• Topping it off with milk. In addition to water, fat-free and low-fat milk also are smart ways to help young athletes meet their fluid needs. But that’s not all. Just one cup of milk packs 15 to 24 percent of the protein most school-aged kids need in a day. It also delivers important nutrients of which most young athletes don’t get enough, such as calcium, which is critical for building strong bones, transmitting nerve impulses, and helping muscles contract, as well as potassium for fluid balance.