If you are looking for an alternative to commercial sports drinks to rehydrate after a tough workout, you just might want to crack open a coconut. Coconut water comes from green, or young, coconuts. It is a good source of the electrolytes potassium and sodium, which can become depleted during strenuous workouts. Coconut water also contains magnesium. A magnesium deficiency can negatively effect your exercise endurance. Furthermore, coconut water only contains 46 calories and less than one gram of fat per one cup serving. The combination of sugar and minerals found in coconut water aids in its absorption, allowing for quick rehydration.

Note: Coconut water is not the same as coconut milk, which is an emulsion of the white meat obtained from mature coconuts. Coconut milk contains more fat and calories than coconut water (48 grams of fat and 445 calories per one cup serving of canned coconut milk).

Reference
Species Profiles for Pacific Island Agroforestry: Cocos nucifera (Coconut)

USDA Nutrient Data Base: Coconut Water and Coconut Milk

Coconut: Its Role in Health; Wendy Snowdon, et al.; 2002