Gut Health & The Microbiome

1. Thesis

Gut health is mostly determined by what you eat, not by what you supplement — and the commercial market has this almost exactly backwards.

The microbiome is real, important, and genuinely one of the more exciting frontiers in medicine. That scientific excitement has been converted, with remarkable speed, into a consumer category that promises far more than the evidence supports. The two facts sit uncomfortably together and both must be said: the science is fascinating, and most of the products sold on the back of it do not work.

Three claims organise this monograph:

1. Fibre is the intervention with the best evidence, and almost nobody sells it. Dietary fibre

improves bowel function and reduces cardiovascular disease, metabolic disease, and mortality. Psyllium specifically improves constipation and IBS, and lowers LDL cholesterol and blood glucose.

1. "Probiotics" is a category, not a treatment. Effects are strain-specific and indication-specific.

A few strain–indication pairs have genuine evidence. General "gut health" in healthy adults is not one of them.

1. Consumer microbiome testing is ahead of the science. There is no established healthy reference

microbiome to compare a result against, and no evidence that acting on such a test improves any outcome.

The most consequential thing on this page is not a supplement at all. It is the distinction between irritable bowel syndrome and inflammatory bowel disease — two conditions with confusingly similar names, entirely different mechanisms, and radically different stakes. One is a disorder of gut–brain signalling. The other physically damages the bowel. Faecal calprotectin tells them apart.

2. What The Gut Actually Does

The gastrointestinal tract digests food and absorbs nutrients, but it is also the body's largest immune interface and the home of the gut microbiome — trillions of microorganisms that ferment what we cannot digest, produce short-chain fatty acids, metabolise drugs and bile acids, and communicate continuously with the immune and nervous systems.

The gut–brain axis is not a metaphor. There is bidirectional signalling between the enteric nervous system and the brain, which is why stress genuinely alters bowel function and why psychological therapies genuinely treat bowel disorders. Both directions of that sentence matter.

3. The Microbiome: What Is Established, and What Is Not

Established. The gut microbiome differs between people. It is shaped substantially by diet. Low diversity is associated with several diseases. Disrupting it — with antibiotics, for instance — has consequences. Faecal microbiota transplantation is genuinely effective for recurrent Clostridioides difficile infection, which is the clearest proof that the microbiome can be therapeutically targeted.

Not established. That there is a single "healthy" microbiome composition. That an individual's microbiome can be usefully graded against a reference. That taking a probiotic capsule meaningfully or durably changes an established adult microbiome. That "rebalancing" the microbiome treats disease outside a few specific settings.

The gap between those two lists is where the entire consumer microbiome industry operates.

4. Consumer Microbiome Testing

Direct-to-consumer microbiome tests sequence a stool sample and return a report — often with scores, rankings, and personalised food recommendations.

BioSignal's assessment: not currently useful for health decisions. There is no validated reference range for a "healthy" microbiome, results vary substantially with sampling and method, recommendations differ between companies given the same sample, and no trial has demonstrated that acting on such a test improves any clinical outcome.

This is why BioSignal does not publish a Biomarker page for consumer microbiome testing. A Biomarker page implies a number that can inform a decision. Creating one here would lend the test a legitimacy the evidence does not support.

5. Fibre: The Intervention That Actually Works

Fibre is not one substance, and this is the single most useful practical distinction in gut health.

Soluble, gel-forming, minimally fermented (psyllium): holds water, normalises stool form in both directions, produces little gas. Improves constipation and IBS. Also binds bile acids, modestly lowering LDL cholesterol, and slows carbohydrate absorption, modestly improving glycaemic control.

Insoluble (wheat bran): adds bulk but ferments and can increase gas, bloating, and pain — and can make IBS worse. This is why the blanket advice to "eat more fibre" fails so many IBS patients, and why they conclude, wrongly, that fibre is not for them.

Fermentable fibres (inulin, FOS, resistant starch): feed the microbiome and produce short-chain fatty acids, but also produce gas — a benefit in general health, a problem in IBS.

Most people eat well under recommended fibre intakes. Closing that gap with food does more for gut and whole-body health than any supplement in this monograph.

6. Probiotics: Strain-Specific, Not Category-Wide

Asking "do probiotics work?" is like asking "do drugs work?". The answer depends entirely on which one, for what, in whom.

Reasonable evidence: reducing antibiotic-associated diarrhoea (specific strains); preventing recurrence of pouchitis (specific multi-strain preparations, clinician-directed); infant colic (specific strains).

Mixed or weak: irritable bowel syndrome — some strains help some people, inconsistently.

