When it comes to bowel movements, fibre is the gold standard for ensuring a smooth ride, so to speak. Known for its poop bulking and regulating properties, fibre is also a vital source of nourishment for the microbes residing in the gut. Thanks to ardent research surrounding the microbiome, it’s now understood that certain microbes play a vital role in the functioning of the human body. Feeding these microbes a diet rich in fibre provides them with the energy and necessary materials to do their job, so we in turn can do ours. Fortunately, eating a high-fibre diet is readily achieved, often requiring a few discerning food swaps rather than a complete diet overhaul.

The following blog post will discuss what exactly fibre is, how it impacts the microbiome, its role in health and suggestions on how to incorporate more of this important nutrient into your diet.

What is fibre?

Found predominantly in fruits, vegetables, whole grains, legumes, nuts and seeds, fibre includes the parts of plants that can’t be completely broken down by the human digestive system. Thanks to our inability to digest or absorb this plant roughage, it is able to pass relatively intact through the stomach, small intestine and colon, providing many health benefits along the way.

In previous decades, fibre has been commonly classified as either soluble or insoluble:

Soluble fibre: dissolves in water forming a viscous, gel-like material, the properties of which can help lower blood cholesterol, increase satiety, maintain bowel health and facilitate the production of essential health-promoting compounds known as short chain fatty acids (SCFAs).

Insoluble fibre: doesn’t dissolve in water, remaining intact as it travels through the gastrointestinal tract. This aids in the movement of material through the digestive system and helps bulk up stool. Basically it’s what helps keep you regular and constipation free.

Moving into the present, new research into the microbiome has enabled a new classification of fibre to be added to the list:

Prebiotics (including resistant starch): non-digestible fibres that travel through the gut, stimulating the growth and/or activity of probiotics – aka the “good” bacteria in our guts – thereby positively impacting health.

How does fibre impact the microbiome?

All the microorganisms present in the gastrointestinal tract, including bacteria, viruses and fungi, are collectively known as the ‘gut microbiota’. Through the process of fermentation, fibre has been shown to modulate the microbiota, impacting gut diversity and microbial metabolic activities.

Fibre and microbial fermentation

Fibre is a source of sustenance for the trillions of microbial cells residing within our guts. The process by which fibre is broken down by gut bacteria is known as fermentation. As well as providing energy for the microbes, fermentation which takes place in the large intestine, also releases beneficial by-products such as SCFAs which not only promote a healthy colon but can influence glucose homeostasis, lipid metabolism, appetite regulation and immune function (8).

Microbes are somewhat picky when it comes to their food. Some bacteria have many different enzymes that allow them to metabolize a variety of complex carbohydrates, while other microbes are only able to utilise a certain type of fibre. Therefore, the impact fibre has on the microbiome will vary based on the type of fibre consumed. For example, insoluble fibre is generally poorly fermented by gut microbes, whereas prebiotics and resistant starch are readily fermented.

The amount of soluble, insoluble and prebiotic fibre varies in different plant foods, which is why it’s important to eat a wide variety of high-fibre foods to ensure all your microbes are getting their fill so that gut diversity is maintained.

Fibre and microbial diversity

Because the gut microbiota plays an essential role in immunity, homeostasis, neurodevelopment and metabolism (1), keeping it in balance is important to good health. Research has shown that diet is one the most significant environmental factors responsible for shaping the microbiome (2), with dietary fibre being one of the main ingredients responsible for modifying the delicate balance of microbes residing in our gut. In fact, significant shifts in bacterial diversity has been seen in as little as 24 hours in humans switching between a high fibre diet to a meat-based diet devoid of fibre (3). In addition, studies of human populations across the world reveal that greater dietary fibre intake is associated with increased gut diversity (4). These results are important as high bacterial diversity has been linked to good health, while below average diversity has been linked to poor health outcomes (5).

Fibre and metabolic activities

Along with the direct influence fibre can have on microbial diversity, fibre can also have an indirect impact on the microbiome through the promotion of metabolic interactions among bacterial species. Growth of certain microbes in the gut can occur through the utilisation of by-products of other community members; a process called cross-feeding. During cross-feeding, the products produced from fermentation of fibre by one bacterial species provides nutrients for growth of other bacteria present in the community (6). For example, cross-feeding may explain the phenomena whereby increased concentrations of butyrate (a by-product of fermentation) occurs post consumption of fructans (a type of fibre) despite the fact that the types of bacterial species that increase as a result of the fructan intake don’t in and of themselves produce butyrate. It is thought that the types of bacteria that utilise fructans produce lactate and acetate during fermentation which is then used by many other bacteria to produce butyrate (6).

