Myths and Truths About Fiber | Chris Kresser

Myths and Truths About Fiber


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For decades, fiber has been touted as an essential component of a healthy diet. The supposed benefits of a high-fiber diet have been drilled into us through recommendations by our doctors, government, and the food industry alike, yet many of these health claims have not been proven by research.

In fact, many studies have demonstrated that excess intake of fiber may actually be harmful, particularly for gut health.

The majority of the research supporting the benefits of dietary fiber come from epidemiological studies that link the consumption of fiber-rich fruits and vegetables with a lowered risk of certain diseases such as obesity, heart disease and cancer, particularly colon cancer.(1)

Yet when tested in the lab, controlled intervention trials that simply add fiber supplements to an otherwise consistent diet have not shown these protective effects. (2) (3) (4)

The Institute of Medicine recommends a daily fiber intake of 38 grams for men and 25 grams for women, which may come from dietary fibers, both soluble or insoluble, or the addition of “functional fibers” to the diet. The IOM defines functional fibers as non-digestible carbohydrates that have been isolated or extracted from a natural plant or animal source, or they may be manufactured or synthesized. Examples of functional fibers are psyllium husks, chitin from crustacean shells, fructooligosaccharides, polydextrose, and resistant dextrins. (6)

These functional fibers are often added to processed foods as a way to bulk up the fiber content for consumers looking to meet the IOM intake guidelines.

A recent report by NPR commented that despite the lack of significant evidence linking fiber intake to health outcomes such as reduced heart disease or cancer, many consumers are buying foods that are fortified with synthetic fiber additives under the guise of health promotion. (7) Three grams of added fiber is enough to allow these food products to claim to be a good source of fiber, and the food industry has used these fiber guidelines as a way to increase their sales of grain-based products in particular. (8)

Tan and Seow-Choen, in their 2007 editorial on fiber and colorectal disease, call insoluble fiber “the ultimate junk food”, as “it is neither digestible nor absorbable and therefore devoid of nutrition.” (9) Excess insoluble fiber can bind to minerals such as zinc, magnesium, calcium, and iron, preventing the absorption of these vital nutrients. (10) Large excesses of certain soluble fibers like pectin and guar may also inhibit pancreatic enzyme activity and protein digestion in the gut, leading to an anti-nutritive effect. (11)

The addition of insoluble and soluble fibers to processed foods may actually cause these foods to be even less nutritious than if they were not enriched with any fiber at all.

A high-fiber diet has also been described as a preventative strategy for the development of diverticulosis, a disease that is markedly more common in Western countries. However, when researchers tested the theory that a high-fiber diet prevented diverticulosis, they not only found that a high intake of fiber did not reduce the prevalence of diverticulosis, but that a high-fiber diet and greater number of bowel movements were independently associated with a higher prevalence of diverticula.(12) Interestingly, this study found no association between the presence of diverticulosis and red meat intake, fat intake, or physical activity, which are other factors commonly attributed to diverticulosis. Here is some more information on a diverticulitis diet and how to prevent it naturally.

The researchers hypothesized that one possible effect of a high-fiber diet in the development of diverticulosis could be the quantitative and qualitative changes in gut bacteria due to the excessive fiber intake. Both insoluble and soluble fibers are shown to alter gut bacteria in as little as two weeks. (13) It is possible that the high levels of excess fiber and overgrowth of intestinal bacteria may have contributed to the development of diverticular pouches in the colon.

This hypothesis brings up another side to the fiber debate: the effect of dietary fiber on beneficial gut bacteria, as well as the bacterial fermentation of undigested soluble fiber into short-chain fatty acids such as butyrate. When we eat the soluble fibers found in whole plant foods, the bacteria in our gut ferment these fibers into short-chain fatty acids like butyrate, proprionate, and acetate, and greater amounts of fiber consumed will lead to greater short-chain fatty acid production. (14) In this case, naturally occurring soluble fibers are very important for feeding the friendly bacteria that live in our guts.

One of the risks of long term very low-carbohydrate (VLC) diets, in my view, is the potentially harmful effect they can have on beneficial gut flora. VLC diets starve both bad and good gut bacteria, which means these diets can have therapeutic effects on gut infections in the short term, but may actually contribute to insufficiency of beneficial strains of gut bacteria over the long term.

