The 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.”
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.”
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.