Fiber

Fiber is defined as the sum of plant cellular components that cannot be degraded by digestive enzymes in monogastrics (^{Hetland et al., 2004}). According to the structure, their configuration and size of the carbohydrates that compose them; fibers can be classified into oligosaccharides and polysaccharides (^{Makki el at., 2018}). Oligosaccharides are made up of chains of 3 to 10 monosaccharides, on the other hand, polysaccharides such as cellulose are made up of multiple units. Hemicellulose, inulin, pectin, gums, mucilages and beta-glucans have similar structures, surrounding the cellulose and hemicellulose microfibrils is lignin which is a structural polymer (^{Krás et al., 2013}; ^{Segura et al., 2007}). Fiber is classified according to its degree of water solubility as soluble fiber and non- soluble fiber, soluble fibers retain water producing viscous solutions, the higher the solubility of the fiber the more fermentable it is. Lignin is the intracellular part of fibrous cells that encloses soluble carbohydrates and is used to determine the amount of fiber in a food (^{Segura et al., 2007}). In the laboratory, neutral detergent fiber can be measured which is the partially digestible portion of the fiber (NDF): lignin, cellulose, and hemicellulose, as well as acid detergent fiber (ADF) and crude fiber (CF): lignin and cellulose which indicate the non-digestible fraction of the feed (^{Segura et al., 2007}). Fibers can also be separated into components to recognize their individual properties.

Fibers found in poultry feed ingredients differ in their composition, abundance and in the nutritional value, they contribute to the diet, they are not fully known because fiber has been considered as a negative ingredient for poultry (^{Makki et al., 2018}). However, the characteristics of fibers depend mainly on their plant origin (^{Krás et al., 2013}). In monogastrics, fiber modifies the passage of feed in the intestine, the digestion of nutrients and the gut microbiota in general (^{Zaefarian et al., 2015}).

Fiber digestion

During the digestive process birds secrete amylase enzymes to degrade simple carbohydrates, but they cannot hydrolyse β1-4 bonds of polysaccharide fiber, therefore, the fiber will reach the gut intact (^{Krás et al., 2013}; ^{Zaefarian et al., 2015}; ^{Mohanty et al., 2018}; ^{Raza et al., 2019}). In this body region, anaerobic bacteria attach themselves around the fiber, feed on it and multiply (^{Mahmood & Guo, 2020}). The reproducing bacterial colonies secrete enzymes β-glucuronidase, β-glucosidase and β-mannanase capable of breaking glycosidic bonds to ferment fibre producing organic acids, amino acids, purines and pyrimidines (^{Das et al., 2012}; ^{Makki et al., 2018}). Cecal digestion in poultry is a process where bacterial fermentation of fibre occurs. Cecal development in poultry is directly related to feeding; in some cases, they can have a histological development similar to the intestine ^{Adebowale et al., 2019}).

During fiber fermentation different metabolites are produced which vary depending on the type of bacteria (^{Mohanty et al., 2018}). In the case of Streptococcus and Lactobacillus lactic acid is obtained, Enterobacter produces acetic and formic acid, Clostridium and Corinebacterium propionic, acetic and succinic acid. While Escherichia coli, Salmonella, Shigella and Proteus generate gases such as H2 and CO2 that alter digestive physiology and host welfare (^{Makki et al., 2018}). In poultry, the efficiency of bacteria to degrade fiber depends on its solubility, its size, the polymerization carbohydrate degree, the availability of the substrate in the medium and the ability of microorganisms to bind to it (^{Cardoso et al., 2020}; ^{Mahmood & Guo, 2020}).

Gut health

The gut is the site where digestion takes place and it is responsible for absorbing nutrients through a capillary network that transports them to the portal circulation (^{Kogut, 2018}). Along this organ there is gut-associated lymphoid tissue (GALT) formed by Peyer's patches and cecal tonsils where T-lymphocytes mature. In the lamina propria there are macrophages, granulocytes and lymphocytes fulfilling their function of local immunity (^{Kogut et al., 2018}).

The intestine has villi formed by epithelial folds that project into the lumen increasing the absorptive surface of enterocytes. The villous surface is formed by mucin-secreting Goblet cells; between each villus are formed Lieberkuhn's crypts, which have Paneth cells, secreting antibacterial substances, (^{Kogut et al., 2018}). Epithelial cells in addition to fulfilling their structural function secrete cytokines that regulate the chemotaxis of the GALT system to present selective permeability that limits the absorption of toxic substances with high regenerative capacity (^{Chassaing et al., 2014}; ^{Kogut et al., 2018}).

