The Fiber Manifesto–Part 5 of 5: Busting the Abrasive Insoluble Fiber Myth

November 25, 2013 in Categories: , , by

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fiber graphicThroughout the research for my first book, I kept hitting on a recurring theme.  Dietary fiber is essential for good health.  But I also kept hitting on pieces of information that went contrary to conventional wisdom (even contrary to conventional paleo wisdom).  Tidbits like insoluble fiber is even more beneficial than soluble (in spite of the fact that the vast majority of studies only evaluate the benefits of soluble fiber).  Even more mind-blowing for me was learning that the whole insoluble fiber being abrasive thing is a myth.  In fact, the deeper I delved into this subject, the more information I learned that completely challenged everything I thought I knew about this non-essential nutrient.  And, the more everything started to make much more sense too.

I have been working on this series of posts for six months and am finally ready to share this information with you.  I have broken this immense topic into five posts.  The first post summarized some the correlative and prospective studies that have been done showing the benefits of diets rich in dietary fiber.  The second post detailed the different types of fiber.  The third post summarized the research showing that insoluble fiber has more health benefits than soluble fiber.  The fourth post went into detail into how soluble fiber lowers cholesterol (and why that might not be a good thing).  Now, the climax of this series and some more myth busting…

Part 5: Busting the Abrasive Insoluble Fiber Myth

It was very important to me that every recommendation I made in The Paleo Approach be backed up with modern science.

My starting point for the diet recommendations in The Paleo Approach was the paleo autoimmune caveat as originally outlined in The Paleo Solution, plus the focus on vegetables suggested by Dr. Terry Wahls, and of course my own research for many blog posts explaining the why’s behind the paleo Autoimmune Protocol that I had already tackled before I started writing the book (as I tried to optimize my own health).  Through the last year of researching and writing, I made a point of reading through the scientific literature to make sure that every aspect of my recommendations could be backed up by science… and this led to some surprises for me.  Some of the common recommendations made for those with autoimmune disease in the paleo community could not be validated by science.  An example is the traditional recommendation to avoid cruciferous veggies (because of their goitrogenic properties) for those with thyroid disorders, a myth which is busted in this post.  And another of these topics is the avoidance of insoluble fiber for those with severe gastrointestinal symptoms and gut pathologies… turns out that this recommendation is not supported by science either.

As I researched the benefits of fiber, it became clear that a diet rich in vegetables and fruit (but not too high in fructose) was going to be essential for anyone to heal from an immune or autoimmune disease (or even to just be optimally healthy for that matter!).  But, more than supporting the intake of fiber-rich foods in general, I kept finding studies supporting the benefits of insoluble fiber compared to soluble fiber (although there are clearly health benefits to soluble fiber as well, especially as prebiotic fibers).  These benefits of insoluble versus soluble fiber have all been detailed in the previous 4 posts in this series (here, here, here and here).

But, insoluble fiber gets a bad rap.  Both conventional medical doctors and alternative healthcare providers typically warn against consuming foods rich in insoluble fibers for those with a severely damaged gut (so common in autoimmune disease) because insoluble fiber is rough and abrasive.  It has been likened to eating sandpaper, and supposedly causes damage to the gut as it scrapes its way through our digestive tract.  Both conventional and alternative healthcare providers (including some leaders in the paleo community) recommend avoiding insoluble fiber for people with IBD, IBS, diverticulitis and severely leaky guts.

So what is the evidence?

Diets rich in fiber reduce the risk of developing many gut pathologies.  High fiber diets (especially insoluble) reduce the risk of diverticular disease (diverticulitis and diverticulosis).  Very few perspective studies have been performed to investigate diet factors increasing risk of irritable bowel syndrome (IBS), but preliminary studies indicate that low fiber intake is a risk factor. High fiber and high fruit intake are both associated with decreased risk of Crohn’s diease.  High vegetable intake is associated with decreased risk of developing ulcerative colitis.

However, decreasing the risk of developing the disease is different than what happens when you eat insoluble fiber-rich foods after the disease has developed.  So, let’s look at that scenario.

