If you think training for a marathon is a pain in the ass (quads, calves, and hammies), just wait 'till you hear this shocking new finding: The physical stress of running 26.2 miles can cause short-term kidney damage, according to Yale researchers. 

In the study, researchers took blood and urine samples from 22 marathoners (with an average age of 44) before the Hartford Marathon in Connecticut, immediately after the race, and 24 hours later. They checked for markers of kidney injury and found that 82% of the runners showed Stage 1 acute kidney injury after the race, which means their kidneys started to lose their ability to filter waste from runners' blood. The damage resolved itself within two days after the marathon, but it still demonstrates just how much endurance running can strain your body. 

“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,” lead author Chirag Parikh, M.D., explained in a press release. “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. We need to investigate this further." 

While there's no workaround to protect your kidneys during a marathon, be more cognizant of what your body's telling you post-race. And if this news totally freaks you out, don't sweat it. There are plenty of other ways to get in shape and bolster your cardiovascular fitness, like these 11 ways to build your best cardio workout.