… and why the carnivore cultists are dead wrong about fibre

4 August 2025

In the thirty years since I launched my clinical practice, I’ve seen all manner of dumb health trends come and go. In fact, I encountered this destructive force even in my student days: One of my classmates became convinced that all her health problems were due to Candida albicans overgrowth. She put herself on a so-called ‘candida diet’ that was so strict, she became quite severely malnourished, which not only exacerbated the symptoms she had (falsely) attributed to candidiasis, but brought on a whole crop of new ones.

Since then, I’ve seen the alkaline diet craze, the blood type diet scam, the gluten-free and anti-grain fads, the vilification of sugar (especially fructose) for every damn thing, the low-carb and ketogenic diet cults, the subclinical hypothyroidism hyperventilation, the great phytate and lectin jihads, the anti-seed-oil crusade… jeez, have I missed any? It’s hard to keep up!

But of all the dopey health trends that have come and gone, one of the most low-IQ must surely be the latest canard being promulgated by the carnivore crowd: the notion that fibre is bad for us. Yep, fibre. That vast, diverse and complex category of carbohydrates indigestible to humans, which nourishes our gut microbiome, allowing our resident microbes to perform a multitude of important tasks which include the following:

• Producing certain vitamins, including vitamin K and biotin, which we can absorb and utilise;
• Synthesising amino acids – the building blocks of protein – which we can also absorb and utilise to build our own body proteins;
• Carrying out biotransformation of bile acids – metabolic waste products that contribute to bowel cancer and gallstones – so that they can be safely removed from the body;
• Producing short chain fatty acids including butyrate, which decreases intestinal inflammation, heals ‘leaky gut’, modulates the sensitivity of visceral organs and improves intestinal motility (helping to overcome IBS), protects against colorectal cancer, lowers cholesterol and reduces insulin resistance… among many other benefits;
• Secreting antimicrobial substances which inhibit the growth of ‘bad’ bacteria that can cause intestinal infections, leaky gut and inflammatory conditions including cardiovascular and autoimmune disease.
• Interacting with toll-like receptors, helping the immune system to identify and destroy harmful bacteria while tolerating beneficial bacteria, human cells and normal components of food.
• Interacting with dendritic cells, a type of immune cell, helping to calm them and reduce their production of inflammatory chemicals.

If you don’t believe me when I tell you that these carnivore zealots are preaching to their acolytes that fibre is not only completely nonessential for human health, but flat-out bad for us, just search Twitter (yes, I know we’re all supposed to call it ‘X’ now, but why should I obey the whims of a personal data-snarfing, monkey-torturing, brain-chipping technocratic huckster?) for phrases like ‘fiber is not necessary’. A torrent of fibre-is-the-devil-incarnate posts awaits you, from self-appointed nutrition gurus who haven’t the faintest idea how the human body actually works… let alone the fact that the paleolithic humans whose eating habits they claim to emulate, ate somewhere between 70 and 150 grams of fibre per day, with an average of 104 grams. Sigh.

Like the one from this genius, who posts a figure from a study that he does not cite, which was conducted on people with a condition which he does not name¹, for an unspecified period of time, and which uses the phrase “strain opening”, a made-up term that you will not find in any medical dictionary nor published study. Yet 764 people ‘liked’ this exceptionally silly post. 🤦These people can vote, consume alcohol and drive cars, and – god help us – many of them probably have children. Is it any wonder that Idiocracy is increasingly looking like a documentary????

Fortunately, not everyone who saw this post was taken in; this acerbic response was one of my favourites:

Then there’s this one, featuring a clip from an interview with a medical doctor who believes that beta-hydroxybutyrate, a ketone body which is synthesised by the liver both during fasting and when people eat a very low carbohydrate diet, is just as effective at nourishing the cells lining the colon (colonocytes) as butyrate, which is formed when colonic microbiota ferment fibre and other carbohydrates that humans can’t digest:

Dr. Georgia Ede explains to Steven Bartlett on the Diary of a CEO Podcast why you don't need fiber on a Carnivore Diet pic.twitter.com/4ipelOwLCA

— CarnivoreJT (@carnivore_jt) May 5, 2025

Did Dr Georgia Ede sleep through her biochemistry and physiology classes? How else might one explain the fact that she apparently doesn’t know that colonocytes derive most of the energy they require for metabolism from “luminal nutrients” – that is, from nutrients in the gut itself, and not from nutrients circulating through the bloodstream (including beta-hydroxybutyrate)? Chief among these luminal nutrients is butyrate, which is the primary energy source for colonocytes and which, as mentioned previously, is a byproduct of fermentation of fibre by gut bacteria.

That is to say, if you’re not eating any fibre, you (or more accurately, your gut microbiota) are not producing the butyrate needed to supply your colonocytes with the energy they need to maintain all their functions. Which might explain the following (reader caution advised):

Colonocytes synthesise beta-hydroxybutyrate from butyrate, but they do not usually utilise it as an energy source; instead, they use it primarily as a cell signalling molecule, to suppress inflammation and abnormal cell proliferation that could lead to cancer.

But even when it comes to suppressing cancer formation, butyrate is more effective than beta-hydroxybutyrate. Butyrate is a potent histone deacetylase inhibitor, meaning that it controls the expression of genes that play a role in cancer formation and progression. Let’s briefly review a couple of fundamental concepts that you need to understand in order to grasp the significance of butyrate’s anti-cancer activity:

• DNA is the long-term storage vessel for the genetic information required for our development and functioning. It is contained in the nucleus of each of our cells.
• Genes are segments of DNA. Genes contain the ‘instruction manual’ needed to construct proteins that play structural and functional roles in our bodies, and RNA molecules. The genetic code specifies the sequence of the amino acids within proteins.
• To organise and compact the large volume of DNA, proteins called histones act as ‘spools’ around which the DNA winds.
• Histones package and order the DNA into structural units called nucleosomes, and – through the activity of histone deacetylases (HDACs) – play a role in gene regulation by controlling which parts of the DNA are ‘transcribed’, or copied to RNA to make proteins.