Providing adequate levels of carbohydrate and soluble fiber to feed friendly bacteria is important for optimizing digestive health and maintaining the integrity of the gut lining through the production of short-chain fatty acids.

Stephan Guyenet has written an excellent blog post describing the benefits of butyrate and other short-chain fatty acids on the maintenance of healthy gut integrity. (15) Butyrate has anti-inflammatory effects, increases insulin sensitivity, and may delay the development of neurodegenerative diseases. It may also be helpful in the treatment of diseases of the colon such as Crohn’s, IBS or ulcerative colitis. (16)

Stephan believes that butyrate may play a significant role in healthy metabolic function, stress resistance, and the immune response. He also asserts that the epidemiologically observed benefits of a diet high in naturally occurring fiber are likely due to the higher butyrate production from these diets. In this case, a higher fiber diet could be protective and beneficial for health, particularly if the fiber is soluble.

So what does this mean for our own consumption of fiber?

Ideally, dietary fiber should be coming from whole food plant sources. Many foods in the Paleo diet are great sources of both soluble and insoluble fiber, such as yams and sweet potatoes, green leafy vegetables, carrots and other root vegetables, fruits with an edible peel (like apples and pears), berries, seeds, and nuts. Interestingly, butyrate itself is also found in high-fat dairy products such as butter and cheese, and can also be provided by the bacteria found in fermented foods.

Although I recommend that most people get fiber from whole foods, there are some people that may benefit from soluble fiber supplementation – including those that aren’t able to eat fruit or starch due to blood sugar issues or weight regulation, and those with severely compromised gut flora or gut dysbiosis. In these cases I’ve found soluble fiber and/or prebiotic supplements to be helpful.

For healthy people, including a variety of fibrous whole plant foods, fermented foods, and high-fat dairy as tolerated should eliminate the need to supplement with extra fiber, especially those insoluble fibers that are from sources high in anti-nutrients.

A Paleo diet with some level of attention paid to the quality and quantity of vegetables, fruits, and starchy tubers can provide adequate levels of soluble fiber to feed the friendly bacteria in the gut that convert these fibers into beneficial short-chain fats like butyrate.

Recommended supplements if needed:

Prebiotic: Klaire Labs Biotagen

Soluble fiber: Organic Acacia Fiber

Caution: it’s crucial to start with a very low dose of prebiotic or soluble fiber and build up slowly over time.  This will minimize any potential adverse reaction that can occur with significant changes (even positive changes) to the gut microbiome.  For Biotagen, start with 1/4 of 1/8 of a tsp (1/32 tsp.) and increase by 1/32 of a tsp every 4-5 days.  For Organic Acacia Fiber, start with 1/4 of a tsp. once per day and build slowly from there.


Join the conversation

  1. I’ve been suffering with constipation due to low motility all my life and wasn’t until I actually got a copy of “Modern Nutrition in Health and Disease” (Shils), which is a textbook on nutrition used in some medical schools, that I understood how misunderstood fiber is. I find it confusing that Paleo type websites always seem to push soluble fiber, which binds me up and bloats me horribly, even in small amounts. But they dismiss what works for me: NOT-fermentable fiber. Of which there are 3 that I trust and 3 only.

    1. Cellulose (natural, not the additive) from greens.
    2. Methyl cellulose (Citrucel) or hypomellose (for cooking)
    3. Fiber Con pills (some kind of calcium concoction that is not fermentable but only comes in pills)

    There was a time when I wasn’t gluten sensitive, and back then, wheat bran worked. But ONLY wheat bran as far as I know. Rice and oat brans contain fermentables.

    The advice here is helpful, but if someone is suffering from gut pain due to bloating and it is killing their life, it’s not enough. It wouldn’t have saved me the research I had to do to help myself. So keep in mind that “fiber” is massive category of foodstuff, like saying “polyphenols” and it has subcategories, which are themselves massive and poorly understood, and contain sub-sub-categories. The FODMAPS are also considered to be “fiber.” Our failure to understand fiber is about equal to our failure to understand our microbiome.

  2. Chris, I am new to healthy gut flora restoration, you mentioned the benefits of eating resistant starch as well.

    What’s the difference between Organic Acacia Senegal Tummy Fiber which you recommended here and RS such as
    products like Bob’s Red Mill, Potato Starch?

