Fifteen marathon runners walked into a bathroom. Out came what researchers identified this week as an inside scoop into their impressive energy and performance, New Scientist reports. Stool samples from the runners, taken 1 week after the 2015 Boston Marathon, revealed a spike in levels of a bacterium linked to the breakdown of lactic acid, which builds up in muscles and causes fatigue and soreness after intense exercise. Researchers then introduced the bacterium, Veillonella, into the colons of 16 mice. To their surprise, the rodents lasted 13% longer on a laboratory treadmill than mice without the infusion, the researchers report in Nature Medicine. Though the discovery could one day lead to performance-boosting supplements, researchers caution there’s no causal link yet between the bacterium and athletic performance.