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Let me introduce myself. My name is Mark Sisson. I’m 63 years young. I live and work in Malibu, California. In a past life I was a professional marathoner and triathlete. Now my life goal is to help 100 million people get healthy. I started this blog in 2006 to empower people to take full responsibility for their own health and enjoyment of life by investigating, discussing, and critically rethinking everything we’ve assumed to be true about health and wellness...

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May 30 2018

8 Misconceptions About Fiber

By Mark Sisson
37 Comments

inline Fiber.jpegThe tricky thing about fiber is that it’s not a monolith. There are dozens of varieties. Some of them perform similar functions in the body, but others have extremely unique effects. Some rend your colonic lining to stimulate lubrication. Some turn into gelatinous slurries. But we can’t talk about fiber without understanding that the word describes a variety of compounds. As such, anyone making declarative statements about “fiber” without differentiating between the different types and their effects isn’t being accurate (except for me in that exact sentence).

This leads to a lot of confusion. People make blanket statements that might be true for some types of fibers and incorrect for others. 

Today’s post will attempt to illuminate the bulk of the matter. I’ll go through some of the most common misconceptions and myths about fiber from all corners of the dietary world. Whether you’re keto, low-carb, vegan, carnivore, or breatharian, you’ll find something to love and hate in today’s post.

1) “Fiber makes you full.”

This is theoretically sound. Mechanoreceptors in the gut respond to physical fullness by triggering satiety hormones. Big loads of insoluble fiber increase intestinal bulk, while some soluble fibers can gel up and increase the size of the stuff moving through your gut. Both result in added pressure on gut mechanoreceptors.

How does it work in practice?

A review found that while soluble fiber reduced appetite more than insoluble fibers, the overall effect on body weight was quite small, unimpressive, and inconsistent. More recently, a soluble fiber supplement failed to have any effect on satiety hormones, appetite, and subsequent food intake for the first 150 minutes after eating in healthy adults. The plucky researchers aren’t giving in, however, promising “further research… to quantify how soluble fiber influences appetite several hours after consumption.”

Resistant starch, an indigestible type of starch that colonic bacteria ferment, on the other hand does appear to increase satiety in humans, reducing food intake by 15%.

2) “Every diet needs the same amount of fiber.”

As it turns out, fiber becomes more critical the more carbohydrates you eat.

Soluble fiber slows down digestion, reducing the rate at which energy is absorbed. This can be helpful for people with glucose intolerance or type 2 diabetes by slowing the release of glucose into the blood. 

The byproducts of fiber fermentation in the colon by gut bacteria often have beneficial effects on carbohydrate metabolism. Eating resistant starch, for example, lowers the postprandial blood glucose spike. This reduction may also extend to subsequent meals, indicating it’s honing your ability to handle glucose. Everyone can benefit from better glucose management, but it’s far more critical for people eating significant amounts of glucose.

3) “All the healthiest people studied eat fiber!”

Observational studies are fun and all, but they’re not a good way to prove the healthfulness of fiber. Looking at fiber intake is just about the best way to capture the “healthy user”—that person who does everything right, like walk daily, exercise regularly, abstain from tobacco, avoid binge drinking, and eat whole foods rather than refined ones. It doesn’t say anything definitive about the health effects of the specific dietary variable they’re observing.

That said, the fact that most healthy populations eat whole foods containing fiber indicates that fiber probably isn’t actively harmful.

4) “Fiber is just roughage for big impressive poops. No functional use.”

That’s mostly true of insoluble fiber, which is pure waste material that shreds your intestinal lining and increases stool volume.

There’s considerable evidence that people with type 2 diabetes can really benefit from prebiotic fiber supplementation:

  • Chicory-derived inulin, a potent prebiotic fiber, reduces liver enzymes and HbA1c, improves blood pressure and fasting glucose, and increases calcium homeostasis.
  • Inulin improves immune markers and glycemic control.
  • Resistant starch lowers insulin resistance and inflammation.