While cross-feeding can be beneficial to some bacteria, it can also have a deleterious effect on other microorganisms, thus potentially affecting diversity. Bacterial fermentation of fibre results in the production of acidic end products that can reduce the acidic environment of the colon which in turn impacts the composition of the microbiota in the gut. In vitro experiments using human fecal samples to model the colon reveal that a reduction in pH can change the microbial composition – more acidic conditions support growth of butyrate-producing bacteria, while at the same time reducing the amount of acid sensitive Bacteroides (6).

Are there health consequences of not consuming enough fibre?

A lack of fibre can lead to a poorly functioning digestive system, which can have both short and long-term effects on health. Low fibre diets can result in irregular bowel movements, constipation, blood sugar fluctuations, lack of satiety after eating, a rise in cholesterol levels and certain inflammatory conditions.

Many studies have indicated a strong link between low levels of dietary fibre and obesity and diseases such as type 2 diabetes, cardiovascular disease, and colon cancer (6). Since a diet high in fibre can improve blood glucose and cholesterol levels that are linked to these types of diseases, the absence of fibre in turn could pose a risk to health.

Eating too little fibre reduces the number of microbes in the gut, resulting in a decrease in SCFAs, molecules known to relieve inflammation and maintain the integrity of the gut lining. A fibre deficiency may therefore result in an increased likelihood of developing conditions that a healthy microbiome prevents.

What is the daily recommended intake of fibre?

According to the World Health Organisation, the current recommended fibre intake is 25g-30g per day. Unfortunately, many people don’t come close to consuming that amount, an issue likely attributed to the rise in processed and fast food consumption; foods notoriously low in fibre.

Fortunately, increasing fibre in the diet doesn’t require a huge life overhaul. With a couple of quick fridge swaps and high-fibre food additions to the pantry, you can be hitting that 25-30g target in no time.

How to increase your fibre intake

When adding fibre into your diet it’s important to take it slow so as to avoid unwanted symptoms such as bloating, cramps and excess gas. The following are examples of some high-fibre foods along with the amount of fibre in typical serving:

Conclusion

Our microbiome and health are inextricably linked. Since diet can modify the types and abundance of bacterial populations living within us, the food we eat can either positively or negatively impact our overall health. While many people associate a high-fibre diet with the perfect poo, thanks to an increased understanding of the microbiome, it’s now known that indigestible plant-based compounds provide more health benefits than previously thought. Consuming a diet rich in different types of fibre encourages microbial diversity and stimulates the growth of “good” bacteria, which are conducive to improved physical and mental health. So when thinking about what to have for dinner, remember that fibre is a popular choice on the microbial menu; and if your microbes are happy, so too will you be.

Brigid xx

References

(1) Zheng, D., Liwinski, T. & Elinav, E. Interaction between microbiota and immunity in health and disease. Cell Res 30**,** 492–506 (2020). https://doi.org/10.1038/s41422-020-0332-7

(2) Singh, Rasnik K et al. “Influence of diet on the gut microbiome and implications for human health.” Journal of translational medicine vol. 15,1 73. 8 Apr. 2017, doi:10.1186/s12967-017-1175-y

(3) David LA, Maurice CF, Carmody RN, Gootenberg DB, Button JE, Wolfe BE, Ling AV, Devlin AS, Varma Y, Fischbach MA, Biddinger SB, Dutton RJ, Turnbaugh PJ Nature. 2014 Jan 23; 505(7484):559-63.

(4) Segata N. Gut Microbiome: Westernization and the Disappearance of Intestinal Diversity. Curr Biol. 2015 Jul 20;25(14):R611-3. doi: 10.1016/j.cub.2015.05.040. PMID: 26196489.

(5) Mosca A, Leclerc M, Hugot JP. Gut Microbiota Diversity and Human Diseases: Should We Reintroduce Key Predators in Our Ecosystem? Front Microbiol. 2016 Mar 31;7:455. doi: 10.3389/fmicb.2016.00455. PMID: 27065999; PMCID: PMC4815357.

(6) Holscher, Hannah D. “Dietary fiber and prebiotics and the gastrointestinal microbiota.” Gut microbes vol. 8,2 (2017): 172-184. doi:10.1080/19490976.2017.1290756

(7) Chambers, E., Morrison, D., & Frost, G. (2015). Control of appetite and energy intake by SCFA: What are the potential underlying mechanisms? Proceedings of the Nutrition Society, 74(3), 328-336. doi:10.1017/S0029665114001657

(8) Koh A, De Vadder F, Kovatcheva-Datchary P, Bäckhed F. From Dietary Fiber to Host Physiology: Short-Chain Fatty Acids as Key Bacterial Metabolites. Cell. 2016 Jun 2;165(6):1332-1345. doi: 10.1016/j.cell.2016.05.041. PMID: 27259147.