    Shall i pick one to eat or I can take both at the same time?


  3. Diagnosed with UC and Crohns in my early 20’s I’ve been creatively trying to keep the disease at bay for over 15 years. I know my trigger foods, I know that sometimes I will eat something that will certainly disagree with me, I’m human and I don’t necessarily want the disease to control my life. I turned 37 this past February and decided I was ready to get in better shape with exercise and a healthy diet, but with IB issues, healthy eating isn’t always very easy or “comfortable”. Lunchtime salads and other leafy vegetables eventually catch up to you and before you know it the inflammation in your bowels has returned. Long story short, I decided to take a chance with Shakeology. I was doing the T25 workout program and having great results, but was feeling fatigued and I knew my nutrient consumption was probably the cause. Shakeology offers a boatload of vitamins and nutrients that eating the plant equivalent I would never be able to match. I make the shakes with almond milk which also reduced bloating and other dairy intolerance’s that go along with an over consumption of milk. There isn’t anything scientific or magical I can or will try to produce, it simply has had amazing results for me and my digestive health is better than it has been in the past 15 years. The product is a little pricey, but when I think about the cost of Asacol per pill per day, the daily cost of this shake is minimal and the benefits encouraging! I’ve suffered through the discomfort of these diseases and I know the desperation I’ve felt at times to find something that might make me feel better. This is an all-natural product and it can’t hurt to see if its something that might work for you too. You can order and get more info at the link below. Yes, I am a coach for this product because I get a discount of 25% on my own purchases. This is not a career for me, but a healthy lifestyle decision I made to try and find relief from my own IB diseases.

    • lol better research before promoting your crap. almond milk would make you more sick.

      • And what qualifies you to remark that Almond Milk would aggravate my condition (or in your words “make you more sick”). I could decide to discontinue use of the shake product tomorrow and I would still use almond milk in place of dairy. The product simply does not bloat my stomach and I have had no negative side effects using almond milk. You think because your internet search may have revealed something to you which suggests your opinion is gospel that it trumps my own experience. If you had any relevant experience with any IBD issues you would know that many people experience different things. If someone wishes to try an alternative approach than why would you care? And who are you to suggest my post is simply about a sale? Post something useful/helpful or keep your misguided opinions to yourself.

  4. Great article, Chris. You really put out top notch info.

    Re: Insoluble fiber being “the ultimate junk food” since the gut can’t derive nutrition from it… that’s actually missing the point..
    any substrate used from fiber, in general, by the bacteria in the gut to make SCFA, etc is a secondary purpose.

    Dietary/plant fiber’s purpose for the plant itself is to give plants their structure and skeleton, so to speak.. Benefits to humans are for the brush/mechanical effect of peristalsis, as a means to create pressure against which the intestines can contract and move. So insoluble fiber does a perfect job when considering it’s primary focus in the gut, motility and bowel regularity.

    Some researchers go to minutiae and miss the bigger, simpler picture.


  5. How much fiber is too much fiber, I suspect I have gut issues since I have every problem under the sun, whenever I add cabbage or beans to my diet I have consistently floating stools.

  6. I am regular for the first time in memory ( I am 62). I definitely feel less bloating,less cramping, and thankfully, less constipation. My personal regimen is to take 1 Lady Soma Fiber Cleanse Capsule first thing in the morning before breakfast. I am good for the day.

  7. Can anyone tell me why it’s cutting me off? Just want to ask..,I have very watery loose infrequent stools…what am
    I supposed to eat, carbs or no carbs? Resistant starch??fiber supplements??? Do I need to give up my kale and healthy greens. I’d so beyond appreciate any knowledgable suggestions or insight!
    Thanks to all of u:)

    • Hi, I had loose stools (cow pat consistency) daily for 12 years. I eventually discovered that home made milk kefir (fermented for 48 hours) gave me my first firm stool in 12 years. After 6 months I got fed up of eating kefir. A few months later I read that eating a low carb diet relieves IBS. I stuck to eating around 72g of carbs daily and my stools miraculously became firm again. It seems that some loose stools could be due to a lack of certain strains of gut bacteria, a lack of which may result in the inability to digest high levels of carbohydrates … I’m sure there’s more to it than this. Hope you find a cure soon.

