The Role of Insoluble Fiber in Animal Gut

We have been actively pursuing research work on dietary fibers, and have also been discussing this topic with several parties involved in producing and using commercial fibers in animal feeds which being an increasing trend. After having discussed this topics on several occasions, it transpired to us that there is a great misconception regarding fibers. It’s not a surprise, asfiber in itself is such a complex topic and there is still lot of new things to be learned about fibers each and every passing day!
The term insoluble is used mostly for cellulose and lignin or lignocellulose to differentiate these fractions from soluble fibers, such as non-starch polysaccharides. As it happens, some groups consider the term insoluble as meaning inert, whereas in reality this fraction of fibers is nothing but inert. As a matter of fact there always has been talks about the behaviour of fibers before they reach the large intestine where the majority of microbial fermentation happens in monogastric animals, such as poultry and pigs.

In the Gut all fibersare known to interact with water molecules..
In general, all fibers interact with water found in the gut butin practical terms it is the nature of their interaction with this water molecules that characterizes them as insoluble or soluble. Soluble fibers take in water in a process that is hard to reverse, forming a gel-like mass; the end result being a highly viscous digesta. Ignoring any effects of microbial fermentation, water trapped in this viscous mass is difficult to retrieve by the animal when digesta reaches the large intestine, where the majority of gut water is reabsorbed. Whereas in contrast, insoluble fiber does not change when it interacts with water. It absorbs water, alright, but it freely gives it back.



To get a visual aspect of this discussion, one may consider paper napkins that are composed mostly of cellulose; these are the good ones. The others that hardly absorb anything are mostly lignins — you can know and experience them because they are brittle in nature. Good paper napkins will absorb a substantial amount of water, but once squeezed, they will release it back. In contrast, we can visualize the very common breakfast item, oats porridge. There, once heated for a while, oat soluble fibersmainly arabinoxylans and beta-gluans take in water and incorporate it into their structure. If heating continues, we end up with a very viscous, almost solid, mass of oats. No matter how much pressure we exercise on this mass, little if any water will be released back.

An oversimplification, perhaps, but I hope it serves the point of understanding that insoluble does not mean inert.