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Dietary fiber - steroids for your gut microbes or all just hot air?

Scientists have long known the gut hosted a huge number of microorganisms from bacteria, fungi and viruses - but the exact makeup of this ‘Gut Microbiome’ was uncertain. That all changed with the Human Microbiome Project. Equipped with better knowledge of the gut microbiome, many of us are now on the quest to nourish our microbiome and unlock a seemingly ever-growing list of health benefits.

BUT, with an explosion of commercial products now claiming to boost your gut microbes, it’s hard to know how to support your precious bugs inside your gut. Luckily for us, a simple and once overlooked product is now considered the steroid of the microbiome world - fiber!

What is Fiber?

Fiber is not a single entity, but a collection of diverse plant-derived carbohydrates, each with their own unique biological effect. There are a number of ways that fiber can be subcategorised based on its resistance to digestion, solubility, viscosity and fermentation. However, common to all dietary fiber is the fact that they are indigestible, passing straight through the stomach and small intestine with little structural or molecular changes.

This means that fiber reaches the large intestine (or colon) relatively intact, increasing viscosity and making for “pleasantly soft stools” says Dr Hermie Harmsen, microbiologist from University Medical Center Groningenin the Netherlands. Without breaking down within the gastrointestinal tract, fiber was historically viewed as fairly useless. However, in the early 70s, an astute physician by the name of Denis Burkitt published a seminal research paper challenging the worthlessness of fiber. His epidemiological evaluations in Uganda revealed that in these communities, where people lived predominantly off home grown food and vegetables, the incidence of common westernised diseases was drastically lower. Critically, he observed that people with high vegetable/fiber intake defecated much more regularly and with less discomfort compared to the westernised world. Dr Burkitt had a strong but simple rhetoric, dietary fiber is central to overall health and he became known as ‘The Fiber Man’.

In the years to come, Burkitt and others failed to show any meaningful benefits of fiber across a variety of clinical scenarios, in particular Irritable Bowel Syndrome (IBS) and his claims were soon considered "hot air". Consequently, fiber science took a significant hit with scientists and clinicians alike ignoring key factors that may explain the inconsistencies between Burkitt's observations and the disappointing clinical results. Now, in the midst of the microbiome revolution, dietary fiber is once again under the spotlight, forcing gastroenterologists and scientists to reconsider the health impacts of dietary fiber and the best diet for it.

Fiber Feeds our Gut Microbes

For both scientists and the health-conscious, the discovery of the human gut microbiome has undoubtedly been the sexiest topic of the 21st century. If you haven’t already come across the concept of the microbiome, here is a quick run-down to get you up to speed. Essentially, we are not as human as we think. In fact, we have over 1 trillion microbes that call us home, with approximately 1.3 times the number of microbes in our body than human cells. Collectively, the microbes that inhabit our gut (the gut microbiome) are particularly prolific, with 95% of all human microbes residing in our intestines. These microbes can weigh up to 2 kg, so every time we go to the toilet, we become more human by expelling alot of bacteria.

The fact that fiber passes through our guts with minimal breakdown means that it is a hugely abundant food source for our gut microbiome. Dr Jens Walter, from the University of Alberta explains “The mutualistic relationship that humans maintain with their gut microbiome has evolved over millions of years in a dietary environment that provided more than a hundred gram of dietary fiber per day. So our gastrointestinal physiology, and even our immune system and metabolism, are adapted to a high provision of dietary fiber”. This means that our gut microbes have become experts in degrading complex fibers.

In fact, our microbes have “adapted to fiber degradation for millennia, [and] do not see “lettuce, carrots or apples”; they see molecular structures such as cellulose, hemicellulose, xylans, pectins… [and as such] have acquired a specialized enzymatic equipment that allows them to benefit from their breakdown”, says Dr Joël Doré, a French microbiologist. It is for this reason that the benefits of fiber are thought to be due to the interaction with our gut microbes.

