Food energy is the amount of energy in food that is available through digestion.
Food energy is typically measured in units of calories, although the International System of Units unit kilojoule (1000 joules) is becoming more common. Some types of food contain more food energy than others: fats and sugars have particularly high food energy levels.
Note that the "calorie" used by dieticians for food is sometimes called a kilocalorie; it is equal to 4.1868 kilojoules.
Measuring food energy
In the early twentieth century, the United States Department of Agriculture (USDA) developed a procedure for measuring food energy that remains in use today.
The food being measured is completely burned in a calorimeter so that the heat released through combustion can be accurately measured. This amount is used to determine the gross energy value of the particular food. This number is then multiplied by a coefficient which is based on how the human body actually digests the food.
For example, pure sugar releases about 3.95 Calories per gram of gross energy but the digestibility coefficient of sugar is about 98% in humans, so the food energy of sugar is .98 * 3.95 = 3.87 Calories per gram of sugar.
* Protein contains about 4 (nutritional) calories per gram