Does a high fibre diet prevent disease?

bran flakes dietary fibre heart disease risk

All doctors (and probably most non-doctors) have heard of Burkitt’s lymphoma, a type of cancer found primarily in children living in malaria-endemic areas in Africa. Denis Burkitt was the first person to describe the disease, and also the first person to propose that there was an environmental cause (now known to be simultaneous infection by both malaria and Epstein-Barr virus).

Most doctors probably don’t know that Denis Burkitt is also almost single-handedly responsible for the now widespread belief that dietary fibre is an important part of a healthy diet. Interestingly, Burkitt developed his ideas about dietary fibre after corresponding with a less well known doctor called Thomas Cleave (who hasn’t been allowed to give his name to any diseases).

Cleave was interested in the connection between diet and disease, and had noticed how the transition from a traditional diet to a modern diet, rich in refined carbohydrates, was associated with a massive increase in a large number of diseases, including cancer, diabetes, heart disease, and obesity. He even wrote a book on the subject. Burkitt was introduced to Cleave in the late 1960’s by epidemiologist Richard Doll (himself famous for discovering that smoking causes lung cancer).

Burkitt was deeply affected by Cleave’s ideas, and in particular his conception that all the “diseases of civilization” had a single underlying cause, but he took the data and went off in a different direction with it. While Cleave believed that it was the refined carbohydrates in the modern diet that were causing harm, Burkitt came to believe instead that it was the absence of dietary fibre that was responsible. Since refined carbohydrates are by definition low in dietary fibre, the two things track together perfectly, and it becomes almost impossible to say from observational data which is the causative factor and which is the confounder.

There was a big fly in the ointment of Burkitt’s hypothesis from the start, however, and that was the fact that the Maasai tribespeople in Kenya and Tanzania, who lived on a diet consisting almost entirely of meat, milk, and blood, showed none of the diseases of modern civilization, even though they had virtually no fibre in their diet. Burkitt, who spent much of his career in Africa, was well aware of this fact, but seemingly chose to ignore it because it didn’t fit his hypothesis.

The world of nutrition was at this time (the late 1970’s) focusing increasingly on dietary fat as the cause of modern diseases (based on atrociously low quality evidence and forceful lobbying by diet-heart hypothesis originator Ancel Keys), and Cleave’s hypothesis was inconvenient, because telling people to cut down on both fats and carbohydrates wouldn’t work – people had to eat something. Burkitt’s fibre hypothesis could however be made to fit together with the diet-heart hypothesis without too much trouble. The two were thus wedded and came to dominate dietary advice for the next couple of decades.

That is how breakfast cereals came to be considered a health food, and why we’ve all been told to increase our intake of dietary fibre. Anyway, it’s now a couple of decades later. One would think that by now there would be plenty of data from actual randomized trials to tell us whether we should be eating more dietary fibre or not. Unfortunately we’re still to a large extent stuck with crappy and confounder-riddled observational studies that cannot separate the presence of refined carbohydrates from the absence of dietary fibre, and that therefore cannot actually say anything about what causes what.

The Cochrane Collaboration tried to do a systematic review and meta-analysis in 2016 to look at the state of the evidence when it comes to the ability of dietary fibre to prevent cardiovascular disease. They found 23 randomized controlled trials with a total of only 1,513 participants. Most of the studies ran for only 12 weeks (the minimum length of time the reviewers had set for inclusion in the review, since short term effects are meaningless from a public health standpoint), and the longest ran for only six months. That’s why I say they tried – there simply isn’t enough data to draw any firm conclusions about what effect dietary fibre has on cardiovascular disease risk. 1,513 people followed for a few months provides far too little data to be able to say anything certain.

With that being the case, the reviewers decided to look at surrogate markers for cardiovascular risk instead of looking at hard outcomes like heart attacks and deaths. In other words, they looked at blood lipids and blood pressure. They included trials of both dietary interventions (i.e. that had people eat more food rich in dietary fibre) and trials of fibre containing supplements. I prefer the fibre supplement trials, since the dietary intervention trials have the same confounding issue that the observational studies have – i.e. that an increase in intake of foods rich in dietary fibre virtually always also means a decrease in intake of foods rich in refined carbohydrates. The supplement trials are also at lower risk of bias, since they can be placebo-controlled and thereby blinded. But I’ll report what the diet modification studies showed too, for the sake of completeness. In total, there were fifteen fibre supplement trials and eight diet modification trials.