The epithelial cell membrane has TLR (Toll-like receptor), NLR (nucleotide oligomerisation domain-like receptor) and PRR (pattern recognition receptors) receptors that respond to stimuli from LPS (lipopolysaccharide) or bacterial endotoxins and certain dietary components (^{Kogut et al., 2018}). When receptors recognize stimuli, they initiate the secretion of cytokines TNF-α, IL-6, IL-1β activating the GALT system (^{Chassaing et al., 2014}; ^{Mahmood & Guo, 2020}). However, intestinal macrophages react to TLR receptor stimulation by secreting IL-10 as an anti-inflammatory that normally regulates the inflammatory response (^{Chassing et al., 2014}; ^{Kogut et al., 2017}).

Rapid growth of birds, excess nutrients in diets, injuries in the digestive tract and constant exposure to LPS can cause chronic inflammation in their gut (^{Kogut, 2017}). Therefore, the function of the digestive tract decreases, it remains in a state of oxidative stress and immune incompetence occurs (^{Cardoso et al., 2020}). The balance between diet, mucosa and gut determines gut health (^{Jha et al., 2019}); its neglect increases the bird's susceptibility to inflammation or infection by pathogenic bacteria (^{Mahmood & Guo, 2020}). In addition, each villus is lined with microorganisms that help maintain intestinal homeostasis; these populations vary according to the age of the host, the diet administered, the use of antimicrobials and even the digestive enzymes themselves (^{Clavijo & Vives, 2018}; ^{Szychlinska et al., 2019}). The expression of proinflammatory cytokines can antagonize growth hormone, increase the amount of glucocorticoids in the blood and alter osteogenesis in birds (^{Tong et al., 2020}).

The intestinal microbiota

Bacteria of the genera Firmicutes (70%), Bacteroides (12.3%), dominate a microbial community Proteobacteria (9.3%) and others (8.4%) that line the gastrointestinal tract (^{Kogut, 2018}). Particularly bacteria of the species Lactobacillus, Enterococcus and Clostridium predominate, but the cecum has the largest population (^{Chen et al., 2020}). However, species pathogenic to birds and bacteria of zoonotic public health concern such as Campylobacter jejuni, Salmonella enterica, Escherichia coli and Clostridium perfringens also inhabit the cecum (^{Clavijo & Vives, 2018}).

The resident microbiota excludes pathogenic bacteria that compete for nutrients and act as a defensive barrier preventing the adherence of other bacterial species. In addition, antimicrobial peptides are produced and stimulate their production by the host (^{Adebowale et al., 2019}; ^{Mahmood & Guo, 2020}; ^{Van der Wielen et al., 2000}). The gut microbiota functions as a metabolic organ that can be compared to the liver; it mainly hydrolyses polysaccharides and produces short-chain fatty acids such as acetate, butyrate, succinate and propionate (^{Chassaing et al., 2014}; ^{Adebowale et al., 2019}; ^{Van der Wielen et al., 2000}).

In the intestine, short-chain fatty acids freely enter enterocytes producing energy that promotes epithelial and intestinal mucosal development (^{Kogut, 2018}; ^{Adebowale et al., 2019}). Butyrate and acetate stimulate goblet cells to regulate mucin secretion (^{Makki et al., 2018}). Resident bacteria are not recognized as foreign agents because they cover TLR receptors by preventing their activation, thus moderating the inflammatory response. In case of receptor activation, the CD4+ T-cell response is stimulated (^{Chassaing et al., 2014}; ^{Clavijo & Vives, 2018}). Furthermore, chain fatty acids generated by bacterial synthesis have been reported to participate in the gut-brain pathway because they serve as energy for astrocytes, thereby regulating appetite and can be transformed to glucose in the liver, decreasing cholesterol synthesis (^{Hu et al., 2018}).

