Your Gut Is Lying to You (And Social Media Is Making It Worse)
What the science actually says about your gut microbiome — and why a patient of mine ended up in hospital after trusting the internet over her gastroenterologist.
A 33-year-old woman came through the doors of my Emergency Department in significant distress. She had Crohn's disease — a chronic inflammatory bowel condition — and was presenting with profuse bloody diarrhoea, diffuse abdominal pain, and a heart rate that told me she was very unwell.
As I sat with her and took a history, she told me she had recently started a carnivore diet. She'd seen it promoted heavily on social media. Influencers were calling it a way to 'reset the gut,' reduce inflammation, and essentially heal autoimmune disease. She had felt great initially. Things seemed to settle. Then she stopped having bowel movements. And then she ended up in front of me.
She was admitted for intravenous corticosteroids. She was subsequently discharged into the care of her gastroenterologist and started on biologic therapy to manage her IBD.
I am not telling this story to blame her. People trying to get better will try things. I am telling it because this case is a near-perfect illustration of what happens when the gap between what the science shows and what social media claims becomes a clinical problem.
The gut microbiome is one of the most talked-about and simultaneously most misunderstood topics in modern health. So let's fix that.
What Even Is the Gut Microbiome?
Your gastrointestinal tract is home to trillions of microorganisms — bacteria, viruses, fungi, archaea, and protozoa. Collectively, this community is called the gut microbiota. The genetic material of this community is the gut microbiome.
Research from Rinninella and colleagues (2019) shows that the human gut harbours an extraordinary density of microorganisms, with the colon containing the most dense microbial community in the entire body. The dominant bacterial phyla are Firmicutes and Bacteroidetes — together accounting for approximately 90% of gut bacterial composition in healthy adults.
What determines this composition? Quite a lot, as it turns out:
• How you were born and whether you were breastfed
• Where in the world you grew up
• Your age — the microbiome shifts substantially from infancy through to older adulthood
• What you eat — one of the most powerful modifiable influences
• Medications — especially antibiotics, proton pump inhibitors, and metformin
• Disease states — including IBD, metabolic syndrome, and neurological disorders
This is not static biology. The microbiome is a living ecosystem, and it responds — sometimes rapidly — to changes in how we live and what we eat.
What Does Your Gut Microbiome Actually Do?
It makes things your body can't make on its own.
The gut microbiota ferments dietary fibre — the complex carbohydrates that reach your colon intact. This fermentation produces short-chain fatty acids (SCFAs), particularly butyrate, propionate, and acetate. These molecules are not a minor detail. Butyrate is the primary fuel source for the cells lining your colon. It maintains the gut barrier, modulates immune responses, signals satiety hormones, and influences how your body handles glucose and lipids.
When fibre intake drops — as it does on a carnivore diet — butyrate production falls. The consequences for the gut lining are significant. We will come back to this.
It trains your immune system.
Approximately 70-80% of the body's immune cells are located in or around the gut. The microbiome helps the immune system distinguish between friendly organisms and pathogens, and modulates inflammatory responses. Disruption of this community — termed dysbiosis — is associated with increased immune activation and systemic inflammation. In conditions like inflammatory bowel disease, dysbiosis is not just a consequence of the disease — it may actively contribute to it.
It talks to your brain.
The gut-brain axis is one of the most fascinating areas of emerging research. The gut and the brain are in constant, bidirectional communication via the enteric nervous system (500 million neurons — often called the 'second brain'), the vagus nerve, immune signalling molecules, and microbially-produced metabolites including neurotransmitter precursors.
Cryan and colleagues (2019) document links between gut microbiota composition and conditions including anxiety, depression, stress responses, and neurodevelopmental disorders. This does not mean your gut microbiome causes all mental illness — the field is still evolving — but the mechanistic connections are real and increasingly well-characterised.
It protects your gut wall.
The gut is lined by a single layer of epithelial cells. A healthy microbiome maintains this barrier through SCFA production. When this barrier is compromised — sometimes called 'leaky gut', or clinically, increased intestinal permeability — bacterial products can translocate into the bloodstream and trigger systemic inflammation. This has been linked to metabolic disease, cardiovascular disease, and chronic inflammatory states. Fibre-rich diets are strongly associated with maintaining gut barrier integrity. Low-fibre diets are not.
Can You Test Your Gut Microbiome?
Here is where I want to be careful, because this is an area where commercial enthusiasm has significantly outpaced the science.
The main methods used to study the gut microbiome include 16S rRNA gene sequencing (broad bacterial overview), shotgun metagenomics (comprehensive but expensive), and metabolomics (measuring SCFA outputs). These are excellent research tools.
