Ten Fibre Myths You Probably Still Believe

I’m sure you’ve heard thousands of times that fibre is the ultimate cure for your constipation. It was probably drilled into you from such a young age that to even question it seems crazy. I’ve had many friends who without any significant knowledge in any area of nutrition confidently state that fibre is helpful for bowel regularity and ameliorating constipation. Many health influencers will go way further, ascribing numerous benefits to increased fibre intake like reduced cardiovascular disease, a healthier and more diverse microbiome, and better glucose metabolism.

Yet, when I was struggling with my own digestive issues like discomfort, bloating, and unpredictable bowel habits, the conventional advice of eat more fibre did not makes things better. While eating more fibre would get me a few extra bowel movements each week, this would usually be accompanied with significantly worse bloating and discomfort, not alleviating my symptoms in any significant way.

When you dive into the research fibre really doesn’t deserve all the credit it gets. In fact, more often than not it can actually be driving many problems of its own. In this post I will overview some common claims we hear about fibre and what the science actually says about their validity.

1. Fibre relieves constipation

This is probably the most universally well accepted claim about fibre. Before we can evaluate the accuracy of this claim, we first need to address a more fundamental problem surrounding the definition of constipation.

Conventional medicine typically defines constipation as fewer than three bowel movements per week alongside symptoms of straining, hard stools and a sense of incomplete evacuation. This misses a more fundamental question: is waste actually being cleared effectively? Someone can have infrequent bowel movements with some straining and still be functionally clearing their bowel, whereas someone could have daily bowel movements and still have profoundly sluggish motility and an inability to effectively eliminate waste and toxins.

With that in mind, what does fibre actually do for constipation? It increases stool bulk and volume, which in a healthy gut may correspond to increased bowel frequency and ease of passage. But adding more fibre to a sluggish, dysfunctional gut without actually addressing any underlying issues is like adding more cars to a traffic jam, and is often accompanied with worsening symptoms. This mirrors what I experienced, with slightly increased bowel frequency, but consistently worse straining and bloating.

The most striking evidence against the fibre and constipation claim comes from a study on patients with chronic idiopathic constipation, meaning long-term constipation with no identifiable cause. They were divided into three groups: high fibre, low fibre and no fibre intake. The high fibre group experienced no meaningful symptom improvement, the low fibre group experienced some improvement in symptoms, and the no fibre group experienced complete symptom resolution in every single person studied.^[1]

So does fibre help with constipation? For people with an already healthy gut, certain fibres may help in the short term alongside adequate hydration. However, if you have chronic constipation and eating fibre isn’t making you feel better, the evidence suggests that adding on even more fibre is not the answer and may even make things worse.

2. Fibre reduces Cardiovascular Disease Risk

Reducing risk is a cause and effect statement, implying that fibre itself is doing something protective. To establish this, researchers would need to control for every relevant variable in a long term human trial. This has never been done and in any practical sense cannot be done.

What we actually have is associative data. That people who eat more fibre have reduced cardiovascular disease. These associations are deeply confounded in two ways.

First, fibre is found almost exclusively in whole foods, while processed foods contain very little. Any group with a high fibre intake will invariably be those who are eating more whole foods and fewer processed foods. Whether the benefit comes from the fibre itself, the nutrients in whole foods consumed, or the lack of processed foods consumed cannot be determined from the data.

Second, people who follow dietary advice tend to be those who are health conscious and are making many choices in service of better health. Decades of public health messaging have made high fibre intake a marker of health consciousness, meaning those who are consuming more fibre also tend to exercise more, drink less, sleep better, and avoid smoking. There is extensive healthy user bias here and the associative data cannot untangle what is actually driving the outcome.

Fibre may correlate with better cardiovascular health, but correlation is not causation, no matter the strength. In this case the confounders are so significant that the correlation tells us very little about fibre intake specifically.

3. Fibre creates a healthier and more diverse microbiome

The gut microbiome is a community of microorganisms, predominantly bacteria, living inside your colon. These bacteria consume substrates that weren’t fully digested in the small intestine. Since fibre is indigestible by humans, it arrives in the colon available for bacterial fermentation. When beneficial bacteria ferment certain fibres, they produce compounds called Short-Chain Fatty Acids, which fuel intestinal cells, support immune function, and help maintain a strong gut barrier. Fibre fermentation also produces other byproducts, some of which are potentially harmful, a point we will return to later in this post.

The problem with this claim is that fibre doesn’t reliably produce a healthier microbiome, it feeds whatever is already there and that distinction has significant implications. A healthy gut with a well established beneficial bacterial community may respond well to certain fibres. A dysbiotic gut with unwanted bacterial overgrowths or pathogens will likely respond poorly, as fibre will feed those organisms just as regularly. In this context fibre doesn’t create a healthier microbiome, it just amplifies an already problematic one. Furthermore, many fibres are not fermented in any appreciable quantity by human gut bacteria and therefore do not contribute to microbiome health or diversity.

