Endurance Fuel contains calories (100 per scoop / 200 per packet) in the form of carbohydrates to provide energy in addition to electrolytes to keep you hydrated. Rapid Hydration contains far fewer calories (45 / serving) and is ideal when you don’t need the sustained energy from carbohydrates. We recommend using Rapid Hydration for your shorter or less intense workouts, while Endurance Fuel is the go-to for your longer, sustained efforts.