge strain on your kidneys. Also, running raises your body’s core temperature, which causes inflammation and starts muscle break down. And because both of these things introduce more proteins into your blood that need to be filtered out, your kidneys have to work harder to do their job. Furthermore, your body diverts some blood from your kidneys while running in an effort to cool you down. This backs things up for your kidneys, so to speak.
Fortunately, Parikh says your body chemistry goes back to normal after a couple of days, assuming you don’t have an underlying condition. So there’s nothing to be too concerned about if you’re a distance runner. That said, the study does highlight the importance of taking some time to recover after a major athletic event.