    • Casey, I hope you find the answer to your dilemma. I’m not sure how to answer though because loose stools is not a good description for my situation. I did have a period of loose followed by hard stools which was so annoying I did something crazy: I fasted on water only for 12 days. I couldn’t last longer than that, but it seemed to help a lot. I’d fasted before for 3-5 days, but it wasn’t quite as effective. Later on, I read that some people “reset” their flora this way, but with a short fast, like 3 days. I did drink kefir as my first food after the fast, and ate normally but sparingly an hour after the kefir. I don’t believe in the “eat only fruit the first day” rule, which I’ve seen, but I think is bogus. Joel Fuhrman, MD is doing some interesting research into fasting as a blood pressure solution. His book is “fasting and eating for health” and he has a website that lists his research studies. It was useful as a primer. And doing the fast saved my digestion from getting even more complicated. I now follow a mostly Paleo diet, but it’s higher in carbs than people expect. I was ketogenic for a while and that helped my gut a lot. To try that, you’d need something like and get your fat % somewhere around 70% at least (by Calories). Technically I’ve calculated that 66% is the 1:1 keto diet, but if you’re that close to the perfect amount, it is safer to go slightly higher. My diet was about 1.3:1. Now I’m going more into a Zone type of macronutrient makeup. But I know which carbs affect me. For me, I have to avoid the fermentables. For you, you might benefit from resistant starch and other fermentable fiber suggestions. Everyone is different and you should be patient. You’ll find what works eventually. I know it’s hard for now.

      • Some people reset their flora via a colonoscopic fecal transplant at Mayo clinic or other places.

        Dr. Orenstein says, and both he and Griesbach feel the procedure’s potential has barely been tapped.

        “Its use in C. difficile has been well established, but much of the rest is mainly anecdotal,” he says. “There is some baseline evidence that it might be effective for IBS, but that hasn’t been looked at in a controlled manner. Some physicians claim to have great success treating ulcerative colitis and celiac disease. And it’s been looked at for obesity, diabetes and rheumatoid arthritis because some of the signals for the gut are pro-inflammatory for RA. But it’s difficult to get real data.”

  8. Wow, thanks to everyone for so much almost too much great info..,I’m certainly more confused than ever but I think I can retread and try to sort it out!
    I’m a very odd case, crazy healthy,majority greens eater for the last 7 years and my gut seems to be in bad shape

  9. Key to a health and long, healthy life: moderation in excess. Too much of anything is not good.

  10. “The majority of the research supporting the benefits of dietary fiber come from epidemiological studies that link the consumption of fiber-rich fruits and vegetables with a lowered risk of certain diseases such as obesity, heart disease and cancer, particularly colon cancer. (1) Yet when tested in the lab, controlled intervention trials that simply add fiber supplements to an otherwise consistent diet have not shown these protective effects. (2) (3) (4)”

    Link #3 is dead.

    Links #2 and 4 are mostly about wheat bran and colon cancer. Wheat bran is not likely to have an effect on colon cancer, given current understandings about the role of SCFAs in protection against colon cancer, and in overall gut health.

  11. Well, this is all a little confusing.

    If we eat fiber, it may decrease the nutrition of food. That’s bad.

    But, if we eat fiber, it may grow beneficial bacteria in our gut. That’s good.

    If fiber is going to decrease nutritional uptake, then why would it matter if it was from a plant source or from an artificial source like polydextrose? Ultimately, this puts us in a confusing place where we aren’t clear what kind of fiber is good and what kind of fiber is bad, and why.

    Overall, this leaves us not having a clear idea of how to move forward.

    • I agree. Seems like a lot of readers are confused by this article .Chris – can you clarify or provide more info ?

      Thank you

    • Fiber and the gut microbiome are inextricably linked. Our confusion about both of these is understandable. If I eat an onion, for example, I will get bloated and will feel feel pain. But do I know exactly which components of the onion did that? And which bacterias ate the components? No. I might know that FODMAPS are in onions and that’s bad for bloating, but the specifics are beyond me and mostly beyond the average researcher even. Life without onions is annoying, but it is less agonizing than life with them. So I avoid onions. A similar story, more complex, may be told about why I don’t eat wheat. Besides the gluten issue, it is also a potent FODMAP containing food.

