Just like a quart is a measurement of volume and an inch is a measurement of length, a calorie is a measurement or unit of energy. The number of calories in the foods you eat is a measurement of the number of energy units that food supplies. Those energy units are then used by the body for physical activity and all metabolic processes (from maintaining your heart beat, blinking your eyes, and growing hair to healing a scraped knee, generating new cells, and building muscle). Only four components of foods supply calories: protein and carbohydrates (4 calories/gram), alcohol (7 calories/gram), and fat (9 calories/gram). Vitamins, minerals, phytochemicals, fiber, and water do not supply calories. The body stores calories as:
1) Glucose: Immediate energy in blood and tissues.
2) Glycogen: Storage form of glucose in the liver and muscle. Used to restock glucose levels.
3) Fat: Long-term storage form of energy and the dumping ground for excess calories.