Milk naturally provides both fluid and electrolytes (sodium and potassium) that assist with rehydration. The sodium concentration of milk is similar to conventional sports drinks, and because of its other nutrients, it’s better at helping our bodies stay hydrated.
A number of studies have shown that milk is an effective drink for rehydration. A 2007 study found low-fat milk helped dehydrated cyclists replace sweat loss better than water or a sports drink.* In four separate trials, volunteers undertook a series of cycling exercises until they had lost about 1.8% of their body mass. They were then given either low-fat milk, a sports drink or water to rehydrate. Four hours after exercise, the cyclists who drank milk were better hydrated by an average 600 ml compared with those who drank water or a sports drink.
*Shirreffs SM et al. (2007) Milk as an effective post-exercise rehydration drink. Brit J Nutr 98, 173-80.