Dietary fibre
No. Even the Dietary Guidelines for Americans (as issued by the US Department of Agriculture, 2015–2020) have acknowledged that: “Other food components, such as dietary fiber, while not essential, also are considered to be nutrients.” Nevertheless, recommendations to eat fibre riddle the document. That quote is hidden away in Appendix 6 — Glossary of Terms.
The argument is that while we do not need fibre, we will be healthier if we eat it. This idea has been around a long time, however it was a medical missionary (from the UK) who worked in Uganda (while it was still a British colony) who became the principal advocate for the Fibre Hypothesis, earning him the nickname of ‘Fibreman’ (Dr Denis Burkitt). His thinking was that Ugandans ate more fibrous foods than westerners, and they had fewer western diseases. This is an association that does not mean anything at all — Ugandan diets, ancestry, lifestyle and society differ from that of Westerners in multiple ways, any or all of which could influence health.
Nevertheless, the concept took off rapidly. Grave nutritionists admonished those not eating ‘enough’ fibre, and Big Food saw a chance to add cheap fibre to their products and promote them as healthy (fibre that might otherwise have gone to cattle feed, for example). Fibre became a commodity in itself (e.g. bran).
However, dietary fibre advice was being given, and accepted, without the science to support it. This is common in the field of nutrition — the science is difficult and time-consuming, and there is a perceived urgency to act on an apparently plausible hypothese without waiting for scientific validation. Unfortunately, because of the time-lag spent scientifically testing an hypothesis, the nutritional advice can become entrenched in public thinking by the time the science becomes available (assuming it ever gets done) — even science disproving the hypothesis doesn’t matter by then (it can easily be marginalised). Certain hypotheses become so entrenched it would take a juggernaut to turn the thinking around — science alone is not up to the task.
Dietary fibre
Dietary fiber is not a single concept, however, it can be thought of as indigestible or poorly digestible carbohydrates and other plant-based materials. Dietary fibres have multiple physical and chemical properties that, when combined with other properties of the foods they are in, or eaten with, have the potential to influence many biological systems.
Regularity
It seemed intuitive that fibre would benefit regularity and colon (large intestine) health — a sort of colonic cleanse.
Throughout history bowel irregularity has been considered to be unhealthy, for example, there’s an Egyptian papyrus on the matter dating to the 16th century BCE. By the early 19th century (of the current era), medical authorities had identified constipation of various degrees as a common denominator of multiple diseases of urbanisation. The ‘intestinal autointoxication’ theory was put forward — that faecal putrefaction in the intestine produced toxins that could be absorbed into the bloodstream and cause malaise, and that daily evacuation was necessary to rid the body of its sewage and reduce autointoxication.
This theory became widely accepted and an anxious public was receptive to foods, drugs and even bizarre devices that promised to improve regularity. This was the golden age for laxatives, and for procedures such as enemas and colonic irrigation. Even surgical removal of the colon was not as uncommon as it should have been.
However, by the 1920s, it came to be realised that bowel toxins did not cross into the bloodstream in healthy individuals, and the theory of autointoxication gradually went out of favour (although modern leaky-gut syndromes may revise that). Still, the fear of constipation did not go away, and high dietary fibre became synonymous with regularity.
This has been questioned by a 2012 publication describing an intervention study of chronic constipation. Even so, this was considered to be a pilot study, and the results interpreted cautiously. Sixty-three patients with medically diagnosed chronic constipation, who were already on a high fibre diet or taking fibre supplements, were asked to go on what the investigators called a ’no fibre diet’ for two weeks. It is not entirely certain what made it ‘no fibre’, however participants were asked to stop vegetables, cereals, fruits, wholemeal bread and brown rice (white bread and polished rice seemed to be allowed, and strained fruit juice). After two weeks, participants were asked to consume as little dietary fibre as they could manage over the longer term (this was an acknowledgement that diets need to work in a lifestyle context). At 6 months, 41 participants had remained on the ‘no fibre diet’. Everyone in this group was regular (an average of 1 movement per day), and none had any of the other symptoms associated with constipation (boating, bleeding, abdominal pain). Sixteen participants maintained a less-strict reduced-fibre diet, and 12 of these (75%) were also constipation-free. Six participants went back to their baseline diet, and all returned to a state of constipation.
