This piece was written by one of our contributors; immunologist and university lecturer – Jenna Macchiochi.
“Disease is rarely eliminated through early diagnosis or good treatment, but prevention can eliminate disease.”
Dr Denis Burkitt, Surgeon and research scientist 1911 -1993
The nutritional importance of dietary fibre has been long recognised. Surgeon and research scientist Denis Burkitt is probably best known for identifying (and naming) Burkitt’s Lymphoma following his observations of swelling in the jaws of children in Sub-Saharan Africa in 1958 (1). But with his remarkable ability to observe patterns of ill health, identify their peculiarities, and develop scientific hypotheses, he also helped put fibre on the health map. In fact, he became known as the ‘Bran Man’ following an important, yet causal, observation in Uganda that local people produce several times more faeces than western people.
In the 1970s, Burkitt hypothesised that a major cause of the growing incidence of non-communicable (non-infectious) disease in western society was due to the low consumption of dietary fibre, comparable to traditional diets he had observed in Uganda. “Western diets are so low on bulk and so dense in calories, that our intestines just don’t pass enough volume to remain healthy”. This sweeping, correlative assumption lacked persuasive scientific evidence at the time (2).
But in 1984, cereal manufacturer Kellogg took Burkitt’s message as dogma, placing a message on its bran cereals claiming scientific evidence linked eating a high fibre cereal with an overall reduction in non-communicable disease risk (3). This perhaps marked the start of exploiting and overselling scientific claims as a marketing tool based on sometimes tenuous links to health. We now know that the links between diet and disease are much more complex, but some of Burkitt’s original fibre hypothesis has helped galvanise today’s science of nutrition.
If 2018 was the year of gut health then 2019 was definitely fibre’s turn to hit the limelight. But the two are not mutually exclusive. Adequate intake of dietary fibre is increasingly being recommended by governmental public health agencies as a means to maintain and increase health and well-being. Some epidemiological studies have shown support for an inverse relationship between dietary fibre consumption and risk of some chronic diseases (4). Fibre has been getting some quality airtime recently in light of this recent landmark study published in The Lancet January 2019 (5). A diet rich in fibre does indeed appear to be important for digestion but also for heart health (6) weight management (7), and reduces the risk of developing diabetes (8), as well as autoimmune disease (9) and many other conditions. In fact, the evidence shows that fibre’s health benefits appear to extend beyond any particular ailment: eating more fibre seems to lower all-cause mortality* (10).
Yet despite now having a clear picture that fibre is good for us, our understanding of the actual mechanisms by which fibre nutrition pertains to health has trailed behind. A primary obstacle to this is that fibre is not a single nutrient but a collective group of carbohydrates or carbohydrate-containing foods. This makes conducting studies on fibre intake challenging and affects the power of scientific analyses. Secondly, there are still some mis-understandings of how much fibre is needed to meet our needs. But also lacking is a comprehensive understanding of many mechanisms by which fibre carries out its biological effects.
*All-Cause Mortality is a term used by scientists and medical professional to refer to death from any cause. The term All-Cause Mortality is used in reference to a disease or harmful exposure in a statistical measure of death during a specific time.
Which brings us to one of the little-known facts about fibre: although not often considered an important ‘immune-booster’, fibre actually plays a vital role in the overall capacity of our immune system to do its job. When people ask me how to boost their immune system – to which my first answer is, “you can’t” – fibre is definitely up there because of it’s important, yet underrated immune strengthening properties.
So, let’s take a deeper dive into what this actually means…
The Immune-microbial alliance!
Most people tend to think of the immune system in its capacity to protect us from infection. But the immune system does so much more than that: from healing and repair to determining the success of a pregnancy and regulating body weight; it’s even our main anti-cancer surveillance.
