Contrary to popular misconception there are four not three macronutrients that provide food calories. Food labelling regulations from the European Food Safety Authority (EFSA) include fibre as a distinct macronutrient group and when calculating energy content of food require the fibre content be allocated 2 kilo calories per gram in addition to the customary 4 kcal/g for carbohydrate and protein and 9 kcal/g for fat.
ESFA list Fibre separately and allocates 2 kcal/g energy content for food labelling. On the other hand, the US FDA allocates zero energy to the fibre in foods. The EFSA arbitrary 2 kcal/g assumes that some fibre is converted to short chain fatty acids by gut microbes which provide energy for their host. The truth is anybody’s guess and more likely be negative net energy because fibre binds many times its weight of fat which increases the amount of energy lost in stool. In any case, short chain fatty acids (SCFA) have multiple physiological effects and benefits outweigh any concern about added calories.
In fact, on a geo-political scale fibre is by far the most important macronutrient on the planet and it may eventually become recognized as the most important for humans as well. Plants are made up mainly of carbohydrates and digestion resistant carbs are generally classified as fibre. These chains of sugar molecules form the structural components of leaves, grasses, trees, etc. and form the major component of biomass on the planet. Fibre is the basis of the food chain for all animals, but they do not have the enzymes to digest it without the help of their symbiotic gut microbes. Virtually all plant eating animals and all of our domestic animals derive their major energy source from short chain fatty acids produced in the gut by bacterial fermentation of fibre. Paleolithic man derived at least 20% of their calories from SCFA’s and earlier hominids likely 50-80% from SCFA as does our closest relatives, gorillas and chimps. The legitimate point here is that fibre fermentation has been a constant environmental influence over millions of years of hominoid evolution and judging by the widespread presence of SCFA receptors throughout human body tissues fibre is important if not an essential substrate for normal homeostasis.