These findings give pause to rethink the fibre-regularity hypothesis, at least in the chronically constipated. The notion that bulking faeces with fibre will ease defecation is not supported by these data. The problem with constipation is difficulty in evacuating a packed rectum, and adding to faecal bulk with fibre is only going to make this worse. As the investigators put it, it’s like trying to clear a traffic jam by putting more cars on the road. Furthermore, fibre will increase microbial fermentation, generating gasses that are not easily released in the presence of bulky faeces and that increase bloating and abdominal pain. In their summary, the investigators say ‘more studies should be undertaken to confirm or repudiate these results.’ As far as I can tell, there have been none. In the field of nutrition, discordant science gets marginalised.
There may be some groups or individuals that would benefit from an increase in dietary fibre. However, a 2019 mini-review of the other end of the spectrum had this to say ‘The use of dietary fibre for the treatment of diarrhoea … is poorly researched’. There are cases to be made for either increasing or decreasing dietary fibre to treat diarrhoea.
Despite all this uncertainty, dietary authorities insist on certain levels of daily fibre intake for the general (presumably healthy) population. Where did these numbers come from? As usual, I will focus on US guidelines because of their global influence.
Adequate Intake
The US Institute of Medicine (IoM) is responsible for setting recommended adequate intakes (AIs) and other intakes (recommended daily intake etc) for a wide-ranging list of foods. Their current report, entitled “The Panel on Macronutrients. Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids (Macronutrients): The National Academies Press” was released in 2005. As the title suggests, this is an enormous report. What did they have to say about fibre?
To begin with, they acknowledge: “… Fibers are not essential nutrients, so inadequate intakes do not result in biochemical or clinical symptoms of a deficiency.” Thus, the Dietary Guidelines for Americans (mentioned at the opening of this post) and the IoM are in agreement — fibre is not essential, but both organisations insist that we are better off consuming it all the same.
Thus, the IoM looked for potential health benefits of fibre consumption. While fibre is hypothesised to have wide-ranging potential health benefits, the IoM focussed on just one — heart disease. They reviewed the literature, and while admitting there were contradictory studies, they nevertheless selected 3 cohort studies to base their recommendations. A cohort study is a type of association study where a large number of individuals are recruited and given a food questionnaire to fill in. They are followed for a number of years to see who develops heart disease, then this outcome is compared with their food choices — for example, does heart disease associate with low dietary fibre. These studies produce evidence so weak that there can be little or no confidence in the conclusions. They cannot show causality and there are multiple confounds — for example, did high-fibre eaters get their fibre from whole vegetables and grains, whereas low-fibre eaters had a diet high in processed junk food. What might be the health consequences of that difference. Statistical attempts are made to correct for these kinds of confounds, however, they are poor assumptions given the multitude of dietary and lifestyle factors and other unknowns in play.
Furthermore, the participants in these 3 studies were not representative of the general US population. One studied Finnish men, another studied registered female US nurses and the third, male US health professionals (the last 2 professional groups being compliant and accessible to followup). Each of these studies managed to find that heart disease was somewhat reduced with a high fibre diet. It is assumed that nothing else in the diet, profession or lifestyle mattered.
Pressing on, the IoM pooled the results and looked at the total range of fibre intake for participants across the studies. This range was divided into five equal sub-ranges (i.e. in 20% steps from lowest to highest). The consumption of fibre in the top (highest) sub-range was set to be the recommended AI.
Two things to note: 1) In the end, heart disease didn’t matter — the AI was simply set to that of the participants with the highest fibre intake; 2) The immediate effect was that most of the population now had, by definition, inadequate fibre intake.
This has led to campaigns to eat more fibre and, in the US, for dietary fiber to be declared a “nutrient of public health concern”. This is a contrived outcome, the IoM methodology created an imaginary fibre deficit.
Finally, to determine actual values, the IoM used calorie intake data from the studies and concluded that we should eat 14 g of fibre for every 1,000 Calories consumed in the diet. Using standard reference values for total Calories, the recommendations became: 38g/day (men, 19–50 years of age); 30g/day (men, > 50 years); 25g/day (women, 19–50 years); 21g/day (women, > 50 years). There were also recommendations for children and adolescents, even though neither of these groups were represented in the cohort studies.
Getting these numbers is a triumph, because now no-one need think about where they came from again.
How achievable is the fibre AI?
An AI of 30g (for a mature male) doesn’t seem as though it would be a challenge to achieve.
