Calories measure the energy your body gets from food. Your body needs calories as "fuel" to perform all of its functions, such as breathing, circulating the blood, and physical activity. When you are sick, your body may need extra calories to fight fever or other problems.
One of the three nutrients that supply calories (energy) to the body. Carbohydrates are needed for normal body function. There are two basic kinds of carbohydrates: simple (sugars) and complex (starches and fiber).