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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.

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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.

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Join the conversation

  1. Do you still recommend the 2 products listed on this post? Or do you have updated recommendations?

    • Please watch my documentary earthlings free on YouTube, no cow, pigs or chickens wanna be murdered for your diet or your taste buds, they are not food they are a living being, shame on you author of this article.

  2. scientists will provide you any outcome you pay for …. high fiber was a way for people for thousands of years before the invention of junk foods, supermarkets & others… now you poop 150-300 grams , when people had a high fiber diet, people poop many times more… american junk diet & lifestyle is to blame for all western diseases

  3. I feel like a lot of people are critical of Chris’ research, however, seem to forget that he’s the one who has dedicated his life to academic research into functional health. I have personally tried everything and believe that there is no answer or one size fits all. HOWEVER, if you do have a compromised digestive system, putting anything into it that makes it work twice as hard is counterintuitive until it heals…which is fiber. Remove, heal then introduce again but not excessively. Fiber from natural foods exists for a reason….added fiber does not. Nature didn’t create fiber supplements. Simple.

  4. Do not believe any of this. The problem with the American diet is that Americans do NOT consume enough fiber and the reason for the high incidence of colon cancer and other cancers of the gastric system.
    With the American diet as it is, the amount of fiber in our diets is way too low compared to foreign countries where fiber is high on the list in their diets.
    We eat too much processed foods that contain no fiber or very small amounts.
    It is perfectly normal to have 2 or even 3 bowels movements a day. These bowel movements rid the body of toxins that we accumulate in our bodies. There are some people who have only 2 or 3 bowel movements a week! Sooner or later these people will come down with some kind of catastrophic illness in their gastric system, primarily colon cancer.
    I don’t buy any of what Kresser has said here. The more fiber in your diet the better you will feel and your overall health will benefit from it.

    • I’m convinced you didn’t read the entire article. Chris didn’t write “against” fiber intake. He shows that the effects (positive and negative) of fiber consumption on the digestive system need to be studied further, and scientific study has so far revealed that fiber itself isn’t a miracle food that does away with disease. It’s a fact that excessive fiber intake (especially insoluble fiber) can aggravate the intestines. Excessive amounts of any good thing, be it vitamin A or potassium or any other nutrient, has a negative effect on the body.

      Chris actually recommends upping your fiber intake at the end of the article, but your statement that “the more fiber [you get] the better you will feel” is most certainly not true. Try eating a few servings of whole sunflower seeds and see how that feels.

      • 2 or 3 bowls a day is healthy?

        Is this guy healthier than me just because I only dump once every two days on average?

        • You should be pooping several times a day. I didn’t start to have daily bowel movements until 6 months after starting changing my diet. I used to eat a bunch of processed crap like the typical american and I only pooped every other day. Now that I’m eating a bunch of fruits and vegetables and no processed food or refined sugar or dairy, I have 1-3 bowel movements a day. So in my experience, 2-3 bowel movements a day is healthier than pooping only once every couple of days.

          Out of curiousity, what does your diet currently consist of?

          • I just started eating a much healthier diet with more fruits and veggies. I’m only pooping once every 3 days which is scaring me. I’m thinking about taking fiber supplements to help things move along. Any recommendations?

          • thats great you don’t eat dairy, but whole milk plain yogurt, sorry, but im not giving that up. There are a lot of great benefits and things to be said about whole-milk/fat dairy. Everyone is different, I think a lot of articles like this are generalized and they simply shouldn’t be….we all have different bodies and tolerances for things. I can tell you that fiber, sugar, nightshades, salt have no place in my body. They have ruined my health, amongst other things.

    • The Japanese eat a relatively low fibre, high carbohydrate (white rice) diet and levels of colon cancer aren’t anywhere near as high as in the US.

        • They also, categorically, eat only to about 80% fullness, leaving energy for proper digestion.

          • It is a myth that the Japanese subsist on large amounts of rice. They eat relatively small portions of rice supplemented with a very broad variety of vegetables and seafood. Modern Japanese diets are obviously changing, but please don’t assume the Japanese eat Sushi all day.