What if we decided to get the 30g of fibre from fruit? Everyone knows that fruits are high in fibre. So, take a medium (182g) apple and a medium (118g) banana as examples (data taken from the USDA database). The apple has 4.4g fibre, and the banana 3.1 g. To get 15g of fibre from each fruit (to make up the AI of 30g), it would be necessary to consume 3.4 apples and 4.8 bananas — in other words 8–9 pieces of fruit with a total weight of 1.2 kgs (620g apples, 566g bananas). Furthermore, this amount of fruit contains 134g of sugar — the equivalent of about 27 teaspoons of table sugar. Combined with other carbohydrates in the fruits, it comes to over 800 Calories. Remember, this is a daily intake.
Well, we know that fruits are high in sugar, so what about a ‘whole’ grain instead — brown rice, for example. To get to the 30g AI, it would be necessary, on a daily basis, to consume almost 10 cups of brown rice (1 cup of brown rice contains only 3.1g fibre). While brown rice is not high in sugar, it is high in starchy carbohydrates that contribute calories. Ten cups of brown rice contain 2,300 Calories — virtually the entire recommended daily caloric intake for most adults.
It doesn’t much help to switch to a non-starchy, low-carbohydrate plant food such as spinach. One cup of raw spinach has just 4.3g of fibre, so 7–8 cups would be needed to reach the AI. Perhaps, this might be achievable (and varied with other low-carbohydrate vegetables), although it would still amount to 310 Calories in the case of spinach. As well, there is always the risk of unintended consequences — spinach is high in oxalic acid that inhibits the intestinal absorption of metals (iron, calcium, magnesium, zinc etc). Furthermore, the official dietary guidelines are not recommending a low-carbohydrate diet, rather the opposite. Hence, the Institute of Medicine’s guidelines for fibre are not easily reconciled with the US Department of Agriculture’s guidelines for a high-carbohydrate diet. Presumably, this is a result of the involvement of two different government organisations that may not have strong lines of communication.
There may be other ways to achieve the recommended AI, however, to incorporate natural fibre into a diet at the recommended levels usually means taking something else out that will almost inevitably be more nutritious than the fibre. The potential for negative health consequences of meeting the AI has not been addressed.
This leaves, for most people, fibre supplements such as ‘All Bran’ (Big Food is delighted). However, be aware that manufacturers usually add sugar to make these palatable (a 50g serving of ‘All Bran’ contains 15g fibre and 10g added sugar), and that they are likely to be eaten with other sugars (e.g. milk lactose, seasonal fruits, dried fruits, sweetened cereals, honey).
Further, a safe upper limit for dietary fibre has not been established. Conceivably, with supplements and determination, such a limit could be breached. It would be expected that excessive intake and colonic bulking would come with risks to the health of the intestinal system. A postulated example is diverticulosis — a kind of outward pocket or bulge of the colonic wall (most common in the elderly), that could be exacerbated by the transit of roughage (that could get trapped in the bulge), increased force on the colon wall as fibre (soluble) takes up water and expands, and internal pressure from the gaseous byproducts of fermentation. Another example is constipation, as discussed earlier.
Rather than risking health to achieve an AI, it would be better for guidelines to admit that their AI is not achievable for most people. That it is not achievable naturally should serve as a warning. Furthermore, guidelines generalised at the population level do not acknowledge our biodiversity at the level of the individual. In the case of regularity, individuals can decide for themselves. In general, guidelines for specific nutrients or foods (e.g. fibre, but also salt, fat, cholesterol, water) are ill-advised, as we don’t eat nutrients in isolation but rather in complex combinations with different consequences.
Does fibre protect against colorectal cancer (CRC)
This hypothesis has been around for decades. According to the US National Cancer Institute (2018 report): “There is no reliable evidence that a diet started in adulthood that is low in fat and meat and high in fiber, fruits, and vegetables reduces the risk of CRC by a clinically important degree.”
Other health claims — the Europe experience
The European Food Safety Authority allows manufacturers to indicate dietary fibre content on their food labels, but does not allow manufacturers to attribute health benefits to the fibre. Specifically (2019 report), it is not permitted to claim that fibre helps with any of the following: a healthy immune system; blood lipid levels; the cardiovascular system; cholesterol; blood sugar levels; glycaemic response; reduced fat absorption; body weight or; normal colonic function. In their evaluation, none of these health claims have been substantiated.
Fibre and gut microbes
Our gut microbes, collectively known as the microbiota, are diverse and numerous communities that can directly or indirectly modulate our biology in favourable ways. Therefore, it is in our best interest to maintain the function and diversity of this community. Dietary fibre is frequently promoted as an energy source for the microbiota and as a means for supporting their functions.