If insoluble fiber were abrasive and irritating to the gastrointestinal tract, clinical trials where patients with diseases such as IBD, IBS and diverticulitis are supplemented with insoluble fiber should show exacerbation (or worsening) of their diseases.  But, the handful of studies that have investigated the effects of insoluble fiber in these severe gut pathologies have not borne this out.

The impact of insoluble fiber on disease activity is best understood with diverticular disease.  In diverticular disease, tiny pouches form in the colon due to weakening of the wall of muscles around the colon.  These pouches are called diverticula, and the diagnosis when these pouches are discovered is diverticulosis.  When these pouches become inflamed, diverticulitis is diagnosed.  When inflammation is severe, diverticula can bleed.  Traditionally, a diagnosis of diverticulitis or diverticulosis comes with a lifetime ban on insoluble fiber-rich foods like corn, popcorn, nuts, and seeds, and often low-residue diets (very low fiber) are recommended.  However, the evidence supporting these diet recommendations has never actually existed.  And in fact, the opposite has been proven:  high dietary fiber intake not only prevents new diverticula from forming, but it also reduces the risk of complications (such as bleeding and diverticulitis) in patients with diagnosed diverticular disease.

One fascinating prospective study looked at whether dietary fiber increased or decreased the risk of complications in patients with diagnosed diverticular disease.  This study showed that high intake of fiber from fruits and vegetables decreased risk of complications, but not cereal grain fiber (they were also able to show that it was the fiber in fruits and vegetables rather than other nutrients that made the difference).  They further showed that high insoluble fiber intake, and especially cellulose (but also lignin and to a lesser degree hemicellulose)—but not soluble fiber—correlated with lowest risk of complications of diverticular disease.  Even patients who suffered such severe diverticulitis that the diverticula were bleeding benefited from high insoluble fiber intake.

What about conditions affecting the small intestine rather than the large intestine, like Inflammatory Bowel Disease and Irritable Bowel Syndrome?  In one large study, patients with Irritable Bowel Syndrome were blindly and randomly assigned either a soluble fiber supplement (psyllium), an insoluble fiber supplement (bran), or placebo. While soluble fiber significantly improved symptoms of IBS, insoluble fiber had only a marginal effect compared to placebo—but, it didn’t make things worse.  A study using chitosan and vitamin C supplementation in Crohn’s disease patients showed no real benefit but also no detriment to disease activity.

This topic has been more thoroughly investigated in animal models than in humans (specifically animal models of ulcerative colitis).  Hemicellulose supplementation has been shown to greatly improve disease activity in mice with colitis.  Not only did hemicellulose supplementation reduce symptoms (like diarrhea), but it accelerated repair of the mucosal barrier and reduced inflammation, attributable at least impart to improvements in the gut microflora.  Cellulose supplementation was also shown to improve colitis in mice.  Again these benefits were attributable to changes in gut microflora; and very interestingly, when cellulose supplementation was stopped, the beneficial effects only lasted about 10 days before the disease started to worsen again.

There have been no studies performed on the role that insoluble fiber may have on healing the gut in celiac disease patients.  Celiac disease is hallmarked by a shortening (called pruning) of the intestinal villi (the fingerlike columns of gut epithelial cells that increases the surface area of the gut to maximize nutrient absorption).  A study evaluating the effects of two types of insoluble fiber (one rich in both cellulose and hemicellulose versus one rich in lignin) on the morphology of the small intestine in piglets (not celiac, but rather the developing intestine which is still likely very relevant) showed that insoluble fiber increases the height of the intestinal villi in both the jejunum and the ileum (and that’s a good thing).

It’s worth noting that within the relatively limited body of scientific literature evaluating the actual effect of insoluble fiber on the gut barrier, there is one situation in which insoluble fiber has been shown to be potentially damaging:  NSAID-induced small intestinal ulcers.  In one study, cats were fed diets that differed in fiber content (high, low, normal) and type (either cellulose or pectin) before being given very high doses of NSAIDs.  The researchers found that the cats developed more ulcers if they received the high fiber diet compared to control and fewer ulcers if they received a very low fiber diet.  They also found that cats that received cellulose-enriched diets developed more ulcers, but not pectin.  The authors conclude that insoluble fiber contributes to the development of NSAID-induced ulcers.  So, I guess if you are taking large doses of NSAIDs (and especially if you are a carnivore), this may be an argument for a low fiber diet.

