We need energy for doing work. Energy is provided by carbohydrates and fats. Carbohydrates and fats on oxidation liberate energy. As compared to fats carbohydrates get easily oxidized and converted into glucose and provide energy. One gram of carbohydrate yields about 17 kilo-joule of energy, whereas one gram of fat produces 37 kilo-joule of energy. This is the reason why carbohydrates and fats ate called ‘energy food’.