Let’s get to the results. The fibre supplements were associated with a marginal 0.04 mmol/L reduction in LDL (“bad cholesterol” – yes I know that description is dumb and technically incorrect) that was just about statistically significant. The effect was similar (0.03 mmol/L) in the diet modification trials. For an average person with an LDL of 4 mmol/L this would represent a reduction of just 1%, in other words nowhere near enough to be expected to have any noticeable impact on cardiovascular disease risk.

And LDL is anyway a poor predictor of risk of cardiovascular disease. A much better predictor is triglycerides. The fibre supplements were associated with a 0,03 mmol/L reduction in triglycerides (not statistically significant), while diet modification was associated with a 0,02 mmol/L increase in triglycerides (also not statistically significant). Considering that the average person has a triglyceride level of around 2 mmol/L, this amounts to no noticeable effect on triglycerides whatsoever (and when the supplement and diet modification trials were meta-analyzed together, the difference between high-fibre and low-fibre was exactly zero mmol/L).

When analyzing blood lipids, the final piece of the puzzle is HDL (“good cholesterol”). While you want LDL and triglycerides to be low, you want HDL to be high. Unfortunately, fibre was associated with a reduction in HDL – 0,04 mmol/L in the supplement trials and 0,03 mmol/L in the diet modification trials. Fortunately, that difference was again so small as to be utterly inconsequential.

Ok, so the overall picture is that fibre doesn’t meaningfully impact blood lipids one way or the other. What about blood pressure?

Both the supplement studies and the diet modification studies reported a 2 mmHg reduction in systolic blod pressure and a 2 mmHg reduction in diastolic blood pressure. Considering that the average person has a systolic blood pressure of around 130 and a diastolic blood pressure of 80, this again amounts to such a marginal difference that it’s not going to have any noticeable impact whatsoever on an individual’s risk of cardiovascular disease (in other words, the story when it comes to fibre is the same as the story when it comes to salt – the impact of diet change is far too small to have any noticeable impact on an individual’s heart disease risk).

So the evidence to support the notion that fibre is “heart healthy” is weak. Kellogg’s should definitely stop marketing bran flakes as a “heart healthy” food.

Fifty years ago, when Denis Burkitt started researching fibre and it’s possible health benefits, the first thing he focused on was it’s potential to prevent colon cancer. The notion that fibre might have a role in preventing colon cancer makes a lot more intuitive sense than the notion that it might prevent heart disease, for the simple reason that fibre doesn’t move from the intestine in to the body proper (technically the contents of the intestine are considered to be outside the body), but fibre does have various effects on the intestine, not least of which is the fact that it interacts with the bacteria that reside in the colon.

Colon cancer is thus a good test case for the many claims made about dietary fibre’s health benefits. The Cochrane Collaboration carried out a systematic review in 2017 that looked at the ability of dietary fibre to prevent colon cancer. The review included randomized trials of people who had had polyps removed and that then followed them over time to see if they developed new polyps and/or colon cancer.

Colon cancer usually progresses in an orderly fashion, beginning as a polyp that over time (if you’re unlucky) progresses to full blown cancer, so studies that want to determine risk of colon cancer progression can usually get away with looking at whether new polyps develop rather than having to wait and see whether the participants develop cancer (which saves time and allows for shorter, smaller studies).

Five trials were identified, with a total of 4,798 participants. The average age of the participants was around 60 years at the start of the studies, and they were followed for two to four years. As with the heart disease studies, there was quite a bit of variation in terms of the intervention used, with four trials providing dietary supplements while one attempted diet modification. The trials were for the most part able to at least double people’s fibre intake.

Ok, let’s take a look at the results.

Over the course of follow-up, participants in the high fibre group were 4% more likely to develop at least one new polyp in their colon than participants in the control group, although the difference wasn’t statistically significant. Hmm. Odd. We’d have expected at least some signal of benefit. The trend definitely shouldn’t be towards harm.

