Marathon runners are always talking about "hitting the wall," a.k.a. the loathsome "bonk." It's going to happen: During intense exercise, our bodies burn glycogen (stored carbohydrates), and we only have a limited supply of it, enough for about two hours. The good news is that there are products out there that make it possible to take in carbs even when our bodies are rejecting solid foods. CarboPro is pure complex carbohydrates in powder form, so you just put a scoop into water or a sports drink, and you've got easily digestible, instantly usable calories during race day. We drank the stuff for 13 hours straight once – it works. [$27;]