When we are born, our immune system is pretty much a black canvas. From day one of our lives, immunity is continually shaped and educated, nurtured by our encounters and adventures, educated by our changing emotions and surroundings (11). The impact of our lifestyle on the immune system is more than the genes you are born with (12). This is common sense. No other system in the body places such a premium on adaptability (except possibly the brain). If our immune systems can’t counter the diversity in our ever-changing environment by adapting and responding we’re dead ducks. And with the majority of our immune system located in our digestive tract, much of this environmental interaction is conducted by the rich and dynamic ecosystem of microbes that live in us –– collectively known as our microbiota.
Despite the fact that many of us might actually fear germs, we have co-evolved in a microbial world and these fibre-loving gut bugs are actually our biggest health allies. When operating optimally, this immune-microbial alliance interweaves the various arms of immunity in a dialogue that selects, calibrates and terminates our immunity as we need it.
During digestion, the food we eat bathes in digestive enzymes which break it down and permit absorption of nutrients. We actually make a fairly limited range of digestive enzymes, leaving some of the fibrous parts of our food undigested. But fibre is indigestible only to us. Those healthy gut bugs eat what we eat, fermenting fibres we are unable to digest. Thus, dietary choices and patterns that are rich and diverse in fibre are reflected in the number and variety of bugs in our gut.
Nurturing your gut bugs with fibre is probably your best bet to getting the most out of your diet, which is only ever going to be as good as the bugs in our guts that eat it too. And it’s not just the fibre in our diet that nurtures our immunity, but also the numerous polyphenols that are often conveniently contained within fibre rich foods.
Educating our immunity from cradle to grave
Our immune system is made not born. And it does not evolve in isolation, rather it ‘grows up’ with germs of all kinds, good and bad. A huge array of studies (13) clearly demonstrate that the composition of our gut microbiota has some quite unexpected effects on our immune system, starting from the day we leave our mother’s womb (and perhaps even before) (14).
The first years of our lives is a period of critical immune development stimulated by environmental exposures that shape the composition of the microbiota from the first minute of life as we enter this microbial world.
Experimentally, when animals are reared without a microbiome their immune systems look distinct, right down to the anatomy of immune organs such as lymph nodes and gut border patrol stations known as Peyer’s Patches. Depending on how our own unique microbial fingerprint develops during these formative years, certain elements may be introduced to the developing immune system with lasting effects, potentially predisposing us to allergy (15), autoimmunity (16) and inflammatory disease later in life (17). While many of the factors influencing microbiome are out of our control (e.g. type of birth, geographical location, frequency of antibiotic use, amongst others), we know that dietary fibre is an accessible tool to rapidly and reproducibly nurture our gut microbiome, and consequently influence the developmental course of our immune system (18).
This all starts with breast milk, which contains indigestible simple fibres called prebiotic glycans (19). For a long time, scientists puzzled over the role of these glycans in breastmilk until it was discovered that they are specifically designed to feed the growing gut biome and encourage correct development of the immune system. As we mature through childhood our food sources diversify, continually shaping our immunity blueprint for our long-term health (13). This relationship between the immune system, dietary fibre and our microbiome continues throughout our lifetime.
One’s trash, another’s treasure
Not only does our microbiome carry genes for some of the enzymes needed to fully digest our food, helping us liberate many of the phytonutrients, vitamins and minerals from our diet, but they also produce a veritable banquet of metabolic by-products known as ‘postbiotics’ (20). Consider it your own personalised pharmacy. A fibre-rich diet is one way to take full advantage of the health potential of postbiotics, and we need to give the microbiome the foods they need to maximise production. In light of this, the study of how our microbiome influences our health has shifted from cataloguing individual members of our microbial community, to unravelling the interplay between how what we eat influences production of these bioactive compounds.
There are numerous postbiotics which have been identified with a plethora of immune-nourishing effects that we are only just beginning to understand. For example, it was recently shown that one of the beneficial effects of omega 3 fish oil supplements is actually down to our gut bacteria producing an anti-inflammatory metabolite called n-carbamyl glutamate (21). But not everyone can produce this chemical from their mix of microbes, explaining why these supplements don’t always work (and perhaps why some of the clinical trials have given conflicting results).
