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Fibre, health and the role of the gut microbiota
Diets commonly consumed in developed countries, such as the ‘Western diet’, are often characterised by high intakes of animal protein, saturated fat, refined grains, sugar and salt and low intakes of fruits and vegetables, which often result in inadequate fibre consumption. In the UK, adults are recommended to consume 30g of fibre every day, but on average are only managing to eat 19g a day.
Fibre is an important nutrient for health. Insufficient amounts of fibre are associated with poor health outcomes, and conversely, increased fibre intakes have been associated with reductions in mortality risk, cardiovascular disease, blood pressure, type 2 diabetes, and some cancers.1
In a recent review by Koç and colleagues, they explored the relationship between fibre and health through its impact on the gut microbiota.2
An important role of the gut microbiota is to break down undigested dietary fibre, which the body would otherwise not be able to do. The gut microbiome encodes genes for many carbohydrate-active enzymes (CAZymes) which are responsible for breaking down complex carbohydrates into fermentable monosaccharides.3
Consumption of a high fibre diet increases gut microbiota diversity; a characteristic of a healthy gut microbiota. This is particularly evident when looking at the gut microbiota of people who consume a Western diet compared to a more traditional diet. For example, children living in Burkina Faso in West Africa, consume a diet consisting mainly of cereals, legumes and vegetables, which is higher in fibre compared to children in Italy who typically eat a Western diet (14.2g/day vs 8.4g/day respectively). Analysis of the gut microbiota of these children showed that children living in Burkina Faso had greater microbial richness, biodiversity and proportion of Bacteroidetes to Firmicutes ratio, compared to children living in Italy. Furthermore, the total amount of short chain fatty acids was significantly higher in children from Burkina Faso, particularly propionic and butyric acid which were nearly four-times more abundant, compared to children living in Italy.4
In the review, Koç and colleagues describe some of the interactions by which the gut microbiota may have an effect of health, namely: production of short chain fatty acids and maintenance of the mucus layer, as well as looking at the effects of prebiotics, as discussed further below.2
Short chain fatty acids
Fermentation by bacteria results in the production of short chain fatty acids including acetate, butyrate and propionate which each have a number of benefits for human health, particularly in relation to metabolic health, satiety and weight. Furthermore, butyrate has been associated with having anti-inflammatory and anti-carcinogenic effects.2
Mucus layer
The colonic mucus layer helps defend against pathogens. However, in times of low nutrient supply, the gut microbiota will use the mucus layer as an alternative energy supply. Evidence from animal studies suggests that low fibre diets lead to a thinning of the mucus layer, reducing its protective capabilities and thus increasing susceptibility to pathogen invasion and inflammation.5
Prebiotics
Certain dietary fibres could be classed as prebiotic; a substrate that is selectively utilised by host microorganisms conferring a health benefit.6 Prebiotics affect the composition and functionality of the gut microbiota, and research from human and animal studies suggest that prebiotics may have a number of benefits for health, including; mental health, intestinal inflammation and obesity.2
The benefits of fibre for health have been known for centuries, but more recently it has been suggested that part of these effects may be attributed to the interaction between fibre and the gut microbiota. As research continues, this relationship will become better understood.
References
1. Stephen et al. (2017) Nutrition Research Reviews 30: 149-190
2. Koç et al. (2020) Nutrition Bulletin 45: 294-308
3. El Kaoutari et al. (2013) Nature Reviews Microbiology 11: 497-504
4. De Filippo et al. (2010) PNAS 107(33) : 14691-14696
5. Desai et al. (2016) Cell 167(5): 1339-1353
6. Gibson et al. (2017) Nature Reviews Gastroenterology and Hepatology 14: 491-502
25/11/2020