But polyps are really just a surrogate marker, like blood pressure is when it comes to heart disease. What we really want to know is whether the high fibre diet protects against colon cancer. Two of the studies were big enough to provide data on this more meaningful outcome.

In these two studies, participants in the high fibre group were 170% more likely to develop colon cancer than participants in the control group. Yes, more. Not less. That difference was statistically significant. So… that’s strange. Admittedly, this result is based on a few thousand participants followed for a few years. It could be wrong. But what it means is that the highest quality evidence currently available suggests that a high fibre diet might actually increase your risk of colon cancer, not decrease it.

How do we square this finding with the observational data that shows a decrease in colon cancer risk with a high fibre diet? As mentioned, the observational data is heavily affected by confounding variables, not least of which is the fact that a high fibre diet usually means a diet low in refined carbohydrates. These results support the notion that Burkitt was wrong and Cleave was right – that the harms associated with a diet rich in refined carbohydrates are due to the presence of refined carbohydrates, not due to the absence of fibre.

So, what can we conclude from all this? Does fibre prevent the so-called “diseases of civilization”?

Well, maybe. That is certainly the impression you would get if you look at the observational data, which find a correlation between a low fibre diet and pretty much any chronic disease you care to look at. The randomized trials that have been done have however for the most part failed to show evidence of a benefit of increasing intake of dietary fibre.

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47 thoughts on “Does a high fibre diet prevent disease?”

  1. Ask some different questions on the physiological effects of prebiotic fibre and the microbiome.
    Are short chain fatty acids, in particular butyrate beneficial for health?
    What is the role of SCFA receptors across the spectrum of organs they are expressed?
    Have you reviewed the studies on fibre in alleviation of diabetes and metabolic syndrome?
    What is the main stimulus for the release of GLP1 and PYY from colonic L cells.
    What is the role of butyrate in colonic epithelial cell energy provision and cell signalling?

  2. I admire you for burying yourself into all these dietary questions, the purpose of which is to find out what in our diet is beneficial, what is likely to be less so. My diet has certainly been modified in my lifetime, but mainly by my wife’s taste as she provides all my meals. Her thoughts and practices in turn have been focussed on organic food production, both vegetable and carnivorous. I suppose we’ve ended up with a ‘balanced’ diet (whatever that is) but I like the statistical approach you use, and the way that it usually shows that there are rarely culprits in our intake, just amounts.

  3. Excellent summary. Thank you.
    But I am mystified by the conclusion of the column. We know that there are some societies in the world that consume zero fiber and they have zero diseases of civilization.
    In this case, doesn’t the lack of association with good health prove the negative—- fiber is not essential for good metabolic health?

      1. Is there any fibre in breast milk?

        I know that comparing the diet of babies to older people with a much higher likelihood of developing cancers or heart disease is not especially useful, but it seems that if fibre were highly protective, then one would expect it to be in breast milk.

    1. …there are hundreds if not thousands of studies showing improved all cause mortality with higher fibre intakes and not one showing any harm….


  4. Very interesting. In general, I am very grateful for your articles, Sebastian.
    I qualified as an M.D. in 1970 and especially in the last 15 years I have not met many colleagues who think critically and dare to go ouside the generally accepted behaviour. Kudos to you!!!

  5. I think you will find the book, “Nutrition and Physical Degeneration – A Comparison of Primitive and Modern Diets and Their Effects”, by Weston Price very interesting. WP wrote of his research in the 1930s among peoples eating “traditional” (not modern) diets all around the world including the Masai (chapter 9).

      1. Marianne – you likely got that impression from mainstream media, which tends to ridicule and demonize anyone who has an idea different than the prevailing conventional wisdom. Read about Weston Price form a wide variety of sources, then draw your own (more informed) conclusions.

      2. On the contrary, Weston A Price was a man way ahead of his time. In his book, just look at the pictures of the people he studied, both before and after the introduction of a modern western diet. A picture speaks a thousand words!

      3. Mainstream media folks probably think of Dr. Bryan Tyson as a quack for treating high risk patients early with antivirals.