But the International Consensus Statement on Microbiome Testing in Clinical Practice (Lancet, 2025) is admirably direct: the current evidence does not yet support routine microbiome testing for the diagnosis or monitoring of most conditions in clinical practice. Their reasoning:
• There is no agreed-upon definition of a 'healthy' microbiome due to substantial inter-individual variability
• Reference ranges have not been established for clinical use
• Therapeutic recommendations based on microbiome testing alone are premature outside a small number of specific indications
The notable exception is Clostridioides difficile (C. diff) infection — where faecal microbiota transplantation (FMT) has strong evidence and is endorsed by major gastroenterological guidelines.
The direct-to-consumer gut microbiome testing market — where you mail in a stool sample and receive a personalised supplement protocol — is significantly ahead of the science. If a test comes bundled with a supplement protocol, that should tell you something about the business model.
How to Actually Improve Your Gut Health
Tier 1 — The Non-Negotiables
1. Eat more fibre. A lot more fibre.
This is, without question, the single best-evidenced dietary intervention for gut microbiome health. Current intake in Western populations sits at around 15-17 grams per day. Guidelines from the American Heart Association and the 2025 Dietary Guidelines Advisory Committee recommend 25-38 grams per day. Most of us are not meeting that.
Practical targets:
• Aim for 30 different plant foods per week (vegetables, fruits, legumes, nuts, seeds, whole grains, herbs, and spices all count)
• Include legumes — lentils, chickpeas, black beans — several times per week
• Choose whole grains over refined grains
• Include good prebiotic sources: oats, garlic, onion, leeks, asparagus, and green bananas
2. Eat fermented foods regularly.
Research from Sonnenburg's group at Stanford showed that a high-fermented food diet increased microbiome diversity and reduced inflammatory markers in a randomised trial. Think plain yoghurt with live cultures, kefir, kimchi, sauerkraut, miso, and tempeh.
3. Limit ultra-processed foods.
Highly processed foods are associated with reduced microbial diversity, increased abundance of pro-inflammatory species, and degradation of the gut barrier. Artificial emulsifiers commonly found in processed foods — including carrageenan and polysorbate 80 — have been shown to disrupt the mucus layer lining the gut.
4. Exercise. Sleep. Manage stress.
Regular physical activity increases microbial diversity and butyrate-producing organisms. Disrupted sleep and chronic stress alter gut motility, increase intestinal permeability, and are associated with dysbiosis. The gut-brain axis works in both directions: a stressed brain changes the gut environment, and a dysbiotic gut can worsen stress and mood.
Tier 2 — Worth Considering
• Condition-specific probiotics — useful for antibiotic-associated diarrhoea, some IBS presentations, and C. diff prevention. General probiotic supplementation in healthy people has a more limited evidence base.
• Low-FODMAP diet — supervised by a dietitian, this has good evidence for reducing IBS symptoms. It is a diagnostic and symptom-management tool, not a permanent way of eating.
• Prebiotic supplements (inulin, FOS, GOS) — can be useful if whole food fibre intake is insufficient, but are not a substitute for dietary diversity.
The Carnivore Diet: Prebunking the 'Gut Reset' Claim
Back to my patient. And back to the claim she was sold: that a carnivore diet — eating only meat, fish, eggs, and animal products, with zero plant foods — is the optimal way to reset the gut.
I want to be fair. Some people report feeling better initially on a carnivore diet. That's real, and worth understanding. But feeling better temporarily is not the same as the diet being safe or beneficial long-term. Here's why the evidence gives us serious cause for concern.
The 'gut reset' mechanism doesn't hold up.
The claim goes: your gut is inflamed because of plant compounds, lectins, or oxalates. Remove all plants. Reset the gut.
This is a dramatic oversimplification. The gut microbiome is a complex ecosystem that depends on diversity of substrate — particularly fibre — to maintain health. A carnivore diet eliminates the primary substrate that feeds the majority of beneficial organisms. The lectin and oxalate toxicity claims are not supported by clinical outcome data in normal individuals. Lectins are denatured with normal cooking. Oxalate concerns are relevant only in a small subset of patients with specific metabolic conditions.
The evolutionary argument doesn't hold up either.
The narrative is: humans evolved as carnivores. Meat is natural. Plants are a modern mistake. The archaeological and anthropological evidence does not support the claim that humans are obligate carnivores. The human digestive tract — including the length of our colon and the enzymes we produce — is consistent with an omnivorous diet with significant plant matter. The healthy human microbiome is structured around the fermentation of plant-derived fibre. In the absence of fibre, the organisms that produce butyrate decline. Natural does not equal optimal.
The long-term trajectory is concerning.
