How to Calculate Kcal of Fiber

Dietary fiber consists of plant cell wall constituents that are indigestible by mammalian intestinal enzymes. Unlike digestible carbohydrates, which yield 4 kilocalories (kcal) per gram from aerobic glycolysis, fiber is processed differently and thus has a different caloric value. Fiber is fermented by the intestinal bacteria to produce short-chain fatty acids. If 100 percent of the fiber is converted and absorbed as fatty acids, then it would theoretically provide 3 kcal per gram consumed. However, since this is not the case, final caloric yield is much lower. The currently accepted caloric value of fiber is approximately 1.5 to 2.5 kcal/g, or 2 kcal on average.

Instructions

• 1

  Obtain the total number of grams of fiber contained within a given food. Only the fermentable forms fiber can be utilized by the intestinal bacteria. These are the soluble pectins, gums, mucilages and some hemicelluloses. Structural fibers are, for the most part, insoluble. These include cellulose, lignin and some hemicelluloses. If the food in question specifies the quantities of both soluble and insoluble factions, calculate only the soluble, as that is the one that will have any caloric contribution.

• 2

  Multiply grams of fiber by the factor of "2." This gives you the number of calories you can expect to obtain from the fiber component of the food. For instance, if a slice of whole grain bread contains 3g fiber, then 6 kcal would be made available after bacterial fermentation and absorption of the resultant fatty acids.

• 3

  Remember to calculate fiber calories separately from those of other carbohydrates. Subtract grams of fiber from total carbohydrates when calculating caloric yield of digestible carbohydrates, such as sugars or starches. For example, if a cereal contains 15g carbs per serving, and 5 are fiber, then you would only count the 10g as the digestible portion. You can then add the two values (calories from fiber and calories from starch) to determine total amount of calories provided by the carbohydrate component of the food. In this case, this would amount to 50 kcal.