When you take out at the end of the day weigh yourself again. If you're down a pound, or two, or three, what you're measuring is water loss. Prevention is the best cure, so to prevent that washed-out feeling, start taking in fluids well before you launch. Fluids don't include coffee, because that will leach the liquids right out of you. Water's good, fruit juices work, sports drinks fill the bill.
Some athletes add a bit of sugar and a bit of salt to their water.
Also, nibble on sports bars or fruit as you cruise along. You'll feel better, you' ll paddle more efficiently with less apparent effort, and you'll think better. On the food side, carbohydrates should be eaten before, during and after exercise. Sports folks believe that you replace glycogen stores by consuming carbohydrates. The hormone regulating glycogen is insulin, which increases the transport of glucose from the bloodstream into the muscle masses and fires up the enzyme that converts the glucose to glycogen.