A March 2017 study by Yale University found that, “the physical stress of running a marathon can cause short-term kidney injury. Although kidneys of the examined runners fully recovered within two days post-marathon, the study raises questions concerning potential long-term impacts of this strenuous activity at a time when marathons are increasing in popularity.”
https://www.eurekalert.org/pub_releases/2017-03/yu-mrm032317.php