A review of studies found that while the prebiotic inulin reduces LDL-C (an imperfect biomarker of dubious utility) in all populations, only in type 2 diabetics does inulin improve HDL and blood glucose control. 

Prebiotic fiber may also help certain patients with non-alcoholic fatty liver disease (NAFLD). The usual therapy for NAFLD patients is weight loss. You lose enough body fat elsewhere and the fat you’ve accumulated in the liver starts to disappear, too.

What about lean NAFLD patients without any real weight to lose?

In lean patients with NAFLD, a synbiotic—blend of prebiotic fiber with probiotic bacteria—reduces liver fat and fibrosis by improving inflammatory markers. Pre-emptive consumption of prebiotics may even protect against the development of NAFLD.

Another function of fiber that occurs in everyone is the production of short chain fatty acids by gut bacteria. When gut bacteria ferment prebiotic fiber, they produce short chain fatty acids, many of which have beneficial metabolic effects.

Butyrate is the most important short chain fatty acid. It fuels colon cells and may prevent colon cancer. Its relationship with existing colon cancer cells is more controversial. Read more about that here.

One interesting line of research is studying the interaction between the ketone body beta-hydroxybutyrate and the short chain fatty acid butyrate. Initial indications suggest that the two may have synergistic effects on cognition, inflammation, and overall health. That alone may be a reason to make sure you get prebiotic fiber on your ketogenic diet, just to hedge your bets.

Now, might a low-carb or ketogenic diet work better for people with type 2 diabetes than adding fiber to their normal diet? Sure. Could such a diet reduce the need for fiber? Yeah, I could see it. The same goes for NAFLD—low carb diets are also excellent in this population. And perhaps people who aren’t eating so many carbs don’t need the short chain fatty acids to improve their metabolic function and insulin sensitivity. But the evidence for fiber in type 2 diabetes and NAFLD stands, and I suspect short chain fatty acid production matters even in low carb or keto dieters.

5) “Fiber cures constipation.”

It depends.

In one 2012 study, patients with idiopathic constipation—constipation without apparent physiological or physical causes—had to remove fiber entirely to get pooping again. Those who kept eating a bit or a lot of it continued to have trouble evacuating. The more fiber they ate, the worse their constipation (and bloating) remained.

A 2012 review found that while fiber may increase stool frequency, it doesn’t improve stool quality, treatment success, or painful defecation. Similarly, glucomannan, a soluble fiber, moderately improves defecation frequency in constipated kids, but has no effect on stool quality or overall treatment success.

However, galactooligosaccharides, a class of prebiotic fiber, do appear to improve idiopathic constipation. And inulin, another prebiotic fiber, improves bowel function and stool consistency in patients with constipation.

6) “Fiber aggravates gut issues.”

Some say fiber cures gut issues like IBS and IBD. Others say fiber aggravates them. Who’s right? Maybe both.

Both IBS-D (irritable bowel syndrome with diarrhea) and IBS-C (irritable bowel syndrome with constipation) patients can benefit from soluble fiber (psyllium) while insoluble fiber (bran) is far less effective.

Wheat bran works okay for IBS, if the patients can tolerate it. They tend to tolerate something like hydrolyzed guar gum much better.

For IBD, the evidence is mixed. One survey of Crohn’s patients found that those eating more fiber (23 grams/day) had fewer flareups than those eating less (10 grams/day), while colitis patients reported no difference in symptoms based on fiber intake.

On the other hand, studies indicate that a low-FODMAP diet, which eliminates most sources of fiber, especially fermentable prebiotic fiber, is an effective treatment for IBS and IBD. Low-FODMAP diets have been shown to reduce bloating, abdominal pain, quality of life, and overall symptoms in intestinal disorders.

These contrary results may not even be contradictory. If your gut’s messed up, one solution could be to add back in the fibers you’re missing. Another could be to take all the fiber out and start from scratch.

7) “Fiber reduces nutrient absorption.”

For a long time, the consensus was that fiber tends to bind with minerals in the gut and thus reduce their absorption. These days, researchers understand that many of these fiber-bound minerals become available after fermentation in the colon.