The short chain fatty acids (SCFA) acetate, propionate and butyrate are probably amongst the best-studied examples of these beneficial microbial by-products called postbiotics which act at the interface between our diet and our immunity (22). A particular group of microbiome bugs called Bacteroidetes are super producers of these SCFAs. One well-documented study compared microbes in the poop of 15 European children with 14 children from an African traditional agricultural community (23). Researchers found a significant difference in the amount of Bacteroidetes and Firmicutes between the two groups—the African children had more Bacteroidetes than the European children. Moreover, the African children had significantly more SCFAs than European children.
Our personalised antimicrobial pharmacy
When bacteria break down dietary fibre into these postbiotic metabolites they act as signals to the immune system, setting our immune rheostat at a healthy level of responsiveness against infectious bacteria and viruses. In fact, postbiotics from dietary fibre help us fight infections by turning up the specific virus fighting cells to put us quickly on the road to recovery (24), and have even been shown to help align our circadian clock, which plays a major role in governing our immunity to infection (25).
You’d be forgiven for wondering if antibiotics change the functioning of the immune system. The reality is that taking frequent or long-term antibiotics, which wipe out many of our vital microbiome species, can leave us more vulnerable to infections (26). In a study of 85,000 patients, those taking frequent or long-term antibiotics were more than twice as likely to suffer colds and other upper respiratory tract infections as those patients who weren’t on antibiotics (257. It seems paradoxical – surely antibiotics are there to treat infections, not cause them! Yet although a course of antibiotics might cure one infection, they may also leave us open to others.
Changing inflammatory personality
Now, while inflammation is a vital immune response and fundamental to our health, it is only ever designed to be an acute short-term assault. Triggered incongruously it becomes more than a short-term problem, emerging as the sine qua non of negative health issues and chronic disease. The main day-to-day job of our immune system is not fighting germs but ‘tolerating’ our environment and regulating how we respond to the world around us. Rather than needing a boost, this vital regulatory function actually restrains our immune system, helping prevent chronic inflammatory conditions like allergies and autoimmunity.
Developing this healthy, balanced tolerance and regulation is intimately entwined with our diet and induced by our microbiome. Postbiotics produced when we eat foods rich in fibre influence the personality of our immune cells, changing their function from being pro-inflammatory to one that is pro-regulatory, extinguishing unwanted inflammation. Short chain fatty acids (27) dial down the inflammatory response (reducing all the unpleasant symptoms we experience when sick) and switch on our own internal anti-inflammatory immune regulatory cells (13). This helps activate healing cells that help repair damage and maintain the status quo.
Not just acting locally in the gut, short chain fatty acids circulate in the blood, affecting whole body regulation of the immune system. Hence, a low-fibre diet can cause low-level inflammation not only in the gut, but throughout the body (28).
If dietary fibre affects the composition of the microbiota, and the microbiota regulates inflammatory responses, then it follows that fibre should have beneficial effects on inflammatory disease. And results of studies in this area are highly promising. There is now known to be a significant correlation between low dietary fibre intake and elevated blood markers of inflammation (29, 30), a possible predictor of future heart attacks, and other chronic inflammatory diseases including asthma and allergy (31), autoimmunity and inflammatory bowel disease (32).
Indeed, a study of children who had been given antibiotics before the age of two showed that a startling 74 per cent of them were on average nearly twice as likely to have developed asthma by the time they were eight (33). The more courses of antibiotics the children received, the more likely they were to develop asthma, eczema and hay fever.
Collectively, it may be that the increased risk for inflammatory disease seen by consuming the so-called ‘western diet’ is due to a deficiency in fibre, not because of the much-debated high sugar, salt or fat content.
