Fiber is defined as the edible parts of plants or analogous carbohydrates that are resistant to digestion and absorption in the small intestine and have complete or partial fermentation in the large intestine. Although there is no dietary requirement for fiber in dogs and cats, there are health benefits of having certain fiber sources in the diet. Fiber is resistant to hydrolysis by mammalian digestive secretions but is not an inert traveler through the GI tract. Increased levels of fiber in diets increase fecal output, normalize transit time, alter colonic microflora and fermentation patterns, alter glucose absorption and insulin kinetics, and, at high levels, can depress diet digestibility.
The diverse nature of fiber has led to numerous classification methods. One way fiber is classified is based on its solubility. Soluble fibers have greater water-holding capacity than insoluble fibers. Fiber sources such as beet pulp, cellulose, and rice bran have low solubilities, while gum arabic, methylcellulose, and inulin have high solubility. Psyllium contains both low-soluble and high-soluble fiber. Although the classification of fiber based on its solubility is still used, fiber is better classified based on its rate of fermentability. Fermentation is defined as the capacity of fiber breakdown by intestinal bacteria, and this definition more accurately assesses the potential benefits of fiber in the GI tract. Fermentation of fiber produces the short-chain fatty acids acetate, propionate, and butyrate. Short-chain fatty acids have numerous benefits, including supplying energy to the large-intestinal epithelial cells, stimulating intestinal sodium and water absorption, and lowering the pH in the large intestines—an environment that favors survival of beneficial bacteria in the GI tract.
Conversely, fermentation also produces less desirable substances such as gases, ammonia, and phenols. Highly fermentable fibers are rapidly metabolized by intestinal bacteria and produce large amounts of gas that can result in cramping and diarrhea. Production of less desirable fermentation products can be minimized by using a moderately fermentable fiber source; examples include beet pulp, inulin, and psyllium. Beet pulp provides good fecal quality in dogs without affecting other nutrient digestibility when included at ≤7.5% (dry-matter basis).
Dietary fermentable fiber also functions as a prebiotic in dogs and cats. Prebiotics are defined as nondigestible food ingredients that selectively stimulate the growth or activity of beneficial bacteria in the intestines, such as Bifidobacterium and Lactobacillus. They also inhibit the survival and colonization of pathogenic bacteria. The beneficial bacteria produce short-chain fatty acids and some nutrients (eg, some B vitamins and vitamin K). Beneficial bacteria also function as immunomodulators and reduce liver toxins (eg, blood amine and ammonia).
Dietary fructooligosaccharides (FOS) and mannanoligosaccharides (MOS) also promote the survival and growth of beneficial bacteria in the GI tract. FOS are nondigestible oligosaccharides consisting of chains of fructose molecules. Dietary sources of FOS include beet pulp, psyllium, and chicory. Beneficial bacteria are able to use FOS as a metabolic fuel, whereas pathogenic bacteria cannot. FOS also enhance the effectiveness of the GI immune system. MOS are similar to FOS, except the predominant sugar molecule in MOS is mannose instead of fructose. Dietary sources of MOS include natural fibers found in yeast cells. MOS use a different mechanism than FOS to inhibit the growth of harmful bacteria. Pathogenic bacteria attach to the intestinal wall using finger-like projections called fimbriae. Fimbriae bind to specific mannose residues on intestinal cells. Fimbriated mannose-specific pathogens can also bind to MOS instead of adhering to the intestinal epithelium, and harmful bacteria are then excreted in the feces.
Several chemical methods are used to determine the fiber level of a food; all extract the components of fiber to different degrees, which results in different estimates of fiber level for the same feedstuff. Crude fiber, which is what is listed on pet food labels, quantifies insoluble dietary fiber, which is primarily cellulose, some lignin, and a small amount of hemicellulose. However, it does not measure a large portion of insoluble dietary fiber, nor any of the soluble dietary fiber. Therefore, crude fiber is not an accurate measure of total dietary fiber. The physiologic effects of fiber are not uniform across all fiber types, and relying solely on fiber content listed on pet food labels does not accurately reflect either fiber content and fiber types in commercial pet foods, or the physiologic effects from a diet.
Source: https://www.merckvetmanual.com/management-and-nutrition/nutrition-small-animals/nutritional-requirements-and-related-diseases-of-small-animals