The microbiome benefit of fibre is real but conditional. Whether it applies to you depends entirely on the state of your gut, the specific fibres you are consuming, and the amount of them that your gut can effectively ferment.

4. More fibre is always better

The fermentation of fibre by colonic bacteria isn’t a perfectly clean process. The byproducts of this fermentation include gases like carbon dioxide, hydrogen, and methane, as well as small amounts of ethanol and acetaldehyde, both known carcinogens.

In a healthy, well-balanced gut ecosystem these byproducts are produced in quantities the system is designed to manage. Cross-feeding between microbial species means that the outputs of one organism become the inputs of another. Fermentation gases are consumed by methanogens and acetogens. Ethanol is cleared rapidly by the liver at the trivial concentrations produced under normal conditions. The net result is a functional fermentation economy with tolerable waste. The byproducts are real but the ecosystem contains them.

The problem with the “more is always better” logic is that it assumes this containment capacity scales linearly with substrate when it doesn’t. Excessive fibre intake can cause fermentation output to outpace the gut’s ability to clear it. Gas accumulates faster than it can be consumed or expelled, producing bloating and distension. Ethanol and acetaldehyde production can now reach non-trivial levels, potentially damaging local tissue and burdening the liver’s detoxification capacity. More importantly, the people most often advised to eat more fibre such as those with IBS, chronic bloating, dysbiosis, are precisely those whose ecosystem is least equipped to handle it. In a dysbiotic gut the cross-feeding networks that manage fermentation byproducts are disrupted, and the same fermentation that a healthy ecosystem contains quietly can become a source of significant harm.

Furthermore, the actual amount of fermentable fibre required to support meaningful microbial activity is remarkably small. This is a microbiome, not a macro-biome. Research on Galacto-oligosaccharides (a prebiotic fibre) found significant increases in beneficial Bifidobacterium species and meaningful butyrate production at doses as low as 0.5 to 0.75 grams.^[2] The organisms doing this work are microscopic and need very little of their preferred substrate to function. You do not need industrial quantities of fibre to feed a healthy gut; you need the right organisms and a small enough substrate load that fermentation stays within the bounds of what the ecosystem can manage. The ceiling isn’t higher fibre intake, it’s a well-functioning fermentation economy that doesn’t overwhelm itself.

5. Only plants contain fibre

Fibre is defined as medium-long chain carbohydrates that are not fully digested in the human small intestine by endogenous enzymes. To put it simply, they are carbohydrates that we cannot digest ourselves. Many of these such carbohydrates are plant based, but there are other sources of such fibres as well.

The first food humans consume is breast milk, which contains over 200 distinct human milk oligosaccharides (HMOs).^[3] HMOs are indigestible complex carbohydrates that exist specifically to feed beneficial gut bacteria and shape an infant’s growing microbiome.

Animal connective tissue, cartilage, and bone broth also contains compounds that fit this definition. They contain glycosaminoglycans, indigestible polysaccharides that gut bacteria ferment to produce short chain fatty acids, the very same compounds that plant fibre’s advocates will point to as evidence of its intrinsic value. There may not be as much of these fibres as there are in plants, but as we discussed above, more is not always better, and these fibres can be sufficient in maintaining a healthy gut microbiome in the right context.

6. Fibre is an essential nutrient

Fibre is not digestible by humans and I’m not sure we can call something that we cannot digest to be a nutrient. A counterargument to this would be that because fibre feeds bacteria to produce Short Chain Fatty Acids, then this is by proxy a nutrient and adds nutritional value. This is a fair point, but then we’d have to acknowledge the second part of this claim, “essential” meaning something we must derive from our diets that we cannot produce endogenously. Our gut epithelium is lined with goblet cells, specialized mucus producing cells. The mucin they produce contain glycans that establish a specific cross-feeding network for beneficial bacteria and result in Short Chain Fatty Acid production.^[4] There is no evidence that supplemental fibre is beneficial or necessary beyond what this process already produces.

7. All Fibres are created equal

The blanket recommendation of eating more fibre treats fibre as a single compound. Fibre is a broad category encompassing a wide range of indigestible polysaccharides with very different structures, behaviours and effects in the body.

The most fundamental distinction is between soluble and insoluble fibre. Soluble fibre dissolves in water, forms a gel in the digestive tract, and is generally fermentable, meaning that it can feed gut bacteria. Insoluble fibre doesn’t dissolve, adds bulk to stool, and passes through largely undigested without feeding bacteria in any meaningful way. These should not be thought of as remotely the same thing as they behave almost entirely differently in the gut.

Within soluble fibre there are important distinctions. Some soluble fibres are classified as prebiotics for their tendency to selectively feed beneficial bacteria. Others ferment more indiscriminately, feeding a broad range of organisms including pathogenic and opportunistic bacteria.