        Tyson is now up to 6,000 patients treated–maybe a third are high risk. Of the high risk patients he’s treated early, four have been hospitalized and none have died. Three patients died who showed up at his clinic very late and very sick. Some others who showed up late were sent to the hospital and survived.

        Tyson isn’t criticized directly because of his success. The pharma-captured media don’t want to give Tyson publicity. Tyson has been reported to his state medical board and he received a commendation (just kidding about the commendation–his fellow doctors may commend him privately for his courage).

      4. He is a “quack” if you work in the refined carbohydrate industry or have a limited understanding of science/nutrition.

      5. Weston Price was an impeccable scientist. He was relentless in his search for data, and went to extraordinary lengths to gather evidence about the effects of different types of human nutrition on dental and general development. I recommend his book *Nutrition and Physical Degeneration* to everyone with an interest in health.
        I’m not quite so sure about the current curators of the Weston A Price website – but the work of the man himself is unassailable.

  6. Personal anecdote: 2 colonoscopies in my early 40s due to strong family history of colon cancer; several polyps removed. Always ate high fiber, lots of fruit/veg, chicken/fish, little red meat – had ongoing digestive issues (constipation, IBS). Dug deeper into the science and did some self experimentation and found that vegetables & fiber caused most of my ongoing digestive issues. Now eat mostly red meat, almost no fiber – digestion much improved. Colonoscopy in my mid 50s found no polyps.

    1. Yeah, it’s stories like this that make me wonder whether the complexities of body type and diet are still far beyond our ability to quantify in any meaningful way. If people react differently to foods at such a fundamental level, how can we possibly get to the bottom of what’s “healthy” or not?

  7. I watched a lecture on YouTube by Dr. Paul Mason a few years ago entitled ‘From fibre to the microbiome: low carb gut health’, which had some very interesting observations.

  8. I really enjoy your thoughtful articles Dr. Rushworth. My experience with this subject was cancerous polyps after years of vegetarian/vegan eating. I was surprised and asked the surgeon if he saw less cancer in those eating plant based diets. He indicated that wasn’t his experience.

    I realize that my experience is an N=1 anecdote which is not possible to quantify. I do wonder however, if it is possible to work with people’s experiences in a scientific manner. Or even design useful studies using existing tools.

    In my case, I kept exact records of my food intake for a number of years in Fitness Pal. It was interesting watching my macro ratios change from carb dominant to fat dominant and tracking weight loss after I transitioned to an animal based low carb diet.

    I am not a scientist, but wonder if this kind of data would be useful in a well designed study, where a significant number of people would commit to this type of tracking for a period of time. It actually was not hard and took only a few minutes each day.

    Thanks for all the articles. Much appreciated!

  9. Conclusion? Dr. Rushworth continues to show his credibility by refusing to give a conclusion. Just not enough evidence…but he certainly increased our knowledge and our ability to reject cure-alls…
    I read an old book some 10 years ago, probably Dr. Burkitt’s and decided to try extra fibre, beginning with psyllium and ending up with a blend of 6 parts wheat bran and 1 part ground flax seed, eaten like a grain (e.g. rice) or cereal (e.g. oatmeal), with appropriate spices/additives. My 1st colonoscopy at age 55 was clean enough for the doctor to recommend 10 years till my next one (despite family history of polyps). Is there a relationship? Who knows. But it certainly decreases my appetite while adding negligible calories and seems to benefit me in other ways. As with anything health or medical, do your own experiments and see what happens. It’s only a study of 1, but it’s the 1 that you have the most knowledge of and the most control over…

  10. Although it is claimed that meat has little fibre, is there anything in meat that Acts like fibre? Or is there something about the digestion of meat diet that doesn’t require fibre?

    Thank you Dr. Rushworth for spurring our thinking and questioning, including mine last week of « …and what about the Alpha, Beta, Gamma and Epsilon variants of the pandemic virus? Why do our information sources Only seem to know about the Delta? »

  11. The more I read about the evidence base for doctors’ prejudices on diet, the more I turn to Jeeves for advice. Or Noel Coward: “Something eggy on a tray, please”.