The take-home message:  there is no evidence that insoluble fiber is abrasive to the gastrointestinal tract and there is no scientific rationale for avoiding insoluble fiber for those with gut pathologies including irritable bowel syndrome, inflammatory bowel disease, and diverticular disease.  In fact, eating plenty of fruits and vegetables will likely speed your healing.

Okay, so what do you do if you have a condition like IBS or IBD and eating foods rich in insoluble fiber, like leafy greens, causes you increased gastrointestinal symptoms?  What does it mean if you can see intact food particles in your stool after eating these high insoluble fiber foods?  This is a reality for many people and one that helps to perpetuate the myth that insoluble fiber should be avoided.  But, this isn’t a case of insoluble fiber being the problem so much as it being a symptom—it’s the big red flag indicating the need for digestive support.

The first step in improving digestion is always to follow good meal hygiene:  don’t rush, chew thoroughly, enjoy your meal, relax, don’t stress, don’t rush to the next activity when you’re done, don’t drink tons of liquid with your meals.  Also avoid alcohol, caffeine, smoking, and high stress, which can all hinder digestion (by lowering stomach acid production, which then reduces the signals to the pancreas to release digestive enzymes and the gallbladder to release bile into the small intestine).  Next, consider digestive support supplements, especially plant enzymes which can be particularly beneficial in aiding digestion of fiber-rich foods (I recommend either Enriching Gifts or Thorne Research).  Pancreatic enzymes (also called pancreatin, or simply, digestive enzymes), ox bile, and betaine HCl may also be very beneficial (careful with HCl supplements since these are contraindicated in a variety of conditions so always check with a healthcare professional before taking them) or an all-in-one supplement.  It is generally recommended for patients with gastrointestinal diseases to increase dietary fiber slowly.  That seems like prudent advice to me.  You may also want to limit yourself to cooked or fermented vegetables as these are both easier to digest.  Also, low FODMAP diets have been well documented to be beneficial in treating IBS and SIBO.  But don’t cut out insoluble fiber-rich foods altogether—there’ no evidence that avoiding insoluble fiber-rich foods will help you (it might even slow your recovery!) and you’d be missing out on some of the most beneficial foods you can eat for your long term health.

Aldoori WH, et al, A prospective study of dietary fiber types and symptomatic diverticular disease in men. J Nutr. 1998 Apr;128(4):714-9.

Ananthakrishnan AN, et al, A Prospective Study of Long-term Intake of Dietary Fiber and Risk of Crohn’s Disease and Ulcerative Colitis. Gastroenterology. 2013 Nov;145(5):970-7.

Bijkerk CJ, et al, Soluble or insoluble fibre in irritable bowel syndrome in primary care? Randomised placebo controlled trial. BMJ. 2009 Aug 27;339:b3154.

Hou JK, et al, Dietary intake and risk of developing inflammatory bowel disease: a systematic review of the literature. Am J Gastroenterol. 2011 Apr;106(4):563-73.

Kanauchi O, et al, Germinated barley foodstuff, a prebiotic product, ameliorates inflammation of colitis through modulation of the enteric environment. J Gastroenterol. 2003;38(2):134-41.

Kanauchi O, et al, Dietary fiber fraction of germinated barley foodstuff attenuated mucosal damage and diarrhea, and accelerated the repair of the colonic mucosa in an experimental colitis. J Gastroenterol Hepatol. 2001 Feb;16(2):160-8.

Kanauchi O, et al. Germinated barley foodstuff ameliorates inflammation in mice with colitis through modulation of mucosal immune system. Scand J Gastroenterol. 2008;43(11):1346-52.

Malhotra S, et al, Dietary fiber assessment of patients with irritable bowel syndrome from Northern India. Indian J Gastroenterol. 2004 Nov-Dec;23(6):217-8.

Nagy-Szakal D, et al, Cellulose supplementation early in life ameliorates colitis in adult mice. PLoS One. 2013;8(2):e56685.

Satoh H, et al, Role of dietary fibres, intestinal hypermotility and leukotrienes in the pathogenesis of NSAID-induced small intestinal ulcers in cats. Gut. 2009 Dec;58(12):1590-6.

