How Does Dietary Fibre Benefit Our Gut Health
Article by DR. Fiona Malcomson BSC (HONS) MRES PHD
Dietary fibre intake has been linked with beneficial effects on health-related outcomes, from stress and mental health to reducing the risk of diseases such as type 2 diabetes, cardiovascular disease and bowel (colorectal) cancer. Find out more about what fibre is and where we can get it from here.
One of the ways through which dietary fibre exerts these effects is by modulating gut health. So, how does dietary fibre affect our gut?
Directly, dietary fibre affects the material travelling inside the gut. Insoluble fibres, including those found in wholegrains, nuts & seeds, bulk up stool, helping it pass through the large bowel more quickly and preventing constipation. This reduces the exposure of the lining of the large bowel (named the epithelium or mucosa) to harmful toxins, including those which promote bowel cancer (carcinogens). Soluble fibres, in foods such as oats and beans, slow down digestion by dissolving in water and forming a gel.
Although these fibres reach our large bowel (intestine), undigested, the bacteria in the gut are able to break these down. This is why we are able to digest dietary fibres such as resistant starch that would otherwise be undigestible. Many of the effects of dietary fibre on health arise indirectly through effects of the bugs (bacteria) residing inside your gut, known as the gut microbiota.
Feeding your gut bugs:
Most (but not all) dietary fibres, such as inulin and fructo-oligosaccharides, can be defined as prebiotics (“a substrate that is selectively utilised by host microorganisms conferring a health benefit”) (Gibson et al. 2017).
In the large intestine, undigested dietary fibres are fermented by the gut microbiota to produce short-chain fatty acids (SCFA) such as acetate and butyrate. SCFA contribute to a small proportion of energy absorbed into the blood and used by the body. Butyrate is the main source of fuel for cells in the large bowel (called colonocytes), providing up to 70% of their energy, and therefore contributes to how they grow (discussed in more detail later).
An increase in dietary fibre intake, and particularly in the diversity of dietary fibres consumed, is associated with an increase in the diversity of gut bacteria. Having a low microbial diversity has been linked with gastrointestinal diseases, such as inflammatory bowel disease (IBD), as well as others, such as type 2 diabetes.
Anti-inflammatory properties:
Chronic inflammation is a driver of many gastrointestinal diseases, including bowel cancer, as well as those external to the gut. Dietary fibre and butyrate possess anti-inflammatory properties (Kuo, 2013). For example, they may reduce pro-inflammatory molecules e.g. interleukins and regulate inflammatory pathways. Indirectly, dietary fibre can reduce inflammation via its positive effects on body composition and on the immune system, and by reducing the growth of bad bacteria.
Further support for the role of dietary fibre in inflammation concerns its beneficial effects in patients with inflammatory bowel disease (IBD), including a reduction of inflammatory markers and gastrointestinal symptoms (Pituch-Zdanowska et al. 2015). It must be noted, however, that not all clinical trials have reported a significant effect of dietary fibre supplementation on markers of inflammation.
The body’s defence system:
The gut is the largest immune organ in the body, comprising 70% of our immune cells. Our immune (defence) system develops from early life, particularly within the first 3 years. This starts right from our birth (different bacteria are transferred from our mothers depending on birth delivery (Shao et al. 2019)) and also the intake of prebiotic fibres (human milk oligosaccharides) present in breastmilk. These events shape the gut microbiota and promote a healthy immune system (both in the gut and systemically) and may protect against allergies (van de Elsen et al. 2019).
Increased gut bacterial diversity and abundance of good bacteria, promoted by dietary fibre intake, improves our immune system. It does this in many ways, including stimulating immune activity, increasing immune cells e.g. lymphocytes and raising antibody levels e.g. IgA (Schley and Field 2002).
The great wall of the gut:
Another defence mechanism that is affected by dietary fibre is the integrity of the gut wall (Makki et al. 2018). The gut wall comprises cells that form an important barrier preventing harmful material from moving into the body. These are connected by tight junctions that regulate the entry and escape of nutrients and other factors. SCFA produced from the fermentation of dietary fibre aid in the maintenance of these tight junctions.
The gut epithelium is further lined with a mucus layer. Dietary fibre and SCFA stimulate the production of mucus and maintain this additional protective barrier. Low fibre diets are associated with increased penetrability of the mucus layer, resulting from, for example, the production of enzymes that degrade mucus, increasing the chance of infection.
