Carbohydrates are the primary and most common source of energy, supplying 4 calories per gram. Carbohydrates are complex (starch, dietary fiber) or simple (sugars, fructose, lactose, sucrose). Glucose or blood sugar is the ready form of carbohydrate used by tissues for energy. The brain requires a steady level of glucose.
Proteins supply energy at 4 calories per gram. A protein is broken down into amino acids, which are then used to build, repair and maintain body tissues and to allow new growth and development. Protein is also essential for manufacture of hormones, antibodies, and enzymes.
Fats supply 9 calories per gram. Fats are used to store energy, insulate body tissues, and transport the fat-soluble vitamins A, E, D and K through the blood.