There is emerging evidence that insoluble fibre may be directly abrasive to the gut lining and may contribute to intestinal permeability. Unlike soluble fibre, insoluble fibre does not feed beneficial bacteria and may even damage the microbiome terrain, certainly not a prototypical gut healthy food.

Ultimately, fibre is a diverse category of compounds, some of which may be beneficial in the right context, some neutral, and some potentially problematic. Your unique gut ecology and terrain will dictate the relevant health outcomes.

8. Fibre improves digestion

This depends exactly what you mean by “improves”. Fibre does slow the digestion and absorption of carbohydrates and sugars. In the context of a high carbohydrate diet, blunting blood sugar spikes could reasonably be called an improvement. Fibre can also slow the digestion of liquid fat, which can be beneficial for those who have sluggish bile flow and need additional time to emulsify fats properly. In these contexts this claim has some merit.

If improves digestion means enhances nutrient absorption, the opposite is true. Both soluble and insoluble fibres can bind to minerals like calcium, iron and zinc, potentially reducing how much the body is absorbed, although this seems context dependent. Insoluble fibre can reduce fat availability in the digestive tract and certain soluble fibres impair fat digestion itself, both of which compromise absorption of fat soluble vitamins A, D, E, and K2. It’s also worth noting that high fibre foods typically also contain phytates, lectins, and oxalates, which themselves significantly reduce nutrient absorption.

9. A Carnivore diet is unhealthy because it has no fibre

A well-formulated carnivore diet is not void of fibre. Nose-to-tail eating includes organs, cartilage, bone broth, and connective tissue of animals, which contain glycosaminoglycans like hyalauranic acid and chondroitin sulphate. These are indigestible polysaccharides that colonic bacteria ferment to produce short chain fatty acids through the same pathways as plant fibre.

Animal foods also supply significant quantities of all the necessary precursor nutrients and cofactors for robust goblet cell production in their most bioavailable forms. These include Vitamin A, Vitamin B6, Vitamin D, Zinc, Calcium, as well as the very amino acids that make up mucin glycoproteins. The result in a well-maintained mucus layer that drives its own endogenous cross-feeding network, selectively feeding beneficial bacteria, and generating butyrate without any plant fibre. This is a gut that is actively producing its own substrate when given the necessary raw materials to do so, thus maintaining healthy homeostasis.

The real-world evidence this comes from the Maasai of Tanzania and whose traditional diet consists of meat, milk and blood, with little to no plant fibre. Their microbiota consisted of a distinct, robust, and thriving ecosystem.^[5] A population eating essentially no intentional fibre does not have a devastated microbiome. Rather, they have a different one, shaped by different substrates, with a strong functional capacity.

10. Fibre decreases colon cancer risk

This claim has the same problems as number Myth #2, a heavily confounded dataset with no established causal data. No matter how many times this myth is repeated is not true.

The supposed mechanism of action here is worth unpacking. The claim is based on the idea that butyrate production, a short chain fatty acid byproduct of fibre fermentation has anti-carcinogenic properties. The idea is that cancer cells rely on glucose rather than butyrate as their primary fuel, meaning that butyrate accumulates in cancer cells and inhibits tumour growth.

The problem with this idea is that butyrate is not the only byproduct of fibre fermentation. As mentioned above, fibre fermentation produces some known carcinogens like ethanol and acetaldehyde.^[6] How much damage is done by these molecules is multi-factorial, depending on the specific extent of fibre fermentation taking place, the liver’s detoxification capacity, and the body’s overall antioxidant support. A person with sluggish detoxification, depleted glutathione (the body’s master antioxidant), or an already dysbiotic gut that ferments above its clearance capacity could plausibly increase cancer progression by increasing fibre intake and corresponding fermentation. A healthy microbiome producing modest, well-managed fermentation likely does confer some protection against colorectal cancer, but that outcome is not achieved by maximizing fibre intake. Rather, it is achieved by maintaining the ecosystem conditions under which fermentation stays within an indicated range.

What this means for you?

Fibre is not a nutrient you need to chase. If you are eating whole, seasonal, nutrient-dense foods within the ancestral dietary framework, you will consume some fermentable substrate as a natural byproduct of eating real food. That is exactly how it has always worked. Fibre was never a dietary target; it was a consequence of eating from the earth in a particular season in a particular place.

The goal is not to maximize fibre. The goal is a healthy gut ecosystem with enough substrate to sustain modest, well-managed fermentation, robust goblet cell function, and the microbial community that emerges from both. That ecosystem does not require a high-fibre diet. It requires a nutrient-dense one.

If adding fibre helps you in the short term, that information is useful and worth acting on. If you have been told for years to eat more fibre and your gut remains dysfunctional, it’s time to look deeper at the microbiome itself, rather than force-feeding it indigestible plant-matter. The question worth asking is not how to get more fibre in. It is what is actually disrupting your gut ecology, and what your ecosystem needs to restore itself.

Eat a nutrient dense, ancestrally consistent diet and let fibre take care of itself.