Dietary fibre is the term used to describe the indigestible carbohydrates that are found in plant-based foods — including the celluloses, the hemicelluloses, the pectins, the gums, and the resistant starches that are the structural components of the plant cell wall and that are not digested by the human enzymes in the small intestine. Unlike the digestible carbohydrates (starches and sugars), which are absorbed in the small intestine as glucose, fructose, and galactose, the dietary fibres 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) — including acetate, propionate, and butyrate — that are the primary energy source for the colonocytes, the regulators of the immune function, and the modulators of the glucose and appetite metabolism. This fibre fermentation in the colon is one of the most important metabolic processes in the human body, and the SCFAs that are produced by this fermentation are the mediators of the multiple health benefits that are associated with the high-fibre diet — including the reduced risk of colorectal cancer, the improved glycaemic control, the enhanced immune function, and the reduced appetite that are the hallmark benefits of the high-fibre dietary pattern.
The Gut Microbiota and the Fibre Fermentation
The gut microbiota is the community of approximately 100 trillion microorganisms that inhabit the human gastrointestinal tract, and it is one of the most complex and most important microbial ecosystems on Earth. The gut microbiota performs multiple essential functions for the human host — it digests the dietary fibres and the other indigestible food components that the human enzymes cannot process, it produces the short-chain fatty acids and the other metabolites that regulate the host metabolism, it synthesises the vitamins (including vitamin K, vitamin B12, and the B vitamins) that are essential for human health, and it educates and regulates the immune system through the constant interaction with the gut-associated lymphoid tissue (GALT). The fibre fermentation by the gut microbiota is one of the most important of these functions — it is the process by which the dietary fibre is converted to the SCFAs that are the primary energy source for the colonocytes and that regulate multiple aspects of the host metabolism. When the fibre intake is low (as it is in the typical Western diet, which provides approximately 15g of fibre daily, compared to the 30-40g that is recommended for optimal health), the gut microbiota is deprived of its primary energy source, the fibre-fermenting bacteria decline, and the production of the SCFAs is reduced — with significant consequences for the colonic health, the metabolic function, and the immune regulation.
The SCFAs that are produced by the fibre fermentation — primarily acetate, propionate, and butyrate — are absorbed by the colonocytes and by the liver and are used for multiple metabolic purposes. Acetate is the most abundant SCFA in the colon (it accounts for approximately 50-60% of the total SCFA production), and it is used by the colonocytes as their primary energy source (it accounts for approximately 70% of the colonocyte energy需求) and is released into the systemic circulation, where it is used by the peripheral tissues as an energy substrate and by the brain as a precursor for the synthesis of the neurotransmitter acetylcholine. Propionate is the second most abundant SCFA (approximately 20-30% of the total SCFA production), and it is absorbed by the liver, where it is used as a substrate for gluconeogenesis (it accounts for approximately 10% of the hepatic glucose production) and as an inhibitor of the hepatic cholesterol synthesis (it reduces the HMG-CoA reductase activity and thereby lowers the blood cholesterol levels). Butyrate is the least abundant SCFA (approximately 10-20% of the total SCFA production) but it is the most biologically important for the colonic health — it is the primary energy source for the colonocytes, it has potent anti-inflammatory effects (it inhibits the NF-kappaB pathway and reduces the production of the pro-inflammatory cytokines), and it has been shown to reduce the risk of colorectal cancer by inhibiting the proliferation and by promoting the apoptosis of the colonic epithelial cells.
Fibre and the Glycaemic Control
The SCFAs that are produced by the fibre fermentation have important effects on the glucose metabolism and on the glycaemic control — they stimulate the production of the glucagon-like peptide-1 (GLP-1) and the peptide YY (PYY) from the L-cells of the distal small intestine and the colon, and these hormones are the primary mediators of the satiation and of the glycaemic regulation that are associated with the high-fibre diet. GLP-1 is an incretin hormone — it is released from the L-cells in response to the SCFAs and to the other nutrients in the gut, and it stimulates the insulin secretion from the pancreatic beta cells in a glucose-dependent manner (meaning that it only stimulates insulin secretion when the blood glucose is elevated), it inhibits the glucagon secretion from the alpha cells (thereby reducing the hepatic glucose output), and it slows the gastric emptying (thereby reducing the postprandial blood glucose spikes). PYY is a satiation hormone — it is released from the L-cells in response to the SCFAs and it acts on the hypothalamic arcuate nucleus to reduce the appetite and the food intake. The SCFA-induced stimulation of GLP-1 and PYY production is one of the primary mechanisms by which the high-fibre diet improves the glycaemic control and reduces the appetite, and it explains why the high-fibre diet is associated with the improved weight management and the reduced risk of type 2 diabetes.
Practical Application
For general fibre supplementation, the evidence-based approach is to consume 25-40g of dietary fibre daily from a wide variety of plant-based foods — including vegetables, fruits, legumes, nuts, seeds, and whole grains. The most evidence-based specific fibre supplements are the prebiotic fibres — including inulin (from chicory root), FOS (fructo-oligosaccharides), GOS (galacto-oligosaccharides), and the resistant starches (from green bananas, from potatoes, and from the Hi-Maize corn starch) — which selectively stimulate the growth of the beneficial fibre-fermenting bacteria (including Bifidobacterium and Lactobacillus species) and which thereby enhance the production of the SCFAs. For comprehensive gut health support, fibre pairs well with the probiotic supplements (which provide the beneficial bacteria that ferment the fibre), with the polyphenol-rich foods (which have prebiotic effects and which support the growth of the beneficial bacteria), and with the adequate hydration (which is essential for the bulking and the transit of the fibre through the colon).
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