People (along with other animals, and plants too) start their lives with a certain genetic code, which is determined at conception. However, we have a multitude of genes that hold the codes for production of an astonishing variety of proteins, with widely divergent effects.

The choice of which genes are ‘expressed’, or activated, at any given time or indeed over time, is strongly affected by environmental factors – that is, factors in the environment of the cell itself, such as hormones, neurotransmitters, nutrients and toxins. These environmental factors can change the expression of some genes very rapidly (in fact, within minutes); can cause very persistent changes in the expression of other genes; and can even cause certain changes in gene expression that are passed on to our children.

Cancer cells manifest aberrant gene expression, and this doesn’t just happen because of mutations, or errors in the DNA sequence of the cell. There are epigenetic changes as well – that is, changes in gene expression that occur without altering the DNA sequence itself. Many cancer cells, for example, show ‘silencing’ of genes that code for tumour suppressor proteins such as p53 and p21, which ordinarily act to keep cell division within normal limits. On the other hand, genes that code for cell survival proteins such as Bcl-2 are ‘upregulated’, or more active, allowing cancer cells to proliferate rapidly and form a tumour.

In the last twenty-odd years, there has been an explosion of research into how HDAC dysregulation contributes to these abnormalities in gene expression. Excessive HDAC switches off the tumour suppressor genes, allowing cells to replicate themselves without restraint – one of the fundamental characteristics of cancer – even without mutations in the DNA sequence of the cell. And conversely, there are numerous compounds found in everyday foods or produced in our bodies when we eat certain foods, that act as HDAC inhibitors, limiting the harmful overproduction of HDAC. HDAC inhibitors can turn on the tumour suppressors that have been silenced, causing cancer cells to ‘commit suicide’ via a process called apoptosis.

Butyrate is a potent HDAC inhibitor, effective in multiple cell types. And so is propionate, another short chain fatty acid that, just like butyrate, is produced by gut microbes in the process of fermenting fibre. Both butyrate and propionate differentially influence gene expression in cancer cells vs normal, healthy cells. In healthy cells, these fibre-derived compounds assist with normal metabolism and the protection of the cell’s genome. But in cancer cells, they cause greater exposure of the faulty genes (which decreases the damaged cell’s ability to successfully replicate), and induce apoptosis, or programmed cell death. Pretty clever, eh?

On the other hand, beta-hydroxybutyrate is reported to inhibit HDAC in some studies, but not others. Further research is required in order to settle the question, but even if it turns out that beta-hydroxybutyrate really is an HDAC inhibitor, it’s still not going to be effective in protecting against colorectal cancer in people who don’t eat enough fibre (and other carbohydrates indigestible to humans) to produce enough butyrate to form beta-hydroxybutyrate in the colonocytes.

As the authors of the article on the HDAC inhibitory effects of butyrate and propionate warn, most people don’t eat enough fibre to provide their gut microbiota with the raw materials needed to produce sufficient quantities of these health-promoting, cancer-suppressing short chain fatty acids. Colorectal cancer is on the rise in relatively young people (that is, adults aged under 50)². These are also the most ‘online’ generations, in whom taking dietary advice from some internet rando has become absolutely normalised… and hence, they are the most likely to be taken in by influencers promoting carnivore, animal-based ketogenic, and other no- and low-fibre diets.

Since I’m completely opposed to internet censorship, I don’t argue for the deplatforming of these ill-informed influencers. Instead, the public needs to become much more savvy about the information they encounter online, and the purveyors of such information. Extraordinary claims require extraordinary evidence. If you come across someone who claims that fibre does not promote human health, you should demand a very high level of evidence for this claim. A string of anecdotes about people who ‘felt better’ when they cut out fibre is not extraordinary evidence.

By all means, maintain a posture of scepticism toward accepted wisdom, but always bear in mind the following pearl:

“Keep an open mind, but not so open that your brains fall out.”

Attribution uncertain³

And if the ‘nutrition wars’ are wearing you out so much that you sympathise with Bo_Diddley…

… remember that eating for health is really not all that complicated. Cut out the CRAP (Calorie-Rich And Processed food), eat food that is recognisable as food – single ingredients, no funny numbers, and no processing beyond what you could do in your home kitchen – and you won’t go wrong.

Is your gut telling you it’s time for a healthy change? Need help to get your health back on track? Apply for a Roadmap to Optimal Health Consultation today.

1. One of the commenters on this post stated that the figure was taken from a study of people suffering from IBS; I have not been able to verify this. It is certainly the case that in some presentations of IBS, fibre intake can cause bloating because of visceral hypersensitivity. The solution is to address the visceral hypersensitivity using gut-directed hypnotherapy and specific pre- and probiotics, not to eliminate fibre from the diet! Furthermore, constipation causes bloating, and if constipation is not resolved before increasing fibre intake, the bloating will get worse. Once again, the solution is to address the constipation, not to eliminate fibre. ↩︎
2. This increase predates the roll-out of the mRNA vaccines. Early-onset colorectal cancer incidence has been rising since the 1990s, with the youngest adults – those aged 20-30 – experiencing the most dramatic surge in rate of diagnosis. ↩︎
3. Who first coined this memorable adage is a hotly contested matter; read the Quote Investigator entry for the leading contenders. ↩︎