The invention provides a composition comprising whey protein and optionally Vitamin D and calcium. The composition can be administered to an individual to decrease fat deposition in muscle; increasemuscle density; improve or maintain muscle quality; and/or treat or prevent muscle weakness. The composition can be administered to an elderly individual or an individual having a condition associatedwith increased fat infiltration in muscle to treat the condition, such as obesity. Preferably at least a portion of the whey protein is whey protein micelles, and a daily dose of the whey protein isat least 18 g.