Kilocalories (kcal) and kilojoules (kJ) are units which measure energy provided by food through its digestion. This energy can be used immediately to sustain life functions such as breathing, heart beat etc, or for other activities such as moving the body to do exercises. Energy which is not immediately used is temporarily converted by the body into sugar (glucose). After a short period, glucose is then converted into fat which is the long-term store of energy.
Energy is derived from food through chemical reactions. In accordance with the law of conservation of energy, energy can only be obtained through a change in the chemical composition of food particles. As particles of insoluble fiber do not undergo any change in the digestive system, the body cannot absorb their energy. However, soluble fiber does change through fermentation and in this way provides energy for the body.
Despite trying, dieticians did not manage to reach any agreement on the amount of energy that is absorbed from fiber, though quite a few accepted that each gramme of soluble fiber provides around 2kcal (8.5 kJ). In some countries, it is customary for food labels to omit fiber altogether, on the assumption that it provides 0 kcal. In other countries, both types of fiber have to be specified on food labels, on the assumption that the total calorific value of the food is increased by 4 kcal with every gramme of fiber. (This is because, in chemistry terms, fiber is a type of carbohydrate and other carbohydrates do provide 4kcal/g.) In the USA a distinction is made between the two types of fiber. Soluble fiber is taken to provide 4 kcal/g while insoluble fiber, assumed to provide 0 kcal, is ignored in food labelling.