Another wrinkle is that dietary fiber often comes with phytic acid, which binds minerals and prevents their absorption. Take wheat bran. Often deemed “wheat fiber” and lambasted for its tendency to bind minerals, wheat bran isn’t just fiber. It’s also a significant source of mineral-binding phytic acid.

Prebiotics increase absorption of magnesium, heme iron, and calcium. This makes sense. Even if the prebiotics are binding minerals, they release them once they reach the colon for fermentation by gut bacteria.

Fiber may reduce absorption of plant polyphenols, however.

8) “No one needs fiber.”

On the surface, this appears to be a sound conclusion. The human host digestive system cannot digest it. The majority of the fiber we eat gets pooped out as literal waste material. Certain classes of fiber may improve our gut health, but no one is keeling over from a lack of fiber in their diet.

Some have argued that a sterile gut is ideal if you have the right diet, that employing vast hordes of gut bacteria is just an adaptive measure taken to deal with a substandard diet full of roughage. The problem is that most people throughout history and prehistory have eaten that roughage, employed those gut bacteria, utilized the metabolites those bacteria produce. I suspect thinking long and hard before you consider it immaterial to human health.

If that were true, why would breast milk—the only food specifically designed for human consumption—contain loads of indigestible oligosaccharides that feed the growing gut biome? Even if it turns out that feeding the gut biome is only vital during infancy, that’s still a population of humans who truly need fiber.

Here’s where I come down: Fiber is an intrinsic part of many whole plant foods (and even whole animal foods, if it turns out that our gut bacteria can utilize “animal fibers” like other top carnivores). The Primal-friendly plants, the ones our ancestors grew up eating approximations of, like fruits, vegetables, roots, and tubers, are mostly higher in soluble fiber and lower in insoluble fiber. The only way to get huge doses of insoluble fiber these days is with supplementation or by eating grains. I don’t suggest eating grains or supplementing with insoluble fiber. I do suggest eating fruits, vegetables, roots, and tubers (while managing your carbs).

As for the carnivore issue, I’m open to the possibility that a properly-constructed carnivorous diet (which may, remember, include gristly animal fiber) obviates the need for plant fiber, prebiotic or otherwise. I’m not confident enough to try it myself, though.

Do I think everyone should be supplementing with prebiotic fiber? No. I add inulin to my Primal Fuel protein powder, mostly to improve mouth-feel but also to feed beneficial microbes and increase butyrate production. I add prebiotic cassava fiber to my collagen bars, again to improve texture and feed gut bacteria. And I’ll sometimes use raw potato starch for its considerable resistant starch content, often just mixing it into sparkling water and drinking it straight. But for the most part, the fiber I eat is incidental to the foods I consume. Berries, non-starchy vegetables, jicama, garlic, onions, green bananas, nuts—these are all foods rich in fiber, particularly prebiotic fiber, and I eat a fair amount of them while remaining low-carb and often keto.

As you can see, the fiber story isn’t simple. At all. There are many variables to consider. If you’re confused and unsure of how to think about fiber, you’re on the right track.

What do you think, folks? How has fiber helped or harmed you? I’d love to hear from everyone.

Take care and be well.

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37 thoughts on “8 Misconceptions About Fiber”

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  1. I find that I don’t digest fiber very well if I eat a lot of it. If I eat grains, or if I eat too many raw fruits and veggies, I don’t feel good and my digestion gets really “off.”

  2. Thanks for another informative article, Mark. But it provokes a question from me:
    What do folks use for resistant starch? I used to use raw potato starch (Bob’s Red Mill), but was told it isn’t actually resistant starch. Curious what others are using or would suggest.

      1. Thanks so much for this helpful article, Mack Daddy! It even addressed some of the exact emails I had seen. Seems pretty obvious (as Faith also mentions) that Bob’s Red Mill doesn’t want to involve themselves in the health supplements arena. Cheers!

