"People don't train their gut and intestines to deal with fluid," says Doug Casa, CamelBak hydration expert and COO of the Korey Stringer Institute, University of Connecticut. "On race day, they want to suddenly hydrate well, but they haven't trained for it, and they get gastric upset during the actual race." Casa says this is one of the most common hydration blunders, even among elite athletes. To avoid this problem, Casa always drinks one liter of fluid per hour of exercise, regardless of the type of workout. Training is incomplete, he says, without a proper hydration plan.