Fermentation of fibers releases phenolic compounds; secondary metabolites such as terpenes, phenols and flavonoids, phytochemicals that have antioxidant or anti- inflammatory effects with local or systemic benefits to the host (^{Makki et al., 2018}; ^{Gasaly et al., 2020}). On the other hand, intestinal bacteria synthesise vitamins such as biotin or vitamin K, which are useful for the organism, and even bacteria themselves can be a source of amino acids (^{Chassaing et al., 2014}; ^{Clavijo & Vives, 2018}). In dysbiosis, pathogenic bacteria proliferate causing enteric infections, thus decreasing growth rates and increasing mortality (^{Kogut, 2018}), while their balance can reduce glucocorticoid secretion in birds under stress and involve bacterial metabolites in fatty acid biosynthesis (^{Kridtayopas et al., 2019}; ^{Chen et al., 2020}).

The microbiota can be modified by diet, sex, environmental conditions, age, even animal bedding. Therefore, its regulation is promoted at specific times such as at hatching, during feed changes and in the course of enteric diseases (^{Clavijo & Vives, 2018}; ^{Kogut et al., 2018}). The aim is to improve production parameters by modulating the inflammatory response, avoid pathogen colonization and prevent disease in both animals and consumers (^{Kogut, 2018}). To this end, the use of essential oils, bacteriophages, bacteriocins, enzymes, functional feeds, probiotics, prebiotics and synbiotics as well as physical feed modification have been suggested (Clavijo & Vives, 2018; Kogut, 2018; ^{Kheravii et al., 2018}; ^{Chen et al, 2020}).

Use in monogastrics

The use of dietary fiber in monogastrics has been investigated, showing that its consumption can regulate intestinal motility, modulate the intestinal microbiota, prevent lipid accumulation in the liver, lower blood glycemic index, regulate liver function, prevent colon cancer and improve intestinal mineral absorption capacity (^{Makki et al., 2018}). Due to the evidence obtained and its biological effect, fiber is recognized as a functional food (^{Das et al., 2012}).

In livestock production, fiber is being investigated in an attempt to replace synthetic ingredients. In addition, fiber can be an alternative to reduce pollutant emissions from the poultry industry into the environment (^{Sittiya et al., 2020}). Due to the advantages of fiber when used in the development of the digestive system, it has been sought to understand its energy potential and effect on the digestive process in pigs and poultry (^{Hetland et al., 2005}). Fiber combination with another additive has improved weight gain, feed intake, feed conversion, egg production, egg weight and egg size in laying hens. In addition, it has health effects by lowering blood cholesterol levels (^{Tang et al., 2017}). In rats, soluble and insoluble prickly pear cactus (Nopal) fiber allows greater bioavailability of calcium in the diet, improving bone density (^{Mendoza-Ávila et al., 2020}).

Prebiotics

These are additives made up of fibers that are resistant to the enzymatic action of monogastrics but degradable by intestinal microorganisms (^{Mohanty et al., 2018}). As substrates for bacterial fermentation, they act as competitive exclusion products promoting the proliferation of beneficial bacteria in the gut by inhibiting the growth and adhesion of harmful bacteria (^{Clavijo & Vives, 2018}). These compounds can also increase intestinal osmosis by enhancing nutrient absorption (^{Kridtayopas et al., 2019}).

The use of prebiotics in production units is limited compared to other options used to regulate the intestinal microbiota in poultry (^{Clavijo y Vives, 2018}). Therefore, it can be used as an alternative to decrease the use of antibiotics, reduce residues in products for consumption and in the environment (^{González & Ángeles, 2017}). However, it should be mentioned that the morphological and physiological changes in the digestive tract of chicken depend on the type and components of fiber added as a prebiotic, as well as its solubility.

It has been determined that the prebiotics with the highest digestibility are isomalt oligosaccharides (IMO), galacto oligosaccharides (GOS) and fructooligosaccharides (FOS) to stimulate Lactobacillus reproduction (^{Kridtayopas et al., 2019}; ^{Karimian & Rezaeipour, 2020}). Its administration decreases the negative effects caused by Escherichia coli by increasing the population of Enterococcus and Lactobacillus (^{Tarabees et al., 2019}; ^{Karimian & Rezaeipour, 2020}). It also decreases the presence of Campylobacter in the cecum, reducing the risk of contaminating food, as well as the prevalence of poultry foodborne diseases (^{Froebel et al., 2019}).

Studies on the addition of fiber in poultry are limited, but there is research on its immunostimulatory effect by increasing vaccine titres against Newcastle virus disease and the amount of IgY in serum (^{Mohammed et al., 2016}). It has been reported that administration of prebiotics in ovo can favour gene expression to resist heat stress conditions (^{Slawinska et al., 2019}).