    Anyway, thanks for the piece, doc: more power to your elbow.

  12. Hi Sebastian
    All I can say about Burkitt and fibre is that in 1971 my long term ulcer burst which put me to hospital for an emergency operation which was successful. However, in the late seventies, the ulcer returned and fortunately I found the book THE TRUTH ABOUT FIBRE IN YOUR FOOD by Lawrence Galton and Denis Burkitt which I followed rigorously and which got rid of the ulcer. In 1994 while living in Spain I saw the report that 2 Australian doctors had discovers the tablet called DUAL which, when I told my father-in-law about it it cured his 30 year old ulcer immediately.
    Am I right in saying that fibre is of no use health wise except for ensuring that a daily visit to the toilet is a good sign, as opposed to constipation which, by the way, killed Elvis Presley and could also lead to varicose veins as well as other very serious problems like bowel cancer and diverticulitis et al.
    Many thanks to you and yours
    An avid follower of Dr. Michael Kendrick

    1. “the tablet called DUAL”

      As per, all the hits were ‘phones, iPads & similar Android devices, and the contraceptive pill. Do you happen to know the names of the actual drugs, rather than the brandname?

  13. In the last 10-20 years we have learnt how important it is for overall health to have a thriving Microbiome of the correct composition. It incomprehensible to me that you can have a healthy Microbiome with a low fiber diet. Could it be that the Microbiome composition is another confounding variable?, I would suggest a study where the participants would be chosen to insure they all have a healthy Microbiome and then put half in a high fiber diet and the other half in low fiber

  14. It’s insulin that makes the difference. Hunter-gathers have basal insulin of about 5, Americans and Europeans are 12 or more. Centenarians are on the low side of that interval. I am at 3.5. I insulin drives cell growth and halts autophagy.

  15. I read this article while eating Raisin Bran, and was very disappointed by the end of the article. 🙂

    With age, eating a high fiber diet to prevent constipation pays large dividends, irrespective of any possible reduction in heart disease or colorectal cancer. I have a huge salad with my main meal every day. The vitamins and minerals are good, but the roughage is important too.

    There’s also a difference between soluble (black beans, pears) and insoluble fiber (broccoli, lettuce, celery).

    1. How does eating foods that swell and fill your colon ‘prevent’ constipation? Its fiber that has you bunged up.

      Eat an animal based diet with little to no fiber and you’ll never be constipated again.

      1. Hi Jo
        When fibre, which is of no real help towards ‘healthy eating’ enters the lower bowel the water makes the fibre swell which then triggers a message to the brain that the bowel needs emptying which is no problem because the faeces is very soft. If you have a ‘western’ diet with little or no fibre then the faeces in the bowel stays there in small hard lumps which build up and in time sends that message to the brain only this time it’s too late and you have quite severe constipation which brings along with it a whole gamut of problems.
        Suggest you read THE TRUTH ABOUT FIBRE IN YOUR FOOD by Lawrence Dalton with Denis P. Burkitt
        Best to you and yours

  16. I wonder whether there is more to dietary adaptation than we know. I think just because the Masai or Inuit can live on a certain diet, is not proof the diet works for everyone. I would like to see a study of the role of epigenetics in dietary adaptation, as I believe it could be an important factor.

    1. When Inuit are mentioned as a example of good diet, I always remember that Inuit have the highest incidence of breast cancer on the planet

      1. Since you didn’t post a citation I will post this. Breast Cancer (and all diseases of western civilization) was unknown to the Inuit until the past century. This was well documented by the work of Icelandic American Arctic explorer and ethnologist Vilhjalmur Stefansson.

        This 2011 citation gives a clearer picture of the situation.

      2. Researchers at the University of Manitoba conducted a retrospective study on breast cancer among Inuit and found zero. This was before the advent of chronic disease caused by adoption of refined foods imported from the south.

      1. But the data is from the 1990’s, after the inuit switched to a standard western diet. It is common for indigenous people who have only recently switched to a high carb diet to become much sicker from it than western people, which makes sense, because we’ve had 12,000 years to adapt and they haven’t.

