Carbohydrates (carbs) provide ready energy to the body, and it's in its quickest useable form. Proteins has the same caloric content per mass as carbs, yet it is not as useable. It takes longer to break proteins down into amino acids then denitrate them into sugar components. Most amino acids we get from protein is used to build more protein for the body. Fats (lipids) pack over twice the amount of energy per mass as carbs, but they are more difficult to transport (because they resist water) and to convert to glucose, due to the long nature of fatty acid chains. But lipids are a fantastic way to store extra calories due to their light density.