Symbiotics

Symbiotics are characterised by the combination of two or more functional additives that allow them to enhance their performance for either or both ingredients. As an example, the combination of prebiotic agents is used to aid growth (^{Mohanty et al., 2018}). In addition, they are natural beneficial residents of the gut microbiota with various health benefits. Likewise, probiotics must have the ability to colonise, adhere and reproduce on epithelial cells in the host, survive gastric acidity and bile secretions (^{Clavijo y Vives, 2018}).

The combination of these two agents has potentiated the beneficial effect of each (^{Awad et al., 2009}; ^{Mohanty et al., 2018}). There is evidence that the use of synbiotics reduces the amount of Escherichia coli, Clostridium perfringens, Campylobacter jejuni and Salmonella spp (^{Kridtayopas et al., 2019}). In addition, it can increase post-vaccination Newcastle titres, improves bone density, and promotes growth of intestinal villi in poultry (Kridtayopas et al., 2019). Under stressful conditions, symbiotics can improve the productive variables of animals (Kridtayopas et al., 2019).

Digestive system development

In laying hens, insoluble fiber accumulates in the gizzard increasing feed retention, moderating feed flow and improving muscle development, similar effect of fiber in broilers at 21 days of feeding 2.5 and 3% insoluble fiber allows small intestine development and reduces gizzard pH (^{Hetland et al., 2004}; ^{Amerah et al., 2009}). Muscle development of the gizzard is related to fiber particle size, which can facilitate gastroduodenal reflux and improve the contact of digestive enzymes with the feed (^{Jiménez-Moreno et al., 2019}).

It has been reported that in diets with high amounts of soluble fibre Clostridium perfringens proliferates causing necrotic enteritis and in turn decreases oxygen tension in the gut favouring the development of toxin-producing anaerobic bacteria (^{Clavijo & Vives, 2018}; ^{Raza et al., 2019}). In a study by Van der Wielen et al. (2000) indicate that enterobacteria are susceptible to the amount of acetate, propionate and butyrate in their environment, while lactobacilli are not affected.

Intestinal viscosity

^{Cardoso et al. (2020)} report that high amounts of fiber increase intestinal viscosity decreasing the diffusion of digestive enzymes and the digestibility of nutrients contained in the feed (^{Krás et al., 2013}; ^{Raza et al., 2019}). In addition, increasing intestinal transit decreases feed intake and weight gain in poultry (^{Cardoso et al., 2020}). However, there are contrary observations where the administration of fiber decreases the transit speed in the digestive tract, producing an increase in feed intake due to energy dilution resulting from viscosity and when the amount of fiber is high the effect is the opposite due to the volume administered (^{Krás et al., 2013}).

Digestibility of nutrients

^{Raza et al. (2019}) reported anti-nutritional effect of soluble fiber due to thickening of the gut mucosa, which decreases digestion. The type of bird, its age and fiber type can influence this variable (^{Eggum, 1995}). ^{Donadelli et al. (2019}) reported different results where there was no effect on the productive parameters of birds, but nutrient digestibility increased. There are reports that indicate that adding 3% insoluble fiber increases the availability of metabolizable energy in broiler diets because it improves starch digestion and in laying hens it improves mineral digestibility (^{Amerah et al., 2009}; ^{Donadelli et al., 2019}; ^{Jiménez-Moreno et al., 2019}).

Productive yield

The administration of soluble and insoluble fiber has been reported to decrease feed intake and weight gain in broilers, an effect associated with insoluble fiber (^{Krás et al., 2013}; ^{Leung et al., 2018}; ^{Raza et al., 2019}). On the other hand, it has been described that administering fiber improves carcass yield (^{Tarabees et al., 2019}; ^{Adewole et al., 2020}), increases weight gain at 21 days. Combining fiber with mannan oligosaccharides (MOS) and enzymes increases weight gain (^{Karimian & Rezaeipour, 2020}); and even in birds challenged with Escherichia coli O78:H11 improves weight gain and decreases mortality (^{Tarabees et al., 2019}). ^{Hetland et al. (2005}) report that laying hens seek fiber when feather pecking or ingesting litter to compensate for the lack of this ingredient in diets.