The present invention provides a kind of composition comprising lactalbumin and optional vitamin D and calcium, and the composition can be applied to individual to reduce the fat deposition in muscle;Increase muscle density;Improve or maintain muscle quality;And/or treat or prevent myasthenia.The composition can be applied to older individuals or with fatty infiltration in muscle increase associated illness individual to treat the illness, such as obesity.Preferably, at least part of the lactalbumin is whey protein micelles, and the daily dose of the lactalbumin is at least 18g.