The gut microbiota as the second brain:
Dietary fibre and its modulation of the gut microbiota can affect gut health via interactions with the brain. There is bidirectional communication between the gut and the central nervous system, whereby the gut can send messages to the brain, and the brain can also regulate processes in the gut, including gut motility and immune function (Oriach et al. 2016). Gut hormone (or neuroendocrine) signalling is responsive to the gut microbiota and regulates the production of peptides which communicate with the nervous system. Furthermore, SCFA such as butyrate and propionate can affect brain function, including its regulation of appetite.
Acidity of the gut:
SCFA produced from dietary fibre lower the pH (more acidic, less alkaline) of the gastrointestinal tract as they are weak acids (Scott et al. 2008). This prevents the colonisation or over-growth of bad (pathogenic) bacteria and promotes the growth of good bacteria. A more acidic environment also favours the absorption of important minerals, such as calcium and magnesium, by making them more soluble.
Regulation of blood sugar levels:
Dietary fibre can slow the absorption of sugar and help to regulate blood sugar levels after eating, known as improving the glycaemic response. More recently, evidence suggests that SCFA improve satiety through the production of hormones such as GLP-1 that stimulate insulin secretion and increase glucose uptake (Byrne et al. 2015). Through the regulation of glucose and reduction in insulin resistance, dietary fibre protects against several diseases where this is impaired e.g. type 2 diabetes (Reynolds et al. 2020).
Fibre, keeping you fuller for longer:
Foods higher in dietary fibre tend to keep you fuller for longer, for example by slowing digestion, and are less energy dense, meaning you get more ‘bang for your buck’. In a human study, consuming an evening meal of brown beans (dietary fibre) suppressed hunger hormones e.g. ghrelin and increased satiety hormones e.g. PYY measured the next morning (Nilsson et al. 2013).
By increasing satiety, a high fibre diet may therefore limit the consumption of less healthy foods, such as saturated fats or processed foods which are associated with poor effects on gut health and on body composition (Gibson et al. 2019). However, further research into how fibre intake affects food choices and consumption, and consequent effects on body composition, are needed.
Regeneration of the gut:
The turnover of cells (the replacement of shed dead cells with new growing cells) is particularly important in the large bowel as it is the most regenerative organ in our body. The lining of the large bowel is renewed every 4-7 days, making sure that it is on top form to perform its functions of breaking down and absorbing foods. An imbalance of growing (proliferating) and dying (apoptotic) cells can promote the growth of tumours and cancer development.
Dietary factors, certain bacteria and carcinogens, such as bile acids, can promote cell growth, rendering the epithelium hyperproliferative. On the contrary, dietary fibre, in particular resistant starch, may reduce proliferation of cells in the large bowel, and this is one of the mechanisms through which it may decrease bowel cancer risk (Malcomson, 2018).
Dietary fibre and bowel cancer risk:
The link between dietary fibre and bowel cancer risk was first reported by Denis Burkitt in the early 1970s. Dr. Burkitt observed a low incidence of gastrointestinal diseases in native Africans, who consume high amounts of dietary fibre, particularly resistant starch in the form of maize meals (Burkitt, 1971). Indeed, bowel cancer risk is more than 10 times lower in native Africans compared with African Americans. Since then, researchers have unravelled that this reduction in bowel cancer risk is at least partially mediated by differences in the gut microbiota and their metabolites, including greater concentrations of butyrate, in native Africans (O’Keefe et al. 2015).
In their latest report, the World Cancer Research Fund (WCRF) concluded that there is strong evidence for a protective effect of foods containing dietary fibre against bowel cancer, and their Cancer Prevention Recommendations advise the consumption of >30g fibre per day (WCRF, 2018).
Take home message:
Although there is at present a lot more scientific evidence for the mechanisms through which dietary fibre exerts its effects on gut health, including modulation of the gut microbiota, inflammation and glucose regulation, further research is required to further explain these, particularly from human studies. As not all fibres are equal, similar to the importance of improving the diversity of our gut microbiota, increasing the diversity of dietary fibres consumed (from pectin in peaches to beta-glucans found in oats) will increase the breadth of their beneficial effects on our guts.
This post was written by Dr. Fiona Malcomson BSc (Hons) MRes PhD, a Research Associate at Newcastle University with a PhD in Molecular Nutrition. Her primary research interest is investigating the relationships between diet and lifestyle factors, such as obesity and physical activity, and markers of large bowel health and of bowel cancer risk and the underlying mechanisms behind these. Fiona is passionate about breaking down science research so it’s accessible for everyone and contributing to evidence-based nutritional and lifestyle public health recommendations through her research. You can find her on Instagram @Fiona.Malcomson and twitter @FionaMalcomson
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