Dietary fiber, or dietary fiber, is an important component of the raw materials used in the manufacture of feed for the monogastric market. It is mainly present in by-products, grains, and cereals that are sources of carbohydrates in the feed. Some of these carbohydrates are not digested by the digestive enzymes of the small intestine, remaining available for bacterial fermentation along the intestine, mainly in the large intestine.
As an agent with an important role, but one that is poorly understood by the scientific community, or even little known by nutritionists, recent research has investigated its role and function associated with nutritional digestibility and intestinal health in relation to the microbiota, starting from an understanding of the non-starch polysaccharides (NSPs) that make up the different portions of ingredients, to overcome barriers in understanding the quantitative requirements of different fiber fractions that meet diverse needs.
The polysaccharides that make up the cell wall of plants vary greatly in their chemical composition, intermolecular associations, and linear chain lengths, thus differing in their effects on metabolism and microbiota composition (McRorie et al., 2017; Molist et al., 2014). Due to its heterogeneity, dietary fiber can be classified according to different physicochemical properties, such as water solubility, viscosity, gel formation, water-binding capacity, and fermentability (Blackwood et al., 2000).
Before delving into what the scientific community has been investigating regarding this incredible dynamic of fiber, we need to understand quantification in raw materials. The most accurate estimation method currently available is obtained through the analysis of total dietary fiber, which is the sum of non-starch polysaccharides and lignin. However, the method for obtaining its individual monomers through enzymatic and chemical hydrolysis, with subsequent quantification by gas or liquid chromatography, is time-consuming and laborious. Therefore, the migration from the old concept of crude fiber (CF) or neutral detergent fiber (NDF), which are two methods for quantifying different fiber portions of ingredients widely used in feed formulations, to the new concepts of non-starch polysaccharides (NSPs) is also a costly and time-consuming process. Furthermore, the roles played by fibers at the metabolic level, in intestinal cells, and in microbiota interactions have not yet been fully understood and unraveled.
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Despite the diversity of quantification methods generating results that are difficult to compare in the literature, some aspects at different levels have been investigated regarding the impact of fiber on the nutrition and intestinal health of monogastric animals. According to Spiller (2011), soluble fibers have a high degree of hydration, with the ability to form gels and viscous solutions, as they have a faster fermentation rate compared to insoluble fiber. Kim et al (2012) and McRorie (2015) added that increased digesta viscosity increases retention time in the gastrointestinal tract, delaying the interactions of digestive enzymes and nutrients, which delays the degradation of complex nutrients, the absorption of glucose and other nutrients in the intestinal microvilli (reduced digestibility), increasing the incidence of diarrhea due to bacterial dysbiosis.
However, some portions of soluble NSPs that do not alter or increase the viscosity of the digesta can be used by bacteria in the large intestine, contributing to the production of Short-Chain Fatty Acids (SCFAs) which, when absorbed, are responsible for providing energy to animals. This results in a chain reaction of reducing the net energy inclusion in the diet, as well as decreasing the production of branched-chain fatty acids, which is an indicator of protein fermentation (Kim et al., 2012). In sows, for example, the inclusion of fiber in the diet provides approximately 301 TP3T of the net maintenance energy (Serena et al., 2009).
Furthermore, these SCFAs, mainly butyric and propionic acids, regulate the production of secretory IgA in the lamina propria and the production of IgG in systemic tissues, stimulating the strengthening of the mucosal barrier, as well as the systemic immunity of monogastric animals (Kim et al, 2018).
Insoluble NSPs (cellulose, lignin, and some hemicelluloses), when present in large quantities, are associated with an increased rate of digesta passage through the GI tract due to their ability to retain water molecules. This softens the fecal bolus and increases its volume, facilitating and reducing intestinal transit time (Spiller, 2001). However, they can also reduce nutritional digestibility and increase endogenous amino acid loss. Nevertheless, when included in small quantities (2-4%), the reduction in prolonged digesta retention in the GI tract shows a reduced risk of excessive bacterial growth in the small intestine, in addition to stimulating feed intake by animals, since the rate of digesta passage increases, improving their performance (Molist et al. 2010).
In a more applied and well-known way, other studies have demonstrated that dietary fiber plays an important role in the productive and reproductive (farrowing) performance of sows. Intense genetic selection to increase sow prolificacy has resulted in longer farrowing periods, which in turn has detrimental effects on the sow's productive and reproductive parameters (Björkman et al., 2018), and negatively impacts piglet vitality, survival, and performance (Langendijk; Plush, 2019). Therefore, diet and nutritional management during the peripartum period are among the factors related to farrowing disorders, where fiber inclusion plays a fundamental role in systemic health and balance.
In summary, Fischer (2003) concluded that the animal's response to different types and amounts of dietary fiber is variable and depends on its physiological status, the microbial community, environmental conditions, and health status. Similarly, changes in the microbiota in a herd due to environmental stress or an outbreak of clinical or subclinical disease will alter the animal's response to different fractions of non-fatty proteins in the diet.
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