As fibre is indigestible, it enters the colon unaltered for the most part. When they reach here, some are able to be fermented by colonic microflora, with soluble fibres typically being affected more so than insoluble ones. This happens when intestinal bacteria use enzymes that they synthesise to digest the fibre (and any other nutrients that enter the colon) in a very similar set of reactions to anaerobic respiration detailed in module 2 (in fact one of the main metabolites that these bacteria produce via this process is lactate, just like human cells). The most fermentable fibres, are:
- Fructans
- Galactooligosaccharides
- Pectin
- Gums
- Psyllium
- Beta glucans
- Polydextrose
- Resistant starch 3 (RS3)
This fermentation provides the bacteria mostly with nitrogen, which is an essential part of life, but side-products are also created including short-chain fatty acids (SCFA’s). Both of these result in benefits to the host organism.
Firstly, by providing nutrients for gut bacteria, fermentable fibres act as pre-biotics. A pre-biotic is simply something that promotes the growth and diversity of the gut microflora – resulting in a state associated with a number of health benefits including a potential reduction in IBS symptoms, improved immune system function, and a decreased ability for more harmful bacteria such as certain strains of E.Coli to colonise.
Additionally, fermentation leads to SCFA production, primarily acetic, butyric and propionic acids. The fatty acids produced depend entirely upon the bacteria and the fibre in question, and it would be far beyond the scope of this course to lay out the exact details. Suffice to say that these three are produced upon the ingestion of fermentable fibres and they all play important roles in GI health:
- SCFA are rapidly absorbed into the intestinal walls. This process uses transport proteins on the cells which also allow water and sodium absorption. This of course plays in to retaining hydration and forming more solid stool
- These fatty acids alter the differentiation fate of cells in the intestinal walls. Intestinal wall cells ‘turn over’ rapidly to stay healthy, and to accomplish this there is a layer of undifferentiated cells always ready to replace the cells at the surface. An undifferentiated cell is simply one that has not fully ‘formed’ into the final cell type that it will adopt for the rest of its life, meaning that it is malleable. SCFA’s promote cells to more preferentially differentiate into mucosal cells, meaning that SCFA absorption increases the density of the mucosal layer inside the intestines. This layer is critical for avoiding the absorption of pathogens, so this process has important impacts on disease avoidance
- SCFA’s alter the intestinal pH, making it more acidic. This causes bile acids to become more soluble and ‘deactivates’ certain enzymes present in the intestine. These enzymes are responsible for altering bile acids from their primary to their more harmful secondary form, and so SFCA’s can reduce secondary bile acid formation. Secondary bile acids are associated with colonic cancer and so this is a very important process. Additionally, SCFA acid seem to halt tumour cell differentiation and promote apoptosis (programmed cell death) in these cell populations
- SCFA’s provide energy. In fact, butyric acid and propionic acid are considered to be primary energy sources for the colonic cells. What isn’t used here finds its way via the portal vein to the liver, and is used for glucose synthesis, or transport to peripheral cells for ATP production there. It is estimated that fermentable fibre, though it is not digestible in and of itself, provides 1.5-2.5kcal per gram via this process
- Propionic acid delivered to the liver seems to reduce cholesterol production
- SCFA’s interact with the intestinal nervous system and smooth muscle to increase blood flow to this area
- SCFA’s promote the production of immune cells, while the decrease in the intestinal pH helps to prevent pathogens from colonising
Fermentable fibres are clearly very helpful, but it is a mistake to think that this means non-fermentable fibres are without merit. Non-fermentable fibres include:
- Cellulose
- Lignin
- Plant waxes
- Resistant starch 1 and 2 (RS1 and RS2)
- Some hemicelluloses
These benefit you in the following ways:
- Non-fermentable fibres can adsorb hydrophobic carcinogens, preventing them from interacting with the intestinal wall. These can then be excreted or rendered inert by bacteria
- Most obviously, non-fermentable fibres increase faecal volume. Faeces is primarily comprised of unfermented fibre, salts, water and bacteria. An increase in gut bacteria thanks to fermentable fibres increases stool mass, but un-fermentable fibres do this in a more direct manner. Increased stool mass leads to easier passage, as peristalsis in the intestines is more effective at moving the stool towards the anus
Clearly fibre has a huge number of benefits, and it can impact our health in far more ways than being a laxative. It can even help us to avoid disease.