Many layman online sources claim that fiber has no calories, while some official sources include fiber in “total carbohydrates” to calculate calories in food. The fact is that there are calories in fiber, but not as many as in digestible carbohydrates.
In order to understand why there are less calories in fiber than in digestible carbohydrates such as simple sugars, it is important to understand how they both produce energy.
How do carbohydrates generate energy?
Carbohydrates from food get broken down into smaller units. Its digestion starts in the mouth, continues in the stomach and is completed in the small intestine, where the majority of the breakdown occurs.
In the small intestine, carbohydrates break down completely to form the simplest carbohydrate units (glucose, fructose and galactose), which are then absorbed by the blood.
Absorbed glucose is ready to provide energy, while fructose and galactose are first converted to glucose in the liver.
- Digestible carbohydrates are completely digested and absorbed and converted to energy in the body.
How does energy get produced by fiber?
Calories in fiber are calculated differently to digestible carbohydrates.
Dietary fiber is a type of carbohydrate that isn’t digested by the body. Instead it passes through the entire digestive system and is excreted from your body relatively unchanged.
However, there is one type of fiber (called fermentable fiber) that is fermented by “good” bacteria living in the colon.
The product of this fermentation is short-chain fatty acids and gases. Short chain fatty acids are then absorbed and used to produce energy.
Only the part of the fermented fiber that results in the production of energy can be used to calculate calories. Non-fermentable fiber (the majority of insoluble fiber) provides 0 Cal of energy.
The most accurate and up to date average estimate is 2 Cal (8kJ) per gram of fiber, assuming that fibrous foods contain on average of 70% of fiber that is fermentable.
This amount has been adopted by most countries in the last 10 years. (See the table below)
The U.S remains one of the few remaining countries that still assigns 4 kcal per gram of fiber.
Please note, however, that manufacturers are allowed to subtract insoluble fiber from calorie calculations since insoluble fiber is mostly non-fermentable.
The majority of the online data though (including USDA) includes all fiber in the calculations of calories which is incorrect and misleading. Please also note that most nutritional websites use the USDA database for calorie calculations.
|Country / organization||Cal/g of fiber||Special notes|
|USA||4 kcal (17 kJ)||Dietary fiber is included in total carbohydrates and therefore in the calculations of calories, but manufacturers can subtract insoluble fiber.|
|Australia, NZ||2 kcal (8 kJ)|
|Japan||2 kcal (8 kJ)|
|Nordic countries||2 kcal (8 kJ)|
|European Union||2 kcal (8 kJ)|
|Canada||2 kcal (8 kJ)||Currently in the process of adopting the change to 2 Cal per gram of dietary fiber.|
|FAO/WHO 2002 Recommendations||2 kcal (8kJ)||Assumed that 70% of fiber in foods on average is fermentable.|
Click here to see the list of the healthiest high fiber foods.
- Fiber isn’t digested by the body, but feeds gut bacteria that produces short chain fatty acids that are then absorbed and used to produce energy.
- It is estimated that there are 2 calories in fiber that is composed of 70% fermentable fiber.
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