Food fulfills three basic needs: (1) to provide energy; (2) to support new tissue growth and tissue repair; and (3) to help regulate metabolism. These three requirements are met by components of foods called "nutrients", which consist of six classes: carbohydrates, fats, proteins, vitamins, minerals, and water. Foods that are "nutrient dense" supply a significant amount of these nutrients for their calories. A high-performance diet emphasizes nutrient-dense carbohydrates necessary to maintain muscle glycogen - the primary fuel for most sports.