Unfortunately, we do not know what constitutes a healthy microbiota, how best to promote and maintain it, or how to account for differences in its makeup across individuals. The best we can say is that the degree of diversity in the colonies of microbiota seems to associate with better health. The problem with this is causation: we don’t know whether people are healthy because they have diverse microbial ecosystems, or whether they have diverse microbial ecosystems because they are healthy. It’s the ‘Association dilemma’ again.
If we accept that diversity matters (and this may still not be the case), then why skew that diversity by preferentially providing energy to a subset of gut microbial communities that ferment plant fibre? We know that microbes that ferment proteins (amino acids) also benefit us. Microbes both utilise and synthesise amino acids (including essential amino acids) and produce other byproducts comparable to those from fibre fermentation What’s more, microbes need more than carbohydrate to grow, divide and replicate — all microbes need other building blocks such as nitrogen or sulphur (available from proteins), minerals, metals and fatty acids for their structure, inner functioning and replication.
The gut microbiota are not a homogeneous microbial system, various taxa (microbial classifications) compete, cooperate and respond to differences in macronutrients, micronutrients, and elemental availability as determined in part by the diet, and in part from our own cells lining the intestinal wall (released into the gut as they are replaced), mucus secretions or immune signalling. In the human, microbial colonies adapt within a day following a change in diet from a standard western diet to one based exclusively on plants or exclusively on animal products (that is, within a day after the diet reaches the colon). They adapt back again within 2 days of a return to the baseline diet. This is a complex, dynamic and intertwined ecosystem that almost certainly was a significant feature of our evolutionary success. It is naive to imagine that it’s all about the fibre.
Ketogenic Diets
A well-formulated ketogenic diet will contain fibre from low-carbohydrate above-ground vegetables. However, this may not be all that important. There are three forms of ketones produced by the liver on this diet, and the most studied is beta-hydroxy-butyrate (BHB). The most studied short-chain fatty acid produced by gut microbes is butyrate. Butyrate is beneficial, and a source of energy for cells of the gut wall. However, the liver can also convert butyrate to BHB (BHB is butyrate with an added oxygen-hydrogen (OH) pair). It has been suggested that it is this conversion to BHB that underlies the benefits of butyrate. It follows that a ketogenic diet may not rely so heavily on microbial fermentation for butyrate — there is a large pool of circulating BHB.
Intermittent fasting
Perhaps counter-intuitively, periods of intermittent fasting or time-restricted eating (eating restricted to say, a 6 hour window each day) may increase microbial diversity and may have benefits for the intestinal wall. Cycling the nutrient availability profile may allow microbes normally out-competed by carbohydrate-utilising microbes to regain their numbers. There may also be increased protection against pathogens. Given our ancestral feeding patterns, it is perhaps not surprising that feeding-fasting cycles could benefit our gut communities, or alternatively, that those communities have adapted for the inevitability of feeding-fasting cycles. Less is known about prolonged fasts.
The Carnivore Diet
Many people follow a plant-free, animal-only diet: meat, organs, fat, seafood, eggs and dairy. They remain healthy because animal products are generally nutritious and well-absorbed, and because plant carbohydrates are not an essential part of our diet. Even the Institute of Medicine acknowledges this when they report: “The lower limit of dietary carbohydrate compatible with life is apparently zero, provided that adequate amounts of protein and fat are consumed.” Note the use of ‘is apparently’ rather than ‘is’ — they seem bewildered by this fact. They go on to assert that while it is not needed, some carbohydrate can optimise health (the same rationale they used for dietary fibre). People going on a carnivore diet experience gut (and other) issues during the adaptation phase, but then everything seems to settle down. Some people use this diet to reverse gut symptoms they were experiencing on an omnivorous diet, such as inflammatory bowel disease.
Summary
Dietary fibre is not essential, but may benefit some individuals.
Guidelines for an adequate intake of fibre are skewed towards he highest fibre consumers, creating an automatic inadequacy for the general population.
Guidelines for an adequate intake of fibre are not easily achieved with regular food intake, other than perhaps with a low-carbohydrate diet.
Achieving recommended intakes could lead to adverse health outcomes if other foods are omitted from the diet to maintain caloric balance.
Excess fibre could lead to gut disorders such as constipation.
Our gut microbial ecosystem needs more than fibre to be diverse and to thrive.
Disclaimer:
I am not a medical doctor. Nothing herein is, nor should be taken to be, medical advice.
