Dietary fiber and its effects on production are a very important matter within poultry nutrition. Carbohydrates are the major component of poultry diets. However, they are one of the least studied food components and this is especially true for dietary fiber (DF):
Different reports consider DF as a functional component for the normal functioning of digestive organs, or as an antinutrient.
This is most likely due to the complexity and variability of DF’s physical and chemical structure, which complicates the understanding of this part of the diet from a chemical and physiological standpoint.
From a nutritional perspective, DF has been defined as a group of heterogeneous compounds, including all saccharides (excluding starch, i.e. oligosaccharides, polysaccharides, pectins, gums, waxes) and lignin. Which in turn possess the characteristic of being resistant to enzymatic hydrolysis.
- A simpler definition of fiber describes DF as the sum of soluble and insoluble non-amylaceous polysaccharides (NSPs) and lignin (Choct, 2015).
Both soluble and insoluble fibers have practical implications within monogastric animal feed industries. This is especially true with the current increase in alternative feedstocks use, which are typically rich in fibers. Therefore, understanding the functional role of different DFs in poultry nutrition is a critical step in achieving production efficiency.
Effects of dietary fiber on poultry diets
Fiber’s unique ability to escape digestion and absorption provides the opportunity to:
- Regulate intestinal morphology
- Interact with digest nutrients
- Modulate intestinal microflora
- Interfere with the overall activity of digestive organs
resulting in changes in nutrient utilization and growth yield.
All of these changes present a general modulation of nutrient metabolism that could have an impact on performance. Such parameters will be discussed below.
Intestinal morphology
Birds require a certain amount of DF for normal intestinal physiological functions to be carried out (Gonzalez-Alvarado et al. , 2007).
In all poultry species, a rapid intestinal response to DF changes has been reported, resulting in the modification of:
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It has been noted that there are endogenous amino acid losses in broilers fed high fiber levels(Kluth and Rodehutscord, 2009).
- In such a case, appropriate modifications of the nutrient matrix should be made to compensate for such changes and ensure maximum growth.
Digestive organ changes
Fiber inclusion stimulates the gizzard’s muscular activity. This results in a size increase due to a larger volume caused by a slower food particle passage rate
The presence of insoluble dietary fiber (such as cellulose, lignin, and arabinoxylans), can also modulate the size of the small intestine, pancreas and cecum, which can improve apparent nutrient retention and feed efficiency for the total digestive tract (Kheravii et al. , 2017).
Nutrient digestibility
Oligosaccharides have been considered to impair performance when used in large quantities due to slowing down and diluting nutrient intake (Heywang, 1950). As a consequence, commercial diets are usually formulated to contain a maximum of 2 to 3% Crude Fiber.
Soluble fibers that generate viscosity, such as β-glucans, pectins and arabinoxylans, have the ability to interact with water molecules. This interaction helps slows down the rate of passage in the small intestine, reduces enzymes diffusion and the subsequent breakdown of the substrate. This increases the amount free nutrients within the intestinal lumen, favoring the establishment of pathogenic bacteria (Silva et al. , 2019).
Two of the main factors that affect nutrient digestion efficiency in the presence of soluble fiber are solubility and fermentability.
- Both factors are determined by the type of bonds and the number of branches between sugar units, which allow or prevent interactions with water molecules and/or possible bacterial breakdown.
It is accepted that long β 1-4 chains like those of cellulose are poorly soluble, while β 1-3 branches are highly soluble as is the case with β-glucans.
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Finally, it is critical to note that fiber should be used as a functional nutrient and not as a nutrient per se. Hence, additional modifications should be made when using fibrous foods regarding energy content, protein, and their proportions.
Activity of dietary fiber and intestinal microbiota
After bacterial inoculum is introduced during hatching, diet plays the most important role in determining the composition and density of intestinal microbiota. (Yadav and Jha, 2019).
The cecum is considered the main site of bacterial activity within the gastrointestinal tract of poultry. The carbohydrate fraction is the most important dietary component that regulates intestinal microbial activity in broilers. This especially true for DF, which is able to escape digestion.
- Amongst these, butyric acid has been considered the most beneficial due to its antimicrobial and anti-inflammatory properties. As well as for its use as an energy source by epithelial cells (Vernia et al., 2000).
Growth performance
Growth performance includes the sum of all the aforementioned parameters. Therefore, in order to draw any conclusions about the effects of fiber, different factors need to be considered:
- Fiber source (soluble vs insoluble)
- Particle size
- Inclusion level
- Species
- Age
- Physiological state
- Dietary energy and amino acid levels
- Duration of inclusion
Gut health
Nutrition and microbiota share a very close interrelationship with each other.
With antibiotic elimination, the realization of the fact that not only the animal is being fed, but also the intestine with its different components has become evident. As a result, undigested fibers and other nutrients can be used by harmful bacteria that can cause dysbiosis. These types of fibers and nutrients can also be used by commensal bacteria that can produce short-chain fatty acids which are then used by the animal. Some of these short chain fatty acids like butyrate are associated with gut health.
| Therefore, in the end, the type of fiber (solubility and fermentability) determines the type of bacteria that is predominant within the intestine and the kind of immune response that is activated within the host under such changes. |
Conclusions
Dietary fiber is an intrinsic component of cereals and oilseeds used for the formulation of poultry diets. Both soluble and insoluble fiber components have direct effects on intestinal morphology, organ growth, nutrient utilization and modulation of microflora, to varying degrees.
The results obtained when using dietary fiber depend on factors such as:
- type of fiber
- inclusion levels
- particle size
- diet formulation
Insoluble fibers are considered functional nutrients due to their ability to modulate nutrient digestion and overall intestinal parameters. Due to their insolubility, they have minimal or no effect on the intestinal microflora with significant effects on intestinal development and nutrient digestibility when used in amounts that range from 3 and 5% of the diet.
On the other hand, the soluble fibers’ group have been considered as anti-nutrients due to their hygroscopic properties. As well as due to their ability to modulate intestinal functionality either directly or indirectly through microbial changes. The presence of soluble fibers such as pectin and arabinoxylans can significantly affect the access of intestinal enzymes to their substrates. Which in turn reduces nutrient release.