Over half of bowel cancer cases in the UK are thought to be preventable (1). The link between lifestyle factors (e.g. diet) and the risk of cancers, particularly bowel (or colorectal) cancer, has been a hot topic over the last few years. Recently, factors such as obesity, red meat and ultra-processed foods have got bad press regarding their link to bowel cancer, but what about foods which may be protective such as dietary fibre?
What is dietary fibre?
Dietary fibre encompasses carbohydrates that are not digested in the small intestine and therefore reach the large bowel undigested. You can find it in wholegrains, cereals, and other foods such as fruit, vegetables and pulses. The intake of dietary fibre may impact our risk of developing diseases such as cardiovascular disease, type 2 diabetes and bowel cancer (2). In the UK, 28% of bowel cancer cases are caused by eating too little dietary fibre (1). As dietary fibre reaches the large bowel undigested, it is perhaps not surprising that it has effects on large bowel health and bowel cancer risk and has been quite extensively investigated.
The discovery of the link between dietary fibre and bowel cancer goes a while back. In the early 1970s, Dr. Denis Burkitt was the first to notice that Western African countries had much lower incidence rates of gastrointestinal diseases including bowel cancer (3). Bowel cancer incidence rates are more than 10 times lower in rural South Africans (< 5 cases per 100,000 people) compared with African Americans (65 cases per 100,000 people). Traditional African diets have a high dietary fibre content and differ in preparation, cooking and composition compared with those of African Americans. One study performed a two-week diet swap between African Americans and rural Africans (4). They fed African Americans a diet high in dietary fibre (55g per day) and rural Africans were given a ‘Westernised’ diet (including pancakes and hamburgers) low in dietary fibre (12g per day) for two weeks. Various samples and measurements were collected during and at the end of the study. In the African Americans, eating the high fibre diet for two weeks reduced markers of bowel cancer risk such as cell growth, increased gut bacteria diversity and decreased toxins.
Since Dr. Burkitt’s discovery, the links between dietary fibre and bowel cancer have been quite extensively investigated in research studies including those in cell lines, in animal models and in humans. Aune and colleagues reviewed the research studies in humans and concluded that there was a 10% reduction in the risk of developing bowel cancer for every 10g increase in daily total dietary fibre intake (5). This would be equivalent to eating a bowl of bran flakes cereal with a medium banana for breakfast or eating a jacket potato (skin on) with baked beans for lunch, or having a portion of wholemeal spaghetti with a tomato sauce for dinner. This finding was particularly strong for dietary fibre from cereals and whole grains.
More recently, the World Cancer Research Fund (WCRF) has reviewed the evidence from research studies for the links between diet, nutrition, physical activity and bowel cancer and concluded that probable evidence exists for a protective effect of foods containing dietary fibre (6). It must be noted however that some research studies do exist that have shown no associations between dietary fibre intake and bowel cancer risk or incidence. For example, results from a study in almost half a million participants in the US did not observe relationships between total dietary fibre intake and bowel cancer risk with the exception of fibre from grains (7).
How does dietary fibre modulate bowel cancer risk?
One of the most obvious and simpler ways through which dietary fibre may reduce bowel cancer risk is by limiting the exposure of the lining of the large bowel to harmful, cancer- promoting toxins (carcinogens). Dietary fibre does this in several ways including bulking stool mass and making it pass through the large bowel more quickly (9).
Dietary fibre, particularly those fibres with prebiotic effects, can also modulate the amount and diversity of gut bacteria (10). The friendly gut bacteria process the fibre in your large bowel to produce beneficial short-chain fatty acids such as butyrate. Butyrate plays a key role in maintaining a healthy large bowel as it is the main energy source for cells in the large bowel (colonocytes) and regulates many important processes such as cell growth and cell death. Uncontrolled cell growth or cell death can promote the growth of tumours and cancers and dietary fibre, and butyrate, may reduce cell growth in the large bowel (11). Dietary fibre and butyrate are also anti-inflammatory (12, 13). Inflammation drives several diseases including bowel cancer and people with inflammatory bowel diseases (IBDs) such as ulcerative colitis are at greater risk of developing bowel cancer.
Another process implicated in the development of bowel cancer is the response to insulin and its impairment (insulin resistance). Dietary fibre intake may reduce insulin resistance and improve the body’s response to glucose (14). Although some studies have provided mechanistic evidence, further research is required to enable us to better understand the underlying mechanisms behind dietary fibre’s anti-cancer effects in the large bowel (15).
How can you increase your dietary fibre intake?
The guidelines in the UK now recommend a daily dietary fibre intake of 30g. However, we are only consuming about 18g of dietary fibre per day, suggesting that we should be increasing our intake by almost two thirds (16).
Here are some dietary fibre rich foods that you can incorporate into your meals and snacks throughout the day:
- Breakfast: oats (add fruit, nuts and seeds for a bonus), cereals e.g. muesli (no added sugar), wheat biscuit cereal and bran flakes
- Lunch/dinner: include a variety of vegetables and choose high-fibre carbohydrate sources such as potato (with skin on), wholegrain pasta and pulses e.g chickpeas and lentils
- Snacks: nuts, seeds, fruits (including dried fruit), crudite vegetables, hummus, rye crispbread, and oatcakes
Please note that significantly increasing your dietary fibre intake all of a sudden could have side effects such as bloating and flatulence. So it is advised to slowly increase your intake of fibre and make sure you drink plenty of fluid. If you have any gut issues, you may wish to discuss these with a dietitian!
Take home message
The evidence for a reduction in the risk of developing bowel cancer with greater dietary fibre intake is strong, and more recently potential underlying mechanisms are being discovered. Aiming for an intake of 30g dietary fibre per day is recommended in adults and leading an overall healthy, balanced lifestyle is the best way to stay healthy and reduce your risk of developing preventable diseases. The WCRF have summarised the evidence for additional lifestyle factors that may protect against bowel cancer, amongst others, for example maintaining a healthy body weight, being physically active and reducing the intake of alcohol and of red and processed meats (6). Furthermore, bowel cancer is a multifactorial disease whose risk is influenced by a number of factors and processes including genetics.
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Feature image: Lex Sirikiat via Unsplash