The mystery of fiber in laying hens diets – part 2

In our previous article, we discussed the definition of fiber, its sources, types, and strategies to overcome technical problems associated with increased dietary fiber. In this article, the digestion kinetics of various fiber types will be discussed to touch on the effects of fiber on nutrient utilization, gut microflora, and immunity. 

Fiber digestion kinetics

Kinetics in physics refers to the study of forces acting on mechanisms, and in chemistry, it refers to the rates of chemical or biochemical reactions. Digestion kinetics explains the complex interactions between feed, digestive enzymes, microbial population, and digesta passage rate occurring within the lumen of the digestive tract. The kinetics of digestion affects the metabolic fate of nutrients after absorption. From the structural standpoint, fiber can be categorized into three main sections. Non-starch polysaccharides (NSP), lignocellulose, and other indigestible plant-based carbohydrates such as resistant starch. The NSP part can be further divided into two main parts: cellulose and non-cellulotic polysaccharides (NCP). The NCP encompasses the pectic polysaccharides (including Arabinoxylans in wheat and beta-glucans in barley) and hemicellulose.

Keeping the different parts of dietary fiber in mind, let’s dive into the fiber digestion kinetics in poultry. The function of fiber and its digestion kinetics in the gut should be investigated from the following perspectives.

  1. Solubility 
  2. Viscosity and gelation
  3. Water-holding capacity
  4. Fermentation
  5. Fiber particle size
  6. Passage rate of solid and liquid components of digesta

From the solubility perspective, dietary fiber can be categorized into soluble and insoluble fractions. Viscosity is defined as the property of liquid to resist flow due to internal friction. A historical view has always blamed the soluble NSP for its adverse effects on increasing digesta viscosity and, subsequently, decreasing nutrient absorption. However, recent data show that those negative effects can be managed and turned into opportunities using proper nutritional management. For instance, feed processing or enzyme supplementation can break down NSP into small fermentable pieces, which can be used by the gut microbiome. We will discuss processing and enzyme supplementation in our next article (part 3). Although inulin and wheat dextrin are categorized as soluble NSP, they do not adversely affect digestion; these fiber components can be used as fuel by the beneficial bacteria in the poultry gut (prebiotic effect). The most common fiber products with prebiotic effects are inulin, raffinose, resistant starch, and small fragments of carbohydrates such as oligosaccharides of fructose, xylose, mannose, and galactose. The ability of these oligosaccharides to improve the microbiota is dose-dependent, and oligosaccharides with lower polymerization degrees (short chain oligosaccharides) can get more thoroughly broken down through the fermentation process. These products stimulate commensal and beneficial microbes that are natural habitats in the gut; this ensures supporting the natural beneficial gut microbiota, whereas some supplemented beneficial microbes (commercial probiotics) that cannot establish colonies in the gut for a long time need to be supplemented continuously through the diet. Using natural fiber compounds that have prebiotic effects can increase the utilization of soluble NSP, supporting the beneficial microbes and managing digesta viscosity in birds fed with fibrous diets.

Fermentability of dietary fiber is an important factor that should be considered in formulating high-fiber diets. In addition to the prebiotic role and stimulating the immune system, some fermentable fibers can bind to pathogens and facilitate their exclusion from the gut. For example, mannan-oligosaccharides (MOS) and galacto-oligosaccharides (GOS) can prevent the adhesion of Salmonella and enteropathogenic Escherichia Coli, respectively, in the intestinal cells, cleaning the gut from these pathogens.

Fiber particle size plays an important role in the regulation of intestinal motility which affects nutrient utilization. Feeding insoluble fiber with coarse particles, in some cases, can help improve growth performance by decreasing the digesta passage rate, increasing the retention time in the upper digestive tract (gizzard), and harmonizing intestinal functionality. The recommended standard values for particle size are available in the “Feed sieve shaker” article in the current issue of our newsletter.

Effects of dietary fiber on gut microbiome and nutrient utilization

The poultry gut microbiome consists of 900 species of bacteria, protozoa, fungi, yeast, and viruses located from the crop all the way to the colon, with the vast majority residing in the cecum and colon. The microhabitats of these microbes are the gut lumen, mucus, and mucosal linings.

