Your body gets energy from calories, which come from three sources: fats, proteins and carbohydrates. Although your body can convert the calories from all three of these macronutrients into energy, fat gives you the most energy because it contains the most calories per gram.
Protein and carbohydrates each contain 4 calories per gram. Fat -- the most energy dense of the nutrients -- contains 9 calories per gram. Fat not only gives you the most energy per gram, but your body also has an unlimited ability to store fat so it can call on this energy for later use. Although fat does provide energy, too much is not a good thing. Keep your fat intake at 20 to 35 percent of your calories.