Dietary fibre is the term for the non-starch polysaccharides and the functional oligosaccharides that are not digested by the human enzymes in the small intestine — they pass through the small intestine intact and reach the colon, where they are fermented by the gut microbiota to produce the short-chain fatty acids (SCFAs, primarily acetate, propionate, and butyrate), the gases (hydrogen, carbon dioxide, methane), and the热量 (which is approximately 1-2kcal/g of fibre, but which varies depending on the fermentability of the fibre). The SCFAs are the primary energy source for the colonocytes (the epithelial cells of the colon) — the butyrate is the preferred energy substrate for the colonocytes, and it is also the most important SCFA for the colonic health because it has the anti-inflammatory, the anti-proliferative, and the pro-apoptotic effects that protect against the colorectal cancer and the inflammatory bowel disease. The fibre is also the primary substrate for the gut microbiota — the diversity and the composition of the gut microbiota are directly dependent on the fibre intake, and the dysbiosis (the loss of microbial diversity and the shift in the microbial composition) that is associated with the low fibre intake is one of the most important mechanisms of the fibre deficiency and its link to the metabolic syndrome, the obesity, and the colorectal cancer. The typical dietary fibre intake in the developed world is 10-15g daily, which is well below the recommended intake of 25-35g daily — making the fibre deficiency one of the most common and most underdiagnosed nutritional deficiencies, and one of the most important risk factors for the chronic diseases of the developed world.
Fibre and the SCFA Production
The colonic fermentation of the fibre by the gut microbiota produces the SCFAs (primarily acetate, propionate, and butyrate), which are the most important energy source for the colonocytes and the most important regulators of the colonic health. The butyrate is the most important of the SCFAs — it is the primary energy substrate for the colonocytes (the epithelial cells of the colon), and it has the most potent anti-inflammatory and anti-cancer effects. The butyrate inhibits the histone deacetylases (HDACs) and thereby regulates the gene expression in the colonocytes, promoting the expression of the genes that are involved in the cell differentiation, in the cell cycle arrest, and in the apoptosis — and inhibiting the expression of the genes that are involved in the cell proliferation and in the inflammation. This butyrate-induced HDAC inhibition is one of the most important mechanisms of the anti-colorectal cancer effect of the fibre, and it explains why the butyrate is called the “histone deacetylase inhibitor” and why it is being studied as a potential treatment for the colorectal cancer and for the other cancers.
The clinical importance of the fibre for the SCFA production and for the colonic health is underscored by the observation that the fibre supplementation increases the faecal SCFA concentrations and reduces the risk of the colorectal adenomas and of the colorectal cancer. A meta-analysis of 25 prospective studies in over 2 million participants found that the high fibre intake (compared to the low fibre intake) was associated with a 16% reduction in the risk of the colorectal adenomas and a 12% reduction in the risk of the colorectal cancer — making the fibre one of the most effective dietary interventions for the prevention of the colorectal cancer.
Practical Application
For general fibre supplementation, the evidence-based approach is to consume 25-35g of dietary fibre daily from the diverse food sources, particularly from the soluble fibres (oats, barley, psyllium, apples, citrus fruits, beans, lentils, peas) and from the insoluble fibres (wheat bran, whole grains, nuts, seeds, vegetables). The fibre supplements (psyllium, methylcellulose, wheat dextrin) can be used to supplement the dietary fibre intake when the food sources are inadequate, but they should be introduced gradually (to avoid the gastrointestinal symptoms of the fibre adjustment) and with adequate water (to prevent the constipation and the intestinal obstruction). For comprehensive gut and metabolic health support, fibre pairs well with the probiotics (which ferment the fibre and produce the SCFAs — the combination of the fibre and the probiotics is more effective than either alone for the gut health and for the metabolic syndrome prevention), with the vitamin D (which has immunomodulatory effects and which works synergistically with the SCFAs for the prevention of the inflammatory bowel disease), with the zinc (which is required for the integrity of the gut barrier and for the immune function in the gut), and with the magnesium (which is a cofactor for many of the enzymes of the energy metabolism and which is often deficient in people with the gut dysbiosis).
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