The benefits of fiber on the gut microbiome were elegantly demonstrated in 2014, with research showing that different diets drastically alter the gut microbiome. Authors demonstrated that a plant-based diet, which was very high in various fibers, caused a significant increase in the diversity of the gut microbiome, a feature of the microbiome commonly associated with favourable outcomes for the host. This is because “Each fiber molecular structures will hence be degraded by a limited set of highly specialized microbes” says Dr Doré. This means that “providing a diversity of fibers will open a diversity of ecological niches and thereby promote a diversification of the dominant microbiota. In turn this will promote a diversification of the overall microbiota” he explains.

Does More Fiber = Less Disease?

It is clear that fiber supports our gut microbes, with 93% of our MetaFact Experts agreeing that a high-fiber diet is good for the gut microbiome. But before you clear the supermarket shelves of MetaMucil, we must recognise that ‘it is less clear that this benefit to the microbiome is also beneficial to the human host’, warns microbiologist Dr Volker Mai.

The gut microbiome has been implicated with both the promotion of health and the initiation of disease. Several diseases, both in and outside the gut, have been associated with a ‘injured’ microbiome, characterised by a loss of diversity as well as changes in the amount and types of bacteria present in our gut. However, let’s be clear - in many cases these findings are simply correlative, not causative. Despite the correlative nature of these findings, they have sparked significant interest in the use of fiber to counteract or prevent disease.

Enrichment of the diet with resistant starch (a form of fiber that reaches the colon) has been shown to increase the levels of short chain fatty acids (SCFAs), the by-products of fiber breakdown. These SCFAs can help dampen inflammation, enhance the integrity of the intestinal lining and inhibit the growth of pathogens. It is via these mechanisms that we think fiber may be useful in promoting infection resistance, minimising the risk of colorectal cancer, easing the burden of IBD and even helping prevent extra-intestinal diseases such as allergies, obesity, diabetes, Parkinson’s disease, depression/anxiety and cardiovascular disease. However, it is important to note that while there is supportive evidence for the use of fiber in these diseases, contradictory evidence also exists, and more rigorous human trials are required.

How much fiber a day?

With the large majority of our experts agreeing that a high fiber diet is good for your microbiome, it is important to emphasise that more does not always mean better, and like most things in life, it’s all about balance says Dr Hermie Harmsen. Current recommendations suggest that adults should consume 30 g of fibre a day (currently most adults consume ~18 g). This can be achieved by increasing foods such as cereals, legumes, nuts, vegetables, fruits and grains. Simple habits such as choosing wholegrain bread and leaving the skin on fruit and vegetables can also help increase your daily intake of fiber. Take note though, excess fibre (>70 g / day) is associated with bloating, gas and even constipation, with long-term excessive fiber intake suggested to “inhibit the growth of some microbiota” as cautioned by Dr J Lu, an expert from the Institute of Disease Control and Prevention, China.

It’s Personal

While fiber science has returned to the spotlight, uncertainty remains on the impacts of fiber supplementation on the microbiome and its potential role in disease prevention. While these inconsistencies have been viewed as disappointing for some, others have cleverly exploited this opportunity to understand exactly why fiber affects people differently.

We know that our gut microbiome is highly individualised. Based on this knowledge, and the intimate relationship between diet and our gut microbes, it has been hypothesised that the variation in response to fiber may in fact be governed by our own unique gut microbiome. A recent study from Denmark supports this concept, demonstrating that the response to fiber supplementation was dependent on the baseline microbiome composition of the individual. These findings, and those of others, have paved the way for personalised nutrition and diets aimed at enhancing the benefits of dietary intervention via an understanding of the microbiome. There is a now a large study underway in Israel tailoring food choices on a very personal level.

Want happy and healthy gut microbes? Skip the probiotics isle at the supermarket. Instead, eat a balanced diet full of fruit, vegetables and whole grains. Aim for 30 grams per day. Simple tricks include choosing wholegrain bread and leaving skin on fruits and vegetables when you can.

Original article from MetaFact (Hannah Wardill)

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