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Some animals, particularly ruminants and termites, can digest cellulose with the help of symbiotic micro-organisms that live in their guts, such as Trichonympha. In human nutrition, cellulose is a non-digestible constituent of insoluble dietary fiber, acting as a hydrophilic bulking agent for feces and potentially aiding in defecation.
However, their ability to extract energy from cellulose digestion is less efficient than in ruminants. [2] Herbivores digest cellulose by microbial fermentation. Monogastric herbivores which can digest cellulose nearly as well as ruminants are called hindgut fermenters, while ruminants are called foregut fermenters. [3]
While foregut fermentation is generally considered more efficient, and monogastric animals cannot digest cellulose as efficiently as ruminants, [5] hindgut fermentation allows animals to consume small amounts of low-quality forage all day long and thus survive in conditions where ruminants might not be able to obtain nutrition adequate for their needs.
Trichonympha and other endosymbionts in the hindgut of these organisms help with the digestion of wood related particles. These flagellate protists, including Trichonympha, convert cellulose into sugar using glycoside hydrolases. [5] The sugar is then converted into acetate, hydrogen and carbon dioxide via oxidation.
Cellulose is a polymer made with repeated glucose units bonded together by beta-linkages. Humans and many animals lack an enzyme to break the beta-linkages, so they do not digest cellulose. Certain animals, such as termites can digest cellulose, because bacteria possessing the enzyme are present in their gut. Cellulose is insoluble in water.
Cellulose also counts as dietary fiber, though Messer says the amount added to shredded cheese “is so negligible it doesn’t contribute significantly to your daily fiber intake.”
A significant quantity of the cellulose humans consume is not metabolized by gut microbes and therefore cannot be considered a MAC. [2] The amount of dietary MACs found within a food source will differ for each individual, since which carbohydrates are metabolized depends upon the composition of each person's microbiota.
The gut microbiota are very important for the host health because they play role in degradation of non-digestible polysaccharides (fermentation of resistant starch, oligosaccharides, inulin) strengthening gut integrity or shaping the intestinal epithelium, harvesting energy, protecting against pathogens, and regulating host immunity.