        1. Glad you like the link. I should mention (now that I’ve read BoBM’s reply below) that I didn’t tolerate Bob’s Red Mill potato starch very well when just mixing it with water. Adding other fibers helped a bit, but what really made it go down well was just throwing it into a smoothie. I’ve been doing about 2 tablespoons most days of the week for several years now. I don’t know for sure what all difference it does or doesn’t make, but it seems like the various fibers (chia, psyllium husk, and many high-fiber foods) I consume do affect digestion for the better. Good luck to you!

    1. Yes, they claim that it isn’t – I’m sure that they don’t want to get involved in making health claims that they’d have to back up with studies. Who wants to get involved with the FDA more than they have to…

    2. I tried a variety of resistant starch sources and probiotics (such as fermented vegetables, pills) for 5-6 months. I found no benefits, only detriments. I used Bob’s Red Mill potato starch, green plantains, plantain flour (which it turns out I’m allergic to), etc. I tried many different ratios, building up to several tablespoons per day of potato starch and/or plantain flour. Multiple different types of probiotics, including soil varieties. Fermented pickles, sauerkraut, kimchi. Raw sauerkraut and I don’t get along for some reason, though I love the taste (cooked sauerkraut doesn’t seem as bad). I’ve found that hot peppers of any type are bad for me, causing me allergies. Kimchi is out. So, I’m down to fermented pickles, which are OK as long as I don’t eat too many of them; too many also causes me some issues.

      No benefits I could find. Only detriments (gas, IBS were the two most prominent). If there was a beneficial effect on blood sugar, I couldn’t find it using finger pricks. (I got my continuous blood glucose monitor after this testing time, so I have no data other then pinprick data.)

      Personally, I find the less fiber I eat, the better I feel. I still eat some fermented foods (mainly pickles) and some vegetables, but if I don’t have any of these per meal or day, I don’t get upset.

  3. If I want a good bout of diverticulitis, all I need to do is to eat some raw broccoli. The more raw veggies I eat, the worse I feel and the more constipated I become. Of course, the doctors never believe me when I say that.

    1. Interesting. I knew an intelligent, old, and very successful guy, who told me he found that he could eat anything with his diverticulitis as long as it was cooked thoroughly. He said even if he would get a small amount of raw lettuce in his guts that is would pack into the little those diverticulum pouches and ferment in there, and cause all kinds of misery. He never had a problem ever again after finding that all his veggies must be cooked. Also, seeds and nuts can cause a terrible problem for some, they pack in those pouches too and ferment, causing terrible attacks. Staying well hydrated will also keep things moving faster through so they get less of a chance to stick in there and cause problems. Summer is harder for people with that problem
      because you can get dehydrated easier with all the summer activities and eat lots of raw produce on top of it. Literally, the guy I knew never ate a raw cucumber even, everything he grew he cooked before eating and got rid of the seeds. Quite a healthy, active old guy, and so observant about everything. Worked like a horse and became a millionaire when he was very young. He planned life very carefully. He told me he would look in the mirror everyday and ask , “Now what good thing can I get done?”

      1. I agree with this. No raw veggies or nuts. Fruits may be ok for some if seeds are avoided.

      2. Are you could completely heal your diverticulosis by eating a plant-based diet. Both my mom and father-in-law, however they were not able to sustain just eating plants, so it has resurfaced, but at least they both knew what it felt like to be heeled!

  4. Seems to me the amount of fiber needed is very much an individual thing. I used to get constipated now and then, prior to adopting a Paleo lifestyle. Looking back, I realize my diet was too high in wheat products and too low in plant fiber. What got everything moving again was a good-size helping of brown rice. It worked every time without any of the irritation wheat bran caused. Since going Paleo I haven’t had a problem. I don’t eat much fruit, but the veggies I eat seem to provide all the fiber my body needs.

  5. I really appreciate the thoughtfulness and the deep research in this post. I’m not ready to opine yet on the topic, as I have been really confused about what my own body needs when it comes to various types of fiber. However, I’m committing, as we begin a new month, to spend this month of June really focusing on trying different tacks for several days at a time and seeing how my body feels. Thanks for giving us the opportunity to reflect on this particular aspect of nutrition, Mark!

