Dietary fiber and its effects on poultry diets

• 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:

Intestinal length

Height of intestinal villi

The depth of the crypts

As well as the speed and size of passage through different intestinal segments.

It has been noted that there are endogenous amino acid losses in broilers fed high fiber levels(Kluth and Rodehutscord, 2009).

Therefore, an important question to consider is whether improvements in the height of villi should be considered an advantage. Considering the fact that in theory, higher villi would have greater absorption capacity. However, such improvements could be considered a burden because a higher villus height leads to greater cell renewal, which increases amino acid requirements for proper intestinal function(i.e., threonine, aspartic acid, glutamic acid, proline).

• 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).

One of the goals when using insoluble dietary fiber is to increase pancreatic secretions (enzymes amylases, lipases, and proteases) which in turn improve substrate breakdown and subsequent nutrient release (Yokhana et al., 2016).

 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.

However, the inclusion of specific types of insoluble fiber, such as cellulose, in a 3 to 5% ratio within the diet has often been shown to improve nutrient utilization (Choct, 2015).

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.

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.

A balanced microflora in healthy broilers has the ability to produce a diverse number of metabolic end products, including antigenotoxic compounds and short-chain fatty acids such as acetic, propionic and butyric acid.

• 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.

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.

Source: Tejeda y Kim, 2021