    • I recommend the whole 30 program or even Paleo! No added fiber needed❗❗❗

    • Chris Kresser advocates eating a VERY high fiber DIET, that is, lots of fiber-rich FOODS which feed your healthy gut bacteria, and this article is just saying supplementation isn’t the same thing. He also advocates for fiber from veggie, rather than grain, sources. Listen to his RHR podcast episodes “Are High-Fat Diets Bad for the Microbiome?”, “Why You Need to Eat More Vegetables—And How to do it”, “Is a Disrupted Gut Microbiome at the Root of Modern Disease”, and “Are Vegetarian Diets Better for the Microbiome?” and you will hear really great and detailed information about how fiber is pretty much the most important thing to your health, since the you can’t be healthy without a healthy gut. He espouses eating small quantities of very high quality pasture-raised meats & organs, fatty fish, and raw pasture-raised dairy, but the rest (the vast majority) should be unprocessed, whole vegetables & some fruit, while avoiding grains, sugar (except what’s naturally occurring in WHOLE fruits), and industrial seed oils (which are high in inflammatory omega-6).

      The big problem with the predominance of medical literature on fiber is that they fail to differentiate between fiber in grains (which is what most people think of and aim to increase consumption of) versus fiber in vegetables. Autoimmune and other chronic diseases of civilization generally respond very favorably to the omission of grains or, conversely, can be exacerbated by grain. If some people are eating fiber from something that makes them sicker (and their guts unhappy), that’s way different from subsisting on a diet comprised of spinach, broccoli, cucumbers, tomatoes, bell peppers, carrots, plantains, sweet potatoes, berries, and apples with a little high-quality protein thrown in. Eating a high-fiber wheat bran breakfast each day (processed food) with additional meals also made up of packaged, processed foods CANNOT compete with this in terms of health in the gut and bowel movements. Soon, we hope, there will be disambiguation research to put this point to rest. Meanwhile, uninformed people everywhere will continue to say the unqualified, nonspecific (and therefore inaccurate) statement, “more fiber is always better.”

  5. I say soak and ferment all whole grains with saliva as culture for 24hrs or more before consumtion!:-)..

  6. Can you actually propose A NUMBER?
    I mean, the federal RDA for men is 38 grams fiber per day.
    What is an unhealthily high fiber intake?
    What would be your recommended upper limit?

    • If you’re eating a Paleo diet heavy on vegetables, tubers, nuts, and seeds, you’re getting enough. Adjusting and monitoring your fiber intake is something you should do if your bowel movements are unhealthy. If everything is working fine, just keep eating real food while occasionally taking a probiotic with HSOs.

  7. do you realize how many “may”s, could be, hopefully, are in this article? Nobody knows nothin.

      • “may’s, could be, hopefully” words are for the lawyers whom are ready to pounce on any unsuspecting person to sue the crap out of them. (pun intended).

        • Actually, they are the most accurate words that could be used.

          Eating more vegetables may improve your health is far far more accurate than saying eating more vegetables will improve your health.

  8. Maybe somebody can give me advice:

    I can not tolerate fermented foods because of histamine intolerance. So I take a probiotic.

    I suffer from a chronic procitits. At the beginning I felt little pain and had bloody stool, higher calprotectin. Through a Paleo diet (no gluten, low histamine, diary and egg free) I got rid of many symptoms like the bloody stool and most of the pain (was not much pain). But I still suffer from irregular motility and mood and energy swings which correlate with my bowel movements. This means:
    I am often constipated for 2-3 days. Then I have either normal motility like I always had (2-3 per day) or I have one movement per day. When having my old motility back, I feel good energy. But the most time I feel like a little subdepressive and low energy…

    When I had acute proctitis the first and only time, I could not tolerate anything but rice and potatoes. Sugar and diary gave me massive mood swings (anxiety). Had to eat rice and potatoes for a week with a probiotic to get normal again…

    Since I suffer from proctitis I cannot tolerate any Gluten, or FOODMAPS. Foodmaps give me heavy brainfog and constipation (tried inulin)

    I tried Bimuno (GOS) which my gut tolerated well, but I react allergic to it (I react to many things :-/)

    So I ask myself if could use Lactulose as a prebiotic for a longer time. Is this an good alternative to inulin or FOS?

    In my probiotic is FOS, but the bacteria eat it all up during the preactivation- phase (has to be diluted in water 25 minutes before drinking)

    Or should I start with tiny amounts of FOS?! But even a tiny capsule of Inulin gaves me heavy brain fog for days last time I tried it. I have no idea where to go with there issues. There´s no Chris Kresser in Germany…:-/

    Can anybody help me with this?

