Your Instagram feed is likely flooded with photos of triathletes paying homage to their beloved mid-ride espresso, and science explains why it's not just about the pretty foam art. According to the American College of Sports Medicine, studies suggests that caffeine improves performance during both endurance exercise and high-intensity, short efforts in the lab. Pro Timothy O'Donnell uses caffeine "religiously" in his training and racing. He starts his training day with a jolt of java but warns against depending too heavily on caffeine for energy. "I used to rely on coffee in the afternoons before second (or third or fourth) sessions but found that to be a slippery slope," says O'Donnell, who switches to decaf after 1 p.m. "If I needed coffee to get through those sessions then I was probably too tired to do them properly and was forcing myself into overtraining." But on race day, caffeine is central to O'Donnell's fueling plan. In an Ironman, he alternates gels between caffeine-free CLIF Shots and double espresso (100 milligrams of caffeine) CLIF Shots on the bike and uses Salt Stick caps with caffeine and will also drink Coke toward the end of the ride. On the run, it's more Coke and caffeinated citrus CLIF Shots. Training-and racing-with caffeine is something you'll want to fine-tune, though, to keep GI distress at bay.