You may not have even considered water as an ingredient of cow’s milk. However, there is a lot of it in there. Milk consists of approximately 88% water. The remaining approximate contents of milk are lactose (milk sugar) 5%, fat 3.5%, protein 3.2%, and ash less than 1%. The composition of milk depends on the type of cow used to produce the milk. For example, a Brown Swiss cow will provide milk with about 4% fat content whereas a Jersey cow will supply 5.5% fat milk.
The high concentration of water in milk contributes to its ability to hydrate the body. In fact, studies have shown that milk may rehydrate the body better than water or sports drinks. One study found that milk remained in the body longer than water or sports drinks because it releases slowly and is gradually absorbed by the body. This helps the body hold onto fluids longer, which supports extended hydration [source]. It may seem odd but grabbing a glass of milk post-workout may be a better bet than water or Gatorade.