The dietary carbohydrates are a diverse group of substances with a range of chemical, physical, and physiological properties. The primary chemical classification of carbohydrates is by molecular size (degree of polymerization [DP]), the type of linkage (alpha or beta), and composition of individual monomers. This approach divides carbohydrates into 3 main groups, sugars (DP1-2), oligosaccharides (DP3-9), and polysaccharides (DP >= 10), the latter being further divided into starch (alpha-1:4,1,6-D-glucans) and nonstarch polysaccharides (NSP). Dietary fiber (DF) recently has been defined as carbohydrate polymers with 3 and more monomeric units plus lignin, which are not hydrolyzed by the endogenous enzymes in the small intestine of humans. This physiologically based definition is broader than what classically has been considered fiber in animal nutrition and delimitates carbohydrates according to their potential for digestion by endogenous enzymes. Carbohydrates are the principal substrates for energy metabolism but also exert a number of other effects throughout the gastrointestinal tract. The starch structure as well as type and levels of DF influence, to a varying degree, the rate of starch digestion in the small intestine. Some types of soluble NSP are found to interact with intestinal mucus and produce a layer that significantly delays the transport of lipid digestion products. Potentially, the same may be the case for proteinous compounds. The delay in the transport of the nutrients to the gut epithelium can potentially influence gastric emptying and the satiating effect of the feed mediated through gastrointestinal hormones. Dietary fiber compounds that reach the large intestine undigested can, by a targeted approach, be used to modulate the gastrointestinal environment and the production of fermentation products. For instance, resistant starch and some NSP have been found to stimulate butyrate-producing microorganisms and increase the production, luminal concentration, and absorption of short-chain fatty acids and butyrate. Resistant starch in diets further provoked major changes in colonic gene expression, which represents induction of oxidative metabolic pathways and suppression of immune response and cell division pathways.