Dietary fiber or its degraded segments are fermented by gut microbes. The beneficial gut microbes play an important role in gut health and animals’ immune systems by producing short chain fatty acids (SCFA); mainly acetate, butyrate, and propionate. Butyrate increases intestinal villi height to crypt depth ratio. This increases absorptive capacity and mucus secretion, which supports creating a barrier against pathogens. It has been shown that fermentation of xylans in wheat-based diets can produce butyrate and increase the mRNA expression of the tight junction proteins gene in the ileum, preventing the leaky gut issue in chickens. The SCFAs are the extracted energy from undigested nutrients (fiber) and improve the intestinal health of poultry by the following means:

  1. Increase gut motility (movement through the gut)
  2. Increase minerals absorption (for example sodium and chloride) 
  3. Increase cecal crypt proliferation
  4. Support the growth of villi
  5. Suppress the invasion of intestinal epithelial cells by pathogens
  6. Reduce colonization of pathogens such as Salmonella
  7. Decrease ammonia absorption through ionization due to intestinal pH drop

Notice that the lower gut’s normal microbiome is the host’s (bird) best friend that performs the following actions:

  1. Utilizes the residual feed and converts it to microbial protein and useful fermentation products such as SCFA  
  2. Extracting energy from undigested feed for the host through fermentation, providing up to 5 to 15% of the daily metabolizable energy requirements for maintenance (MEm)
  3. Preventing colonization of pathogenic and putrefactive bacteria (decay causing)
  4. Promotion of gut maturation and integrity
  5. Regulation of the immune responses

Gut microbiome composition can be affected by the inoculum passed from breeders (parents) to chicks, surrounding environmental conditions during the hatch, age, diet type, and intestinal environment. It is vital to support our birds’ best friends (their microbiome) through proper nutrition. In this context, gut microbes need two main sources (energy and nitrogen) to grow and survive. Fermentable soluble NSP is used as an energy source (fuel), and undigested protein or amino acids as a nitrogen source for the microbiome. Lack of fermentable NSP shifts the microbial fermentation from saccharolytic (carbohydrate degradation) to proteolytic (protein degradation), producing odorous sulfur compounds, ammonia, and harmful metabolites such as amines, phenols, indols, biogenic amines, hydrogen sulfide, and nitric oxide in the lower gut. More specifically, escaped protein and amino acids from enzymatic digestion will reach the lower gut, be fermented by the gut microflora, and turned into harmful fermentation products. These fermentation products are excreted and can affect the litter quality negatively and cause foot problems in layer pullets. Providing sufficient fermentable carbohydrates for the gut microbial population through dietary fiber is necessary to optimize gut microbial fermentation.

Dietary fiber plays an important role in colonizing gut microbiota by providing them with the substrate (foundation to live on). Lack of fiber decreases microbial diversity and thickness of the mucus layer. If bacteria in your birds’ gut are “hungry” they will use glycoproteins of the mucus layer as an energy source. This will cause leaky gut disorder, malabsorption of nutrients, wet litter, feather pecking, and cannibalism.  

Effects of dietary fiber on the immune system

Research has shown that including Beta-glucan NSP from dried distillers grains (DDGS) and yeast can increase immunoglobulin levels (IgA and IgG). IgA is found in the lining of the respiratory tract and digestive system. IgG is the most common antibody in blood and other body fluids and protects against bacterial and viral infections. In addition, as was previously discussed, some NSPs can be used as fuel for beneficial bacteria (prebiotic effect); these bacteria and their components activate immune cells of GALT (Gut-Associated Lymphoid Tissue). GALT is a component of the mucosa-associated lymphoid tissue, which works in the immune system to protect the body from invasion in the gut.

In conclusion, dietary fiber type, inclusion level, and particle size are worth considering in feed formulation as these factors affect the functionality of the fibrous feed components. Stay tuned for our next article, where the effects of dietary fiber on laying hens’ performance, stress, and feather pecking will be discussed.


Hong T. N., M. R. Bedford, S. Wu, and N. K. Morgan. 2022. Dietary soluble non-starch polysaccharide level influences performance, nutrient utilisation and disappearance of non-starch polysaccharides in broiler chickens. Animals. 12, 547. (Link to the article)

Lázaro, R., M. Garcia, M. J. Aranibar, and G. G. Mateos. 2003. Effect of enzyme addition to wheat-, barley-and rye-based diets on nutrient digestibility and performance of laying hens. British Poultry Science 44 (2): 256–265. (Link to the article)

Rajesh J. and P. Mishra. 2021. Dietary fiber in poultry nutrition and their effects on nutrient utilization, performance, gut health, and on the environment: a review. Journal of Animal Science and Biotechnology. 12:51. (Link to the article)

Tejeda, O. J. and W. K. Kim. 2021. Effects of fiber type, particle size, and inclusion level on the growth performance, digestive organ growth, intestinal morphology, intestinal viscosity, and gene expression of broilers. Poultry Science. 100:101397. (Link to the article)

About the author(s)

Research Associate at Poultry Innovation Partnership | + posts