Shalane Flanagan, the first American woman at the 2014 Boston Marathon, drinks Gatorade Endurance Formula.

"The orange flavor goes down smooth," says Flanagan of her go-to beverage, which has double the sodium and triple the potassium of Gatorade's traditional mix. "I sip it every three miles on tempo runs or anything over two hours."

It contains six percent carbs–a number shown to improve performance and hydration, says Kitchin. Its tasty flavors and high calorie count (213 in 32 ounces) make it easy to overdo. Save it for hour-plus runs.