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Carbohydrates that cannot be hydrolyzed by the endogenous enzymes in the upper gastrointestinal system are called dietary fibers [27]. Functional fiber is a dietary fiber that exhibits beneficial physiological effects in humans [28]. However, there is still disagreement about the definition of dietary fiber [29]. Although Codex Alimentarius described dietary fibers are as carbohydrates contain ten or more monosaccharide units, there is some flexibility in the definition of fiber: each national authority can decide whether to include carbohydrates with three to nine monomeric units instead of limiting the definition with carbohydrates containing more than ten monomeric units in length. For instance, Canada, Australia, China, Brazil, New Zealand, and Europe describe fiber as nondigestible carbohydrates consist of more than three monomeric units [27].

Dietary fibers include edible carbohydrate polymers naturally occurring in the consumed food and carbohydrate polymers obtained from food raw material by enzymatic, physical, or chemical methods [27]. Traditionally, dietary fiber is categorized depending on its solubility. The soluble fibers are generally present at high amounts in oats, fruits, beans, and vegetables while major food sources for insoluble fibers are whole grains [30]. Non-starch polysaccharides and oligosaccharides, such as resistant oligosaccharides, nonstarch polysaccharides and lignin, substances associated with the nonstarch polysaccharides and lignin complex in plants, and other analogous carbohydrates, such as dextrins and resistant starch are natural sources of dietary fibers. Some of the synthetic carbohydrate compounds, such as polydextrose and methylcellulose, are also categorized as dietary fibers because of their health benefit effects approved by the scientific community [27, 30]. The food ingredients with known health benefits if consumed in adequate amounts are called nutraceuticals and dietary fibers can be categorized as nutraceuticals since results of epidemiological studies and clinical investigations suggest that consumption of adequate levels of dietary fiber can reduce the risk of numerous diseases like diabetes, cardiovascular diseases, hypertension, hyperlipidemia/hypercholesterolemia, obesity, and cancer [25, 30]. These benefits are mainly attributed to the fermentation products of more soluble dietary fibers called short-chain fatty acids (SCFAs) [31]. On the other hand, insoluble dietary fibers, such as cellulose, lignin, and hemicelluloses have been shown to help in the prevention of hemorrhoids, constipation, and diverticulosis through stimulating bowel movement and speeding up the removal of waste [25, 28].

Nutritional and health benefit effects of polysaccharides result from their structural properties. With the mechanical action of chewing and the chemical action of salivary enzymes, such as salivary amylase, the digestion of plant polysaccharides starts in the mouth. After passing through the esophagus, these polysaccharides absorb water, swell and get solubilized either completely or partially in the acidic digestive fluid of the stomach. Types of the monosaccharide units, number of covalent bonds and their positions within the polymer, anomeric forms and substitutions on the sugar molecules of the polysaccharide determine the level of the acid hydrolysis in the stomach as well as in the intestine [32]. The small intestine is the major site for carbohydrate digestion. In the duodenum part of the small intestine, the acidic stomach content is neutralized by the alkaline secretions from the pancreas allowing further digestion to occur. Pancreatic amylase from the pancreas breaks down starch polysaccharides into disaccharides. In brush borders of the small intestine, disaccharides are further hydrolyzed by disaccharidases to glucose, fructose, and galactose followed by the absorption by the small intestinal cells. Finally, the sugar units enter the bloodstream by passive diffusion (fructose) or active transport mechanism (glucose and galactose) [33]. Although a significant proportion of the dietary polysaccharides is digested and absorbed in the small intestine, some of them, including dietary fibers, are resistant to hydrolysis in the stomach and the small intestine of humans. These non-digestible polysaccharides are classified into two categories as fermentable and non-fermentable polysaccharides [32]. Non-fermentable polysaccharides ultimately are excreted in the feces. The fermentable polysaccharides are metabolized by the human large intestinal microbiota to yield diverse products or metabolites that can be used as an energy source for the host. SCFAs, mainly acetate, propionate, and butyrate, are the major microbial metabolites found in the human gut and mainly produced as a result of microbial fermentation of non-digestible carbohydrates [28, 32]. SCFA production resulted from fiber fermentation reduces luminal pH and therefore suppresses the growth of pathogens. Prebiotic fiber is described as a sub-class of fiber that acts to beneficially modify the colonic microbiota. Several factors are known to affect the composition of the microbiota, including the physiological conditions of the host (e.g., health status, age), environmental circumstances (e.g., hygiene with antiseptics, antibiotic therapy, etc.), and the dietary compositions [28].

Between the SCFAs, butyrate is the one that mainly utilized as an energy source for enterocytes and colonocytes even in the presence of competing substrates such as glucose and glutamine [34]. Followed by conversion into glucose through intestinal gluconeogenesis, propionate can be used as an energy source. Besides, propionate can diffuse into the portal vein and it can be used as a substrate for hepatic gluconeogenesis. The most abundant SCFA in the circulation is acetate which is capable of crossing the blood–brain barrier. Although some acetate is converted to butyrate by lumenal bacteria, most of it reaches to peripheral tissues where it can be utilized for lipogenesis in adipose tissue or oxidized by muscle [34, 35]. Butyrate has been reported in particular for maintaining health through the regulation of the immune system by regulating inflammatory cell populations and histone deacetylation, maintenance the integrity of the epithelial barrier by orchestrating the tight junction protein complexes and giving a feeling of satiety after meals. Butyrate has been also suggested to be protective against several diseases including type 2 diabetes, cardiovascular diseases, obesity, colorectal cancer, inflammatory bowel disease, and graft-versus-host disease [35, 36]. Furthermore, it is increasingly evident that in the microbiota–gut–brain axis crosstalk, SCFAs might influence brain physiology and behavior. Animal studies suggest that gut microbiota dysbiosis and SCFAs are involved in behavioral and neurologic pathologies, such as Alzheimer’s and Parkinson’s diseases, depression, and autism spectrum disorder, though human studies are inadequate and offer inconsistent conclusions [37, 38].