There are no long-term randomised controlled trials on the carnivore diet showing improved health outcomes. What we do have is strong and consistent evidence that:
• Low-fibre diets reduce SCFA production
• Reduced SCFA production impairs gut barrier function
• Impaired gut barrier function increases systemic inflammation
• Long-term low-fibre intake is associated with increased risk of colorectal cancer — one of the best-established diet-cancer relationships in the literature
• High red and processed meat intake increases production of secondary bile acids and N-nitroso compounds, both pro-inflammatory and genotoxic
The carnivore diet may provide short-term symptom relief — particularly by reducing fermentable carbohydrates, which can decrease gas and bloating. But the longer-term biological trajectory, based on what we know about fibre, butyrate, gut barrier function, and colorectal cancer risk, is concerning.
Back to my patient.
She had Crohn's disease — a condition already characterised by gut dysbiosis and impaired gut barrier function. She adopted a carnivore diet that eliminated the substrates maintaining that barrier. Initially she felt better (reduced fermentation, reduced stool frequency — which can feel like improvement). Then gut motility slowed in the absence of fibre. The beneficial organisms declined. The mucus layer thinned. In a patient with pre-existing compromised gut barrier function, this was predictably bad.
The result: a severe flare requiring intravenous steroids and hospitalisation, followed by escalation to biologic therapy.
There is currently no evidence that the carnivore diet is safe or beneficial for people with Crohn's disease or ulcerative colitis. Major gastroenterological guidelines (AGA, 2024) recommend increasing dietary fibre during IBD remission — the exact opposite of what the carnivore diet provides.
The 30,000ft View
The gut microbiome is a genuinely remarkable system. The science connecting it to metabolic health, immune function, mental wellbeing, and chronic disease prevention is real, growing, and worth taking seriously.
But the discourse around it is noisy. There are people claiming it can cure everything. There are people claiming it doesn't matter at all. Neither is accurate.
The evidence-based picture is this: the gut microbiome thrives on diversity. Diversity of organisms, diversity of substrates, diversity of plant foods. Fibre is the lever. Exercise, sleep, and stress management compound the effect. Fermented foods add to it. The carnivore diet, whatever short-term symptomatic relief it may offer, works directly against the conditions under which a healthy microbiome is maintained.
And if you or someone you care about has inflammatory bowel disease — please work with a gastroenterologist and a dietitian with specialised expertise. This is exactly the kind of condition where complexity demands expert guidance. Not a YouTube algorithm.
How Should This Modify Your Practice?
Whether you are a healthcare professional or someone trying to make good decisions about your own health, here is the practical summary:
• Eat more fibre — target 30 plant foods per week and 25-38g of fibre per day
• Add fermented foods to your regular rotation
• Limit ultra-processed food and added sugar
• Exercise most days, prioritise sleep, and manage stress — the gut-brain axis is bidirectional
• Be sceptical of direct-to-consumer gut microbiome testing, especially when bundled with supplement protocols
• Do not adopt a carnivore diet as a gut health strategy — the evidence does not support it, and for people with IBD, it can be dangerous
• If you have IBD, do not self-manage with social media dietary protocols — see a gastroenterologist and a dietitian
The gut microbiome is a lever. A real, well-evidenced lever. Pull it carefully.
Author: Dr Adrian Cois, MD
Emergency Medicine Specialist | Founder, Mindful Mates Health
@dr_cois
Extend Yourself
Want to go deeper? Listen to Episode 9 of Overheard in the Emergency Room — 'Gut Feelings: Fibre, Bugs & Bloating' — for the full evidence breakdown, including a deeper dive into the gut-brain axis and our Misinformation Watch segment.
Free resources available at DrCois.com
References
1. Rinninella E, Raoul P, Cintoni M, et al. What is the Healthy Gut Microbiota Composition? A Changing Ecosystem across Age, Environment, Diet, and Diseases. Microorganisms. 2019;7(1):14.
2. Cryan JF, O'Riordan KJ, Cowan CSM, et al. The Microbiota-Gut-Brain Axis. Physiol Rev. 2019;99(4):1877-2013.
3. Zhang P. Influence of Foods and Nutrition on the Gut Microbiome and Implications for Intestinal Health. Int J Mol Sci. 2022;23(17):9588.
4. Porcari S, Mullish BH, Asnicar F, et al. International consensus statement on microbiome testing in clinical practice. Lancet Gastroenterol Hepatol. 2025;10(2):154-167.
5. Hashash JG, Elkins J, Lewis JD, Binion DG. AGA Clinical Practice Update on Diet and Nutritional Therapies in Patients With Inflammatory Bowel Disease: Expert Review. Gastroenterology. 2024;166(3):521-532.
6. Scientific Report of the 2025 Dietary Guidelines Advisory Committee. dietaryguidelines.gov.