      We’ve made a symbiotic deal with germs. They help us access more vitamins, and in return, they get to keep some of the energy for themselves, including some of the vitamins and minerals. In the balance, we may come out ahead, but only if the bacterias aren’t prone to giving off toxins, or being pathogenic and killing off the variety/community of bacterias that already exist. In my case, I’m quite certain I’m an example of the “motility loss” effect of bacteria. When I eat fermnetable fiber I can’t “go.” My bowels lock up and won’t move for a week if I don’t use magnesium citrate plus senna to get it out. Luckily as I’ve become more familiar with avoiding those fermentables, I’ve been able to reduce my reliance on laxatives.

      But there’s no shame in using a laxative, just like there is no shame in using Immodium. It was always confusing to me how people stigmatized laxatives, yet didn’t do the same for Immodium, even though it is an opiate. It must be some kind of holdover from stigmatizing anything that teenagers misuse for weight loss.

      Ultimately, if your gut is healthy, you don’t need to think too hard about this. When things go wrong, though, it bears thinking about, and patience and persistence will be the key to finding your unique solution and what applies to your needs. I know that isn’t much help, but fiber and microbiome issues are not one size fits all.

  12. Also eat home made stock which provided glycine from gelatin – eat eggs- which provides cysteine – precursors for glutathione -and heals tbd gut lining.

    Eat collards cooked in lard and homemade broth for more precursors for glutathione

    Eat cod liver oil
    (GAPS/pre-industrialized food preparation-via nourishing traditions- Sally Fallon).

  13. This entire issue has been fully discussed by Sally Fallon and Mary Egnin years and years ago.
    Eat whole plants with butter. Eat Fermented plant foods. Eat raw grassfed butter.

  14. to whom it may concern my name is Carlos Vargas and my question is, is too much fiber bad for you? I am a diabetic amputee, I do not and cannot exercise due to my physical health, my doctor suggested that I had to eat more fiber. thank you very much. Mr Vargas.

  15. Hi Mr. Kresser,
    I was just wondering what your thoughts are on coconut meat, which contains loads of fiber. I’m living a paleo diet lifestyle, and I’m very interested in knowing if this is a food that will be hazardous to my GI health (I eat about 200g/day of coconut meat). I know that you are a busy man, so a simply smiley would suffice if the fiber from a coconut is of benefit.

  16. Hi: I am interested in understanding Pysillum Seed/husk. I am on the SCD diet and they do not like Pysillum because they believe it may be feeding the bad bacteria along with the good.. This is what they say on the website “Pysillum are loaded with cellulose and lignin which some bacteria thrive on. We accept that the cellulose in vegetables and fruit can be handled OK but a concentrated form such as husks would not be in order.” So, does that mean none should be eat at all. For instance I noticed that some recipes are including small amounts of Pysillum to bind together the flours they are using. The Julian Bakery makes a Paleo Coconut Bread made from Coconut flour and the use Pysillum to bind the Coconut flour. Sort of like using “Gums” which you also mentioned above. I personally cannot use any gums. So, the Pysillum is interesting, but I don’t want to use if it’s going to cause an over growth of bad bacteria. Or a problem with not absorbing the nutrients I need from important foods I am eating. I’ve heard that is a problem too. Anyway,, could you please explain to me if Pysillum is safe to use and why for binding baked goods such as bread. Thank you for your help, Cathi G. Ventura, CA

  17. I have a friend who was on standard american diet growing up, never liked vegetables as a kid, etc, etc, etc. Then after she got married and had children, she really got into taking care of her and her family’s health, proper nutrition, etc. Gradually, they started to eat more and more vegetables, berries, nuts, seeds and other high-fiber content foods. When she turned 35, she was taken to a hospital with a terrible bout of diverticulitis, which came as shock to everyone because she is so young and they all ate so healthy. Now she has to stay away from fiber and a lot of foods that are good for healthy people but not for somebody with her condition. Anything that has seeds of some sort in it causes flare-ups now. She can eat only very little of fiber and has to be very selective with the source. I was wondering if Mr. Kresser would have some thoughts on it, recommendations or a link to diverticulitis diet. Just trying to help my friend!

    • My grandma used yellow dock root to control her diverticulitis. It only flared when she quit taking it. She lived 100 years.

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