The reason is simple, says nutritionist Suzanne Girard Eberle, M.S., R.D., author of Endurance Sports Nutrition. Any time you exercise hard enough that it becomes difficult to carry on a conversation-as when you're climbing a ridge or pushing hard through heavy cover-your body is predominantly burning carbohydrates for fuel. "If you don't have enough carbohydrates in your system," she says, "you'll be forced to slow down. Eventually, you'll ¿¿¿bonk'-what marathoners call ¿¿¿hitting the wall.' For hunters way out in the wilds, this can definitely be dangerous. Say you're five hours away from the car. Are your buddies going to carry you back?"