  17. In 1975, while an interne in Saskatoon, I was privileged to attend a lecture by Dr Burkitt at the University of Saskatchewan in Saskatoon, Saskatchewan, Canada, where he presented his views on fiber as prevention for many GI diseases, especially colon cancer, diverticulitis, and appendicitis. He stated that in his 25-year experience as a surgeon in South Africa at a hospital that served many indigenous people (all of whom consumed a high-fiber diet), he had seen only 4 cases of appendicitis. His common-sense statistics and views persist with me 45+ years later! –Carlin G Bartschi, MD

  18. On the Inuit:

    Eat a walrus a day and keep the infarctions away.

    Eating a carb is worse than a harpoon’s barb.

    Jeeves would murmur “If you say so, sir”.

  19. High fiber can cause problems. For me, it happened about 2 p.m. I gulped down 5 psyllium husk capsules like usual, only apparently a couple got stuck and I ended up with an esophageal blockage. I scrambled to find a GI guy at about 4 p.m. when it looked like the blockage wouldn’t clear. Couldn’t find one. Anyway, about 8 p.m. the blockage cleared after lots of water and regurgitation. Whew.

    Be careful with psyllium husk and don’t gulp it like I did.

  20. Yes! Eat more protein and healthy animal fat and you will never have any problems with constipation.

  21. Thanks to the info in Dr. Rushworth’s posts, I have switched to getting my fiber from vegetables, fruit, seeds, and nuts. In the last month, I have lost 12 lbs. (good thing) without changing my net calories or reducing carbs or sugars. I also check labels and avoid high fructose corn syrup in anything. I still have energy to do things.

    Thank you Dr. Rushworth!

  22. So I keep seeing a pattern coming out more and more from folks who are seeking more answers, as the good doctor here. I appreciate his views and ability to lead readers to seek more answers, ask more questions. So I believe that the debate for or against fibre may not, after all, be where we need to look. When many different areas of the world are studied that have centinarians, the ONLY common denominator appeaers to be a healthy, intact microbiome complete with working naturally present digestive enzymes. From one culture to another you see that they eat widely varied diets – and STILL live to around 100 mostly disease free of course, and looking younger than they are. From diets that include even refined carbs and sugars, to high fiber and lean meats, to only fats, oils and meats – and more. Yet that common demoninator still stands. So as I seek healing for myself as an experiment of 1, I have found relief from MANY symptoms by going low carb high fat almost zero sugar (most fruit included). My ailing brain (literally my mind too) and body is less “on fire.” So my next step is to see how I can move forward to recover my microbiome decimated by antibiotics and other such supposedly helpful modern medicine remedies from my infancy. Unbeknownst to my parents and many others, their attempts to protect and prevent, only created a literal bubble against the bad AND the good naturally occurring protections that balance and strength an immune system AND allow for creation of vital neurotransmitters in the gut for brain function and mental wellness. I may still be fighting physical and mental illness – in smaller proportions at least, but now I know where the focus of my battle lies. The researchers who are in this realm are who I have my eye on. My goal is to get to such a place that I can consume any reasonable diet across the world, and my gut is fully stocked with all the good bacteria to break down any foods properly. As of right now, I know exactly what won’t break down in my system, causing immediate pain and inflammation from the release of things like sugars, lectins and oxalates into my body from the digestive tract, because the good bacteria that is expert at breaking those down are simply missing. They too were innocent bystanders- casualties wiped out by modern western medicine, and co-opted by big pharma assisted by big food/sugar/carb industry who partnered up with big government to recommend a faulty diet- all using the playbook from the tobacco industry on making the most money. No more. My quest is laid out before me. I don’t cater to the industries listed above anymore, and I mourn all the great men and women whose valiant desires to help humanity by going into medical and health fields, who were hijacked and decieved in these industries, and who now are trapped into the worst kind of unethical corners, with their livelihoods at stake if they don’t comply. I mourn those who remain trapped or those who consciously give in or choose to join that terrible side, and I rejoice when othees escape and/or speak out. To give up one’s means of financial security for ethical reasons takes a brave soul who truly understands and holds on to their original desires to serve and help humanity. To do no harm.

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