    • Try to remove all vegges, dairy and eggs, and use only rice, almond milk, some steamed corn flakes, meat and fish with olive oil and a pick of salt, then reintroduce eggs and see how it go for a week.
      Yes, is not completely paleo since i use some gluten free grains, but if i don’t i become so weak and my muscle so flat that i’m not able to stand still (i’m 58 kg and i lost about 35 kg after i get the sibo 3 years ago)..

      If is ok and your constipation don’t worse then add dairy, and so on till you find what is bad for you. I immediately notice what foods cause pain since i get bloated, constipated with abdomin al pain (cause peristalsis increase) .

      I can’t eat every kind of veggies for a long time, even zucchini, carrots and pumpkin cause pain. About fruits i eat sometimes an apple or papaya and i use filtered orange juice (homemade from oranges).

      Veggies increses peristalsis and is very bad if you have a damaged gut. Bone broath sometimes cause cramps so i tend to avoid it and limit to 1-2 for week.

      Sometimes i eat gluten free pasta (no preservatives) and white rice, and i’m ok, a bit of honey don’t hurt too. I need them or i feel very weak and bad.

      There is no turn back once your gut is damaged, is our fault to consume refined craps, is our fault because we are ignorant to trust food lobbies and is our fault to use medications instead to fight the root of the cause, so is pratically something irreversible that we have to deal for the rest of your life, it mean low carbs and high fats/proteins because they are the only things that we can eat.

      But i’m fine, i eat because is mandatory to our life, i don’t live for food anymore.

  9. What about the fiber from vegetables like broccoli, cauliflower, bok choy, and lettuce? Do we need to be careful with those as well?

    • Nope, everything about veggies is good and specially if eaten raw, as a snack, etc. Follow the greed diet and you will be just fine.
      One of the things I noticed when reading the article is that it lacks real life cases about treating people with fiber, other than comments.
      I have research the subject of fiber not only to be healthy but to control certain illnesses and disorders of the human body. Healthy people should ingest about 25 grams of fiber daily from food sources and “diabetic” people should ingest about 40 to 60 grams of fiber daily and this has been proved to be effective by a diabetes specialist doctor at the VA Hospital in Lexington, KY. He treated bad cases of diabetes with a heavy dosage of fiber and no medication and the results were excellent. Concerning diet, people should realize that it is not only fiber what the person should eat but also watch out for other things such as sugar, salt, cholesterol, fat, etc. However, for the digestive track natural fiber is the best. Some people cannot tolerate grains; however, there are some grains which those people will be ok eating such as Quinoa, the best grains of all, which can be cooked in a variety of ways and eaten at breakfast, lunch, and dinner. The best thing is to avoid any dairy products, sugar in all forms and from all sources, salt although sea salt is healthy in moderate amounts, fats from all sources, and foods containing cholesterol. Want to be healthy? Stay green in your diet!

      • Bad grammar, very inaccurate information. With inflammatory bowel conditions you absolutely do need to limit your intake of gassy cruciferous vegetables like broccoli. Quinoa is not a grain. This person has no idea what they’re talking about and future readers should be warned not to listen to this garbage. It’s foolish.

      • Vegetables best eaten raw…. avoid all dairy, salt, fat and food containing cholesterol…. Etc …..

        You clearly don’t know enough about nutritional science. Better not giving advises to people again…

        • High fiber foods does not necessarily mean it helps with blood glucose. The best method is to use a blood glucose meter after one and two hours after eating meals with carbs. I found black beans spiked my blood glucose.

        • You Said, don’t eat “salt, fat and food containing cholesterol…. Etc …..”
          You do realize that saturated fat is crucial to maintain a balance with your hormones? If not meat, at least some coconut. If you don’t eat cholesterol foods, then your body will just make the cholesterol from protein.

      • I disagree with you and the doctor who gave 60 grams fiber. I myself did this to reverse type 2 and it worked BUT it caused me to become severely anemic and low minerals and has taken 4 years to get getter. Too mulch fiber binds to iron and mineral and vitamins and you never absorb them. It’s dangerous. I say rat your fiber in food not added supplements

  10. Where does a product like PGX come in here. Some doctors recommend to use it at least once a day. What kind of fiber is it?