Schedle K, et al, Effect of insoluble fibre on intestinal morphology and mRNA expression pattern of inflammatory, cell cycle and growth marker genes in a piglet model. Arch Anim Nutr. 2008 Dec;62(6):427-38.

Strate LL, et al, Nut, corn, and popcorn consumption and the incidence of diverticular disease. JAMA. 2008 Aug 27;300(8):907-14.

Tarleton S and DiBaise JK. Low-residue diet in diverticular disease: putting an end to a myth. Nutr Clin Pract. 2011 Apr;26(2):137-42.

Tsujikawa T, et al, Supplement of a chitosan and ascorbic acid mixture for Crohn’s disease: a pilot study. Nutrition. 2003 Feb;19(2):137-9.


This is the part I was most excited about….I think that this should be required reading for all the GI´s that told me that I should eat white bread and rice for my Crohn´s….I plan to keep eating my veggies! I think that your comment about food particles is very important…..I was worried about this too….but discovered that it had more to do with HOW i was eating, and the fact that my large intestine was so compromised that it didn´t consolidate the food thus hiding the undigested food…..
I am curious, is it possible that the increased symptoms that I have from eating raw vegetables could be because of the fermentation process, and not because of the fiber itself?
Thanks so much for the myth busting! Keep up the good work….your work is helping so many people, myself included!

What an informative, thorough series on fiber! It’s true, there are many misconceptions about fiber – how much is recommended, what types are best, etc. As a natural fiber supplement (with 75% more psyllium fiber per dose than leading brands), we’re always hoping to help debunk the myths and share helpful information. We hope it would be okay with you if we shared your series! As well, if you’d be interested in trying any of our products, we’d love to send some your way!

I just finished the entire series. Wow! I took notes because I was so surprised by the research and what I was learning. Thanks for putting this all together in such an easy to understand way.

So I read that coconut products are no longer a high fodmap food…because I’m lowfodmap and combining that with AIP to “heal my gut,” of course I was thrilled. Coconut flour and palm sugar is still unknown to affect ibs

Also, I’m trying desperately to heal my gut. I’ve been doing aip and lowfodmap for a couple weeks and it got better and now it’s worse than ever! Bloating, pain in my abdomen, and the distention is 24/7. I suspect I not only have ibs (I was told by a gastro I went to one time, never went back– said it was mostly stress, hardly helpful), but I might also have another autoimmune disease like leaky gut and/or sibo. Totally a guess, I’ve been dealing with bloating for two years now. Any advice would be terrific!

Interesting. Coconut makes me terribly bloated too (except for coconut oil). It’s too bad because it rules out a lot of AIP recipes. I can get away with a lot of higher-fodmap items, but coconut is terrible for me.

I can’t tell you specifically what caused it. All I know is I was doing HCG for 40 days as a cleanse (lean protein and veggies only–no oils, only berries- around the 30th day my stomach was slowly getting better) and once I added back oils, good fats, and other fattier grassfed meat or wildcaught fish I bloated horribly–worse than ever before.

Right now my typically breakfast is bacon and a side of berries, lunch is ground beef with sauteed lowfodmap veggies in coconut oil or ghee, and dinner is usually more beef or wildcaught fish with more sauteed veggies. I hardly ever eat raw veggies because I think I read that wasn’t good for those with bad digestion. For snacks I have either homemade kale chips or 1 or 2 tablespoons of coconut butter with a few berries.

My supplements are metagest (HCL), zinc chelate, iron chelate, multivitamin (ultra preventative), pro-5 probiotics, Blue Ice Fermented Cod Liver Oil, and potassium magnesium. I am trying the AIP and low fodmap like you said. My stomach used to be flat and now I wake up looking 3 months preganant and end the days looking 6 months pregnant. Trying to diagnose myself, I know I atleast have IBS. My stomach looks a whole lot like other leaky gut pictures I’ve seen.

What diet should I try? I want to heal my gut! I believe once my gut is healed my acne and everything else will be too. I’m only 19 and I’ve dealt with this for a little over two years.