  6. Plenty of green vegetables, some amount of tubers, small amounts of vegetable fat (avocados) and adequate water intake from the usual sources including high water content fruits and veggies does it for me, always has. Oat meal is fine and has its place, as well as nuts and seeds. No need for very large amounts of grain. A little grain goes a long way, but that about sums it up, fiber-wise.

  7. Soluble and insoluble fiber may have different functions, but it’s not like you can just eat the soluble fiber from a food and leave the insoluble behind. So, when people say that fiber from whole foods helps or hurts their digestion, does it really matter that it’s not an accurate description of the type of fiber? Total fiber is all we have to work with. I am keto-adapted and have had to cut way down on vegetables because they just ruin my digestion (poorly formed stools, gas, bloating, stomach pain) and my skin (acne, perioral dermatitis). After reducing total fiber to less than 8 grams per day, my digestion and skin are fantastic. I also found that keeping the nutrient dense food (like cheese/fish/nuts) volume the same in say, a big-ass salad, while reducing the volume of vegetables makes me feel full faster and for longer.

    1. This is very interesting. I have serious bloating problems and random stomach pains that I can’t connect to any specific foods. Where would you say most of that 8 grams of fiber comes from? You mention keeping nuts the same in your salads, do those not cause problems for you? Are they roasted nuts? Thanks for sharing your experience. 🙂

      1. Hey Tiffany – The vegetable fiber I eat comes from red leaf (or similar) lettuce (can’t do dark leafy greens at all), carrot, bell pepper, cucumbers, onions, radish and anaerobically-fermented sauerkraut. I also eat a quarter cup of soaked then roasted (previously raw) sunflower seeds and an 1/8 cup of raw walnuts. Those are most of the ingredients of my salad and pretty much all the fiber I’ll eat during the day. The other meals are made up of eggs/dairy/meat/fat.

  8. I supplemented with fiber years ago and it made my gut a hot mess. Since then – pre-keto life to keto-adapted – I have done just fine with a large range of 5 grams a day to sometimes 25, from real food. I never notice a differenfe either way.

  9. How useful having this topic discussed. I am two weeks out from having a right hemicolectomy for bowel cancer. Previously very health conscious exercised daily. Have followed LCHF lifestyle for two years often tipping into keto. Was shocked as found after routine bowel screening. Recovering well but been off keto whilst recovering. But new ‘plumbing’ working well introducing fibres etc. But difficult to know what best to eat. This article has helped and given me new ideas. Thank you

  10. My son’s diagnosis of celiac and my discovery of FODMAPS saved my life!
    I suffered from IBD for over 35 years. No doctor could help me…….they all just said “Take as much immodium as you need.”
    ………Then my son was diagnosed with celiac. His GI doc told him everyone in the family should be tested. Well, I had already gone gluten-free thinking possibly that was my problem all those years. I was symptom free in 2 weeks!
    I then occasionally (rather than daily) would have minor bouts of bloating and infrequent diarrhea when a friend told me about FODMAPS. I began experimenting…..
    I have now been gluten free for 5 years and have discovered several FODMAP foods I no longer eat. As a result, I am symptom free!!!

  11. Possible typo: “Low FODMAP diets have been shown to reduce …quality of life…”.

  12. “As for the carnivore issue, I’m open to the possibility that a properly-constructed carnivorous diet (which may, remember, include gristly animal fiber) obviates the need for plant fiber, prebiotic or otherwise. I’m not confident enough to try it myself, though.”

    I’m not either. Moreover, I see no reason to try it. My digestion works just fine on my diet of protein and an abundance of vegetables (mostly cooked but also a lot of salad greens). There are a few things I routinely avoid, such as coffee, raw onions, milk, and coconut. Otherwise, I have no difficulties with my GI tract.

    To paraphrase: If it ain’t broken, there’s no advantage in trying to “fix” it!

    1. But you don’t know that your system is fine. Maybe it’s not? Maybe you’d feel even better if you ate nothing but meat?

