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Fibre

What is Fibre?

Dietary fibres are plant-based carbohydrates that, unlike other carbohydrates, are not digested or absorbed in the small intestine and therefore reach the large intestine (BNF 2018) . Simply put these are edible parts of plants that our bodies are unable to digest. This includes all non-digestible oligosaccharides, resistant starches and polydextrose. 

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Current Recommendations

Current guidelines recommend a fibre intake of 30g/day for adults (aged 17 years and over) and between 15-25g/d throughout childhood, as shown in Table 1 (SACN 2015).

Dietary fibre can be defined and measured as either non-starch polysaccharides (NSP) or AOAC fibre. The NSP method only measures fibre within plant-cell walls, such as wholegrains, fruits and vegetables, whereas the AOAC method measures both NSP fibre and non-digestible carbohydrates, such as resistant starches and lignin (EC 2019). The previous fibre recommendation of 18g/d for adults was based on NSP fibre, which was the equivalent to 23-24g of AOAC fibre, whereas the updated recommendation of 30g/d of fibre is based on AOAC fibre (SACN 2015).

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This increase on the previous recommendation was a result of the growing body of evidence indicating that a diet rich in dietary fibre can have beneficial effects on health (SACN 2015). However, according to the latest data from the National Diet & Nutrition Survey, the UK adult population aged 19 to 64 years are currently only eating 19g/d on average, with those over 65 years of age only eating 17.5g/d  (Roberts et al. 2018), and so more needs to be done to meet the current recommendations.

Dietary fibre is naturally present in many foods, including:

- Fruits: strawberries, oranges, pears, apples, bananas, plums, prunes

- Vegetables: broccoli, carrots, sweetcorn, sweet potato

- Legume: chickpeas, peas, beans, lentils, baked beans

- Nuts & Seeds: almonds, hazelnuts, linseeds, sun flower seeds, peanuts

- Wholegrains & Carbohydrates: oats, wholegrain bread, whole-wheat pasta, whole-wheat cereal, potato with skin

Impact on Health

There are many health benefits associated with the consumption of fibre from the improvement of our gastrointestinal health, such as prevention of constipation, to a reduced risk of developing chronic diseases, such as cardiovascular disease and type 2 diabetes (SACN 2015).

Gastrointestinal health: Evidence from randomised-controlled trials have shown that consuming more dietary fibre  decreases intestinal transit time and exerts faecal-bulking effects, softening stools and decreasing constipation (SACN 2015). It is important to drink the recommended 6-8 glasses of fluid a day, particularly water, as this helps fibre to do its job.

Several forms of dietary fibre are also involved in the production of key metabolites known as short chain fatty acids (SCFA), through gut bacterial fermentation (Makki et al. 2018). These SCFA, such as acetate, propionate and butyrate, play an important role in maintaining gut and immune homeostasis (Tan et al. 2014).

Chronic diseases: Observational studies have provided strong evidence suggesting a significant association between diets high in fibre and a reduced risk of chronic conditions such as cardiovascular disease (CVD), type 2 diabetes and colon cancer (SACN 2015). 

Impact on the Gut Microbiota

Dietary intake is considered one of the main drivers of gut microbial composition, diversity and richness (Voreades et al. 2014). There is a strong body of evidence to suggest that the Westernised diet, characterised by low fibre and high sugar, protein and fat consumption, is associated with an altered and less diverse microbiota, as discussed by Makki and colleagues (2018).

Non-digestible dietary fibres are degraded and fermented in the colon, resulting in the production of short chain fatty acids (SCFA). These SCFA are known to have wide-ranging impacts on host physiology, but their production can be limited by fibre available (Baxter et al. 2019).  

Although some researchers have examined the effects of dietary fibre consumption on the microbiota and production of SCFA, the changes  that are observed due to dietary fibres appear to be very individualised (Martίnez et al. 2010, Baxter et al. 2019).