Sorry for all the comments but I forgot to mention my overall symptoms I have cravings for sugar and carbs (before HCG–not so much now), gas, bloating, and anxiety, brain fog, abdominal pain, nausea, flatulence, and possibly yeast overgrowth (suspect–especially when i had any sugar).

Hi Summer, I have similar issues, and I think SIBO is the culprit. I get horribly bloated whenever I eat high fodmap foods, and generally most vegetables. SIBO appears to be the cause of fodmap intolerance from what I can gather.

“What about conditions affecting the small intestine rather than the large intestine, like Inflammatory Bowel Disease and Irritable Bowel Syndrome?” -> sorry to be nitpicky, but since EVERYBODY gets this wrong ALL THE TIME, I have to insist: inflammatory bowel disease probably affects the colon more frequently than it affects the large intestine. That’s because a) IBD patients can be divided into two main subgroups: Ulcerative Colitis and Crohn’s disease. Ulcerative Colitis affects the colon, not the small intestine, and b) even for the other roughly half of IBD patients, i.e. Crohn’s patients: almost all of them have some part or all of their colon affected. Crohn’s disease often starts in the terminal ileum and spreads to the colon.

I have to wonder if the mouse models of Ulcerative Colitis really allow conclusions about real Ulcerative Colitis. I have Ulcerative Colitis and it is well established that people with my disease have by far too many microorganisms in their gut. They have less variety, but the total quantity is too high. So if you throw a bunch of prebiotic substances into a gut like that, I think it is reasonable to assume that the total numbers will grow even more. Since the bad bacteria don’t know that the food isn’t meant for them, they will unapologetically pig out on the fiber and thrive and cause even more trouble than before. Prebiotics (as sold as a supplement) are not allowed on the SCD diet (designed specifically for IBDers and the basis of the GAPS diet). There aren’t any studies on this, but in the groups I’m in, it definitely seems that UC sufferers don’t tend to do well with fiber, at least not while they’re already having symptoms.

That’s a very interesting point! Do you happen to know the genera or orders of bacteria that are most likely overgrowing in IBD? I think that information would be necessary to answer this question. I’ll definitely look into it once work on the cookbook slows down.

Also, my understanding is that sources of insoluble fiber are allowed on SCD and GAPS (it’s the higher starch and soluble fiber foods that are omitted), cooked for GAPS intro of course.

At the phylum level, it is the Proteobacteria that most often seem to be disproportionately numerous, at least in Ulcerative colitis patients. Both Crohn’s and colitis patients seem to have (this it at the ‘order’ level!) more Clostridiales. Interestingly, at the family level, people with autoimmune disease often have fewer Clostridiaceae (this is a small family within the Clostridiales order), and within that family, the largest genus is that of ‘Clostridium’ – of which Ulcerative colits patients typically have fewer than the average person. All this information comes from ubiome. Maybe this is one of the reasons UC patients are more susceptible to C.difficile infections based on the fact that this species has fewer close relatives to compete with.
The most striking difference compared to the average microbiome is with the Prevotellaceae family, which are far more numerous in UC patients (in my ubiome sample for example, there were 4 times as many of those than in the average sample). This family is a part of the Bacteroidetes phylum and class, which is another phylum (apart from the Proteobacteria) which is often overly big in UC. Not to be confused with the *Bacteroides* family, which, to the contrary, is often diminished in IBD.
Yes, fiber is allowed on the SCD so long as it is part of whole foods. However, Elaine Gottschall often warned against overdoing certain very fibrous foods especially in the early stages of the diet.

Diagnosed with sibo, am also celiac. It has been gone on for 4 yrs. had a burst appendix, soon after became celiac.i am taking xfaxamin almost finished eating no carbs or sugars basically protein and insoluable Veg. Really confused with the different diets. They are contradictory. Taking digestive enzymes ,Hcl ,inflameacor. Glutemien, soil based probiotics. Nothing is working. Read that small bowled dysmotility can be the source of problem. Lost weight having a real problem with food. I have no glue what to eAt at this point. Any suggestions

Hi Sarah,
Thanks for all of your wonderful posts! They are truly encouraging to those of us trying to heal from digestive issues. I’ve recently come across the following website and book that encourages a low fiber diet and asserts that fiber is the cause of most digestive issues. I’m interested to hear your response, if you feel so inclined to look into it. It seems very contradictory to the advice that you and other knowledgeable health/diet bloggers have given. Here is the link to the website and the book:


Sarah, your book has truly been sooooo helpful to my health. I have mast cell activation/POTS/Ehlers Danlos Syndrome and am in remission for thyroid schmanser. I don’t like the “C” word. Anyhow, the mast cell activation has devastated my life. I have always been a very healthy girl, with health hiccups here and there. I have tried about 7 or more diets, including vegan. Nothing has worked like the autoimmune paleo. I have finally quit going into anaphylaxis so much. And it’s high pollen season. I am not sure if I can be cured, but am hopeful! At least to not being going into anaphylaxis is a huge plus. I had no idea how much grains were affecting me, even though I didn’t eat them that often. Also, question is anyone can help me. I have been on the diet almost 7 weeks. Am having terrible constipation. What would be the best food to add that will be less likely to affect this type of nutritional approach? I used to eat organic oatmeal with flax, and I think my body might be missing that fiber or something. I eat a ton of vegas and some fruits, so I know I can’t eat any more of those than I already am. Thanks for the blog and the book!! Healthy and Best Wishes!

I am wondering specifically about fiber supplements. Took Metamucil for years and now have taken psyllium husk powder supplement for help with sometimes diahrrea and sometimes constipation. Psyllium seems to moderate what my gut does. From the little information I’ve found on it, some seems to suggest that it helps the gut and others say it is damaging. Trying to make change to AIP, which is difficult with so many food allergies – I know that these are probably caused by leaky gut. It seems clear that what I’ve been eating doesn’t work. Should I stop psyllium as well? Does the “husk” designation mean that it contains insoluble fiber?

This is such a well-researched and clear series! Thank you so much for sharing what you’ve learned.

I’ve recently been diagnosed with gastroparesis (though I’ve had it for a long time…) – and I’m trying to learn what is acceptable for my gut. Gastroparesis is more about the stomach (slow stomach emptying, can result in excess acid damage and IBS like symptoms) but there’s not a ton of info out there on how to eat for it. The pamphlet my doc gave me was about how to eat for gastroparesis and diabetes (not helpful) and recommended eating a ton of fake food (Kool-aid, Ensure, Tang, pudding, etc). The main “Rules” I find are: low fat, low fiber (!!!), and no raw veggies. This is devastating!

My big question to you is, from what you’ve learned, do you think particular types of fiber might be more digestible (through the stomach) than others? Fiber is so important, and has been a big part of my diet and health (but I guess also contributes to slowing down food passage). It seems like insoluble fiber might be slightly more acceptable based on what you’ve written – but I’m not sure how much info for your intestines can be generalized to info for your stomach. In general, though, what’s a girl to do?

I absolutely agree that all the scare mongering about the “fibre menace” has done a lot of harm. I’ve had IBS all my life and the first thing you hear is how damaging fibre is to your compromised gut. The gas and cramping comes from mixing protein opioids WITH insoluble fibre. It’s not the fault of insoluble fibre that you also ingest breads and cheeses that simply don’t want to move out. I guess if a food opioid paralyses the gut but insoluble fibre tries to push out – yes, there’ll be a conflict and you can see how this would cause gastrointestinal distress. But insoluble fibre on it’s own – or with protein you can tolerate – wholefoods like meat & fish might be safer – won’t harm you as long as you introduce it carefully if you need to. I’m on a low FODMAP specific carbohydrate diet and mainly eating salad vegetables during the day with small amounts of protein and plenty of mayonaisse. It seems to be working well at the moment. Complete evacuation with very little gas.

Sarah, do you have any idea why digestive enzymes (specifically like the Plantizyme that you mentioned) would give me terrible, terrible stomach aches? I have tried several and they all make me sick for a few hours after using them (with a meal). I can use the Pancreatin/ox bile/ hcl just fine. I have not been able to figure out why these plant enzymes make me so sick and I feel like it is a clue to my digestive issues. Thank you for any knowledge you can offer.

I recommend you work with a practitioner who is well versed in digestive issues. Unfortunately I’m very limited as to the advice I can give here. But I have a bunch of great consultants on my team.

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