Fiber has attracted a lot of interest in the field of nutrition in recent years. With the term fiber, we refer to those plant components that cannot be broken down by animal digestive enzymes. The topic of fiber is a very complex one as fiber consists of a diverse array of compounds characterized by different physicochemical properties. These properties change further as fiber passes through the digestive system and by its degradation by intestinal microflora. In this article, important chemical, physical and fermentation aspects of fiber technology, of particular interest to animal nutritionists, are discussed to make this difficult subject more accessible.
1. The four major fibers in common feed ingredients
Plants, lacking a bone skeleton, rely on their cell wall structure to maintain integrity. This structure is based on a plant-specific mix of four major groups of fibers: celluloses, hemicelluloses, pectins and lignins. Celluloses are glucose polymers, just like starch, but with a slight biochemical difference that makes them completely indigestible by animals. Hemicelluloses, found in lower quantities than celluloses in plants, are a diverse group of carbohydrate polymers containing various sugars in addition to glucose such as beta-glucans (abound in barley) and arabinoxylans (abound in wheat). Pectins, also a group of diverse polysaccharides, are lengthy polymers of galacturonic acid, an acid derived from galactose. Sugar beet pulp is an example feed ingredient rich in pectins. Finally, lignins are extremely complex polymers, not of carbohydrates but of aromatic compounds such as coumaryl alcohol, sinapyl alcohol and coniferyl alcohol. Lignins can not be broken down by enzymes secreted by the animal or even by any of the microbes in the intestines. In contrast to lignin, which abounds in wheat straw for example, all other fibers can be used by gut microbes to a varying degree.
Plants, lacking a bone skeleton, rely on their cell wall structure to maintain integrity. This structure is based on a plant-specific mix of four major groups of fibers: celluloses, hemicelluloses, pectins and lignins.
2. Soluble and insoluble fiber
One major physicochemical property that helps to distinguish the different types of fiber is its ability to dissolve in water. For example, pectins are highly soluble in water, forming a gel that increases gut viscosity. In contrast, celluloses and lignins do not dissolve in water at all. Hemicelluloses, however, can be either soluble (for example some beta-glucans) or insoluble (for example many arabinoxylans). Soluble fiber slows down digestion because enzymes cannot penetrate easily into the gel. On the other hand, insoluble fiber speeds up passage rate of digesta minimizing, thus, the time available for enzymes to act on nutrients. This is why excess fiber is deemed, in general, as a negative nutrient. Nevertheless, a moderate amount of the right blend of soluble and insoluble fibers can bring about functional benefits on gut health.
3. Fermentable versus non-fermentable fibers
Fiber can be broken down by bacterial enzymes completely, partially, or not at all. Fermentable is that part of fiber that can be used by bacteria as a source of energy, whereas non-fermentable fiber is excreted as is. Pectins are highly fermentable, in contrast to lignins that resist bacterial break down completely. Celluloses and hemicelluloses can be used by bacteria to a certain degree depending on their chemical structure. The prebiotic effect of fiber in supporting a healthy gut microflora has attracted increasing interest in recent years as a means of replacing antibiotics in animal nutrition. Here, it should be noted that excessive dietary fermentable fiber can be the source of diarrhea due to increased water secretion in the gut.
4. Water-holding capacity, factors and examples
Another physicochemical property of fiber that affects its behavior in the digestive tract is its capacity to hold water. For example, lignin is an inert material that has practically no water-holding capacity. Celluloses and many hemicelluloses, even though they may not be water soluble, can still retain water. A high water-holding capacity of insoluble fiber reduces transit time in the gut and increases fecal weight. In general, ingredients such as sugar beet pulp that exhibit a strong water-holding capacity can play a significant role in piglet diets, supporting better water management. Indeed, diets with an inherent capacity for osmotic (non-pathogenic) diarrhea can benefit from ingredients with increased water-holding capacity.
5. Specific fibers in novel ingredients, macro-algae and wood
Novel sources of fiber, such as algae and wood, offer a new approach to fiber nutrition, in addition to being a source of biomolecules with beneficial side-effects. Algae are rich in polysaccharides that bypass digestion. Such polysaccharides include alginates (up to 45 percent in certain algae), laminarin and fucans. Studies conducted with young pigs indicated that products based on laminarin provide a prebiotic effect similar to that of lactose. In contrast, wood fiber offers a rich source (over 50 percent crude fiber) of a varied mix of polysaccharides. The secret here is to pick a product with the correct balance of soluble versus insoluble fibers, and, of course, those with enough fermentability. Wood fiber is also known for its good water-holding capacity, which needs to be of intermediate strength for piglets so as not to impede feed intake. Current studies focus on wood fiber polyphenols, which might be a source of powerful antioxidants.
Fiber is a complex issue that mystifies even specialists. Seen from the plant’s perspective, it is an integral part of its structure. But, for monogastric animals, it has always been approached with a negative connotation. Today, its role in diets for young piglets cannot be denied as it helps to promote a healthy gut microflora, control water flow and regulate gut motility. It certainly deserves our better understanding despite being a rather intricate subject.