      I still eat raw onions, drink coffee, don’t drink milk (no reason to do so), but will eat coconut. I also feel as if I have little difficulty with my GI tract, after going on a near zero carb diet. I limit onions and other vegetables to maybe once per day, sometimes twice per day, most days zero times per day. I thought carnivores were crazy, but I find for me, more meat, fewer vegetables = better.

      1. In a word, Bob — Nonsense. I DO know that my system is fine. Eating nothing but meat might work for some, but I find the idea too extreme and completely unappealing. As I already pointed out, in my case there’s nothing to “fix.”

  13. I really don’t know how I feel about fiber. Reading so much mixed info about it doesn’t help. I have had chronic bloating for… Pretty much since I had my second baby 18 months ago. Maybe it has more to do with an abdominal separation from close together pregnancies, I’m not sure. One day I want to cut out all fiber to get rid of my bloat, then the next I want to eat tons of prebiotic fiber to make my gut healthy and reduce bloat that way. I still don’t know which one will work for me because I haven’t been able to make a solid decision that I stick to. Cutting out fruits and vegetables is really hard for me. This article was great, but again I’m left unsure of what to do… Tired of everything I eat making me look pregnant again.

    1. ^ this. Lots of info here but struggling to translate into dietary tweaks for myself to reduce bloating. So many acronyms, so little time.

  14. The second study you linked was a group of only 41 people who drank a carbohydrate “beverage” and didn’t fill full. Hardly enough evidence for point #1. Come on Mark, you’re better than that.

  15. The study of caprolites from our paleolithic and early neolithic ancestors, suggests that they were eating between 100 and 200 grams of fiber a day! The average American eats 15 grams day. I eat close to 100 grams a day and believe me, pooping is EFFORTLESS, bulky and like clock work.

  16. “That’s mostly true of insoluble fiber, which is pure waste material that shreds your intestinal lining and increases stool volume.”

    I’ve seen the shredding claim before, but wonder if there are any reputable studies backing it up. The best I’ve seen is that it can be irritating to the gut for some people, a far cry from ‘shredding the lining’. I think we’re made of hardier stuff than that!

    This was something I found discussing the gel that protects the lining, and the effect of fiber on it. Seems like more fiber is a better thing for the lining!

    “”The gel is like a sponge with holes that let material through,” says the paper’s lead author, Sujit Datta, a postdoctoral scholar in the laboratory of Rustem Ismagilov, Ethel Wilson Bowles and Robert Bowles Professor of Chemistry and Chemical Engineering. “We are seeing that polymers, including dietary fiber, can compress the gel, potentially making the holes smaller, and we think that this might offer protective benefits,” Datta adds.”

    Read more at: https://phys.org/news/2016-06-dietary-fiber-microbes-gel-lines.html#jCp

  17. Interesting article. But just so I understand this correctly: How does prebiotic fiber relate to soluble and insoluble fiber? Is it correct that prebiotic fiber is a subset of soluble fibers?

  18. I also very much appreciated the thought and consideration that went into this article looking at fiber from all angles. I have IBD (ulcerative colitis) and I keep it under control with diet and lifestyle, avoiding long-term medication for now. I tried a vegan diet and it didn’t work for me long-term. I’m working with a nutritionist currently who is encouraging me to decrease raw vegetables, limit fruit, enjoy thoroughly cooked vegetables, and increase lean proteins and healthy fats, while cutting out all processed sugar and gluten. I’ve never felt better. I get my fiber and feed my guy bacteria much the way the author is describing- cooked veggies, through nuts and berries, and in my prebiotic supplement.

  19. Another thorough post – thank you, Mark! I get all my fiber from whole plant and animal foods (even while eating low carb paleo-primal). Interesting to read that those with higher carb diets need MORE fiber. And such a bizarre myth that humans need grains, legumes and/or supplements to meet their fiber needs – after not needing the stuff for most of human history!

    Sigh, the answer for me is always so simple: real, whole food, including ample protein, ample fat, lots of plants.

  20. Yeah! You are absolutely right. There are several misconception among mass regarding the daily diet plan and the fiber intake. But, after reading this article many of us get benefited.