But it’s not just physical health that is affected by what we consume. Eating a plant-rich diet prevents inappropriate inflammatory signals entering the brain that could otherwise trigger psychological behaviours normally reserved for an infection like the flu (known as ‘sickness behaviours’ including lethargy, depression, social withdrawal and anxiety). In fact, these fibre-derived dietary metabolites inversely correlate with anxiety levels and are now receiving attention as an improved (34), innovative strategy for prevention and promotion of recovery from mental ill-health such as ADHD, anxiety and depression (35).
The gut firewall
For digestion purposes, humans have developed a very complicated and highly specialised digestive system. Separating our gut contents from our body is a delicate barrier that works hard to both absorb nutrients whilst keeping out undigested food, bacteria and potentially harmful things we might end up swallowing. An enormous fraction of our gut immune system’s efforts is aimed at controlling gut barrier integrity, preventing ‘leaky gut’ (yes leaky gut is a real thing, but perhaps not as Dr. Google would have you believe –more on that another time) (36).
Now, leaky gut is a normal physiological phenomenon. Numerous dietary and lifestyle factors affect gut permeability, like excessive fat or fructose, alcohol consumption, heavy exercise and gut microbiota dysbiosis (e.g. after a round of antibiotics). But the idea of a leaky gut leading to chronic inflammation and kick-starting or exacerbating both physical and mental health conditions is an exciting one for medical science (37). A balanced and well-fed healthy microbiota seems to act as a gatekeeper, reinforcing the integrity of the gut and protecting the sanctity of the body (38). The delicate cells that line our digestive tract rely on short chain fatty acids to work properly, multiplying and keeping the barrier tight, and making a healthy supply of mucus and our own blend of anti-microbial molecules (38). Like a mucosal ‘firewall’, this works to keep any potential nasties at bay.
Also important in keeping our microbiome in peaceful coexistence with the immune system, these microbiota rest atop the gut’s mucus layer at a safe distance from the intestinal wall. Any bacteria that wind up too close get wiped out by our own potent antimicrobial poisons. When we eat a fibre- deficient diet our gut microbes starve, which impairs this peaceful relationship (39). Some bacteria may even switch to feeding on our vital gut mucus. With less food for the bugs, the production of their useful metabolites slows and consequently the health of our gut barrier deteriorates. Our gut barrier integrity fluctuates throughout the day depending on what we do and what we eat. We now know that fibre is the key to controlling our gut barrier, minimising the damage caused through the act of eating itself.
Diversity can do it!
The aphorism, “You are what you eat” seems truer than ever before. The tricky thing about fibre is that it’s not a monolith, but an intrinsic part of many whole plants. There are dozens of varieties of plant based foods containing a plethora of different fibres, which leads to a lot of confusion. Our microbiome, and therefore our immunity, depend on a number of different kinds of dietary fibre and it’s possible that not all fermentable fibres are created equally: each type of fibre feeds a particular set of bacteria, which send their own important signals to our bodies (40).
Examples of fermentable fibre that are eaten by a wide variety of friendly gut bugs include prebiotic foods such as:
- pectins – found in fruits and berries ?
- gums – found in seeds ?
- inulin – found in onions, garlic and artichoke ?
- resistant starch – found in bananas and cooked cooled potatoes ?
- green leafy vegetables which contain a pre-biotic known as sulfoquinovose ?
Many foods that are high in fibre are also high in fermentable fibres known as FODMAPs (which stands for fermentable oligosaccharides, disaccharides, monosaccharides and polyols). These foods can trigger digestive upset in some people such as those with irritable bowel syndrome so alter your fibre with care and under the guidance of a nutrition professional if you are concerned.
The composition of our own unique microbiome sitting at the interface, determining whether we can reap the benefits of one fibre over another. Some specific fibres such as beta-glucans from oats or mushrooms are known to have direct anti-microbial and anti-inflammatory activity. It may not be as sexy as an ‘immune boosting superfood’, and we still have much to learn about fibre to be able to use it as a way to prevent and treat disease, but for those of us striving to feel good, simply adding fibre is a good place to start.
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