Not established: general "gut health" in healthy adults; immunity; mood; weight loss. These are the claims that sell the category.

A practical problem that is rarely mentioned: the strain that produced a benefit in a trial is frequently not the strain in the product on the shelf. Many products list only genus and species, do not guarantee viable organisms at end of shelf life, and have never been tested as formulated. If a product does not name its strain, no trial evidence transfers to it.

A safety point the category also rarely mentions: these are live organisms. In critically ill, severely immunocompromised, or central-line patients, serious infections have been reported. "Natural" is not the same as "harmless in every context".

7. Glutamine and "Leaky Gut"

Glutamine is the primary fuel of intestinal cells. That is true, and it is the foundation of an entire supplement category built on a diagnosis — "leaky gut syndrome" — that is not a recognised clinical entity.

Intestinal permeability is a genuine, measurable research phenomenon, and it is altered in several diseases. What has not been established is that increased permeability causes those diseases (rather than resulting from them), that it can be diagnosed as a standalone condition, or that glutamine supplementation improves it in a way that matters clinically.

Healthy adults are not glutamine-deficient. The body produces it in abundance.

The cautionary tale. Glutamine was once standard practice in critical care on precisely this kind of mechanistic reasoning. A large randomised trial then found increased mortality with high-dose glutamine in critically ill patients. It is one of medicine's cleaner demonstrations that a compelling mechanism, applied confidently, can cause harm rather than benefit.

8. IBS: A Real Disorder With Real Treatments

Irritable bowel syndrome is a disorder of gut–brain interaction — recurrent abdominal pain associated with altered stool frequency or form, diagnosed positively by symptom criteria (Rome IV), not by exclusion of everything else.

Two things are said about IBS that are both wrong. The first is that it is "nothing" — the bowel looks normal, therefore nothing is wrong. The second is that it is "all in your head". Both misunderstand the condition. The gut–brain signalling is genuinely dysregulated, which is why psychological therapies work and why the symptoms are physically real. Those facts are not in tension.

What works, roughly in order of evidence:

The low-FODMAP diet deserves a caveat it rarely receives: it is a three-phase protocol — restriction, structured reintroduction, personalisation. It is not meant to be permanent. People left indefinitely in the restriction phase needlessly narrow their diets and may harm the microbiome they were trying to help.

9. IBD: A Different Disease Entirely

Inflammatory bowel disease — Crohn's disease and ulcerative colitis — is immune-mediated inflammation that physically damages the bowel. Unlike IBS, it causes visible tissue injury, and untreated it leads to hospitalisation, surgery, and increased colorectal cancer risk.

Modern biologic therapy has transformed outcomes. The most important thing a person with IBD can do is take the treatment that prevents bowel damage.

This is the highest-stakes claim in this monograph: IBD is not a condition to manage with supplements. Treating active inflammation with glutamine, probiotics, or peptides instead of proven therapy risks permanent, irreversible bowel damage. Exclusive enteral nutrition genuinely induces remission in paediatric Crohn's disease and nutrition matters throughout — but that is clinical nutrition under supervision, not a supplement regimen.

10. Telling Them Apart: Faecal Calprotectin

Symptom severity does not distinguish them. IBS can be severe and disabling while causing no damage; IBD can be symptomatically mild while quietly inflaming the bowel. This is exactly why an objective test matters more than how bad it feels.

Alarm features — bleeding, weight loss, anaemia, nocturnal symptoms, onset over 50, family history of bowel cancer or IBD — warrant investigation, not self-management.

11. GERD: The Under-Emphasised Lever

Reflux is common and treatable, and two things are consistently missed.

Weight loss is the most effective lifestyle intervention — better evidenced than the dietary trigger lists that dominate popular advice. Trigger avoidance beyond the foods you personally notice has surprisingly weak evidence.

Chronic reflux can cause Barrett's oesophagus, a precursor to oesophageal cancer. This is why long-standing symptoms and alarm features should not be managed indefinitely with over-the-counter acid suppression.

On proton pump inhibitors: they are effective, and for people who need them the benefits generally outweigh the risks. Observational studies have linked long-term use to various harms, but these are prone to confounding and are not established as causal. The sensible position is neither fear nor indefinite unreviewed use: take them if indicated, at the lowest effective dose, and review the need periodically.

12. Constipation: Where Common Advice Is Wrong

Two pieces of standard advice do not survive contact with the evidence.

"Drink more water." Increasing fluid beyond adequate hydration does not improve constipation unless you are actually dehydrated — the excess is simply excreted. Fluid matters chiefly as a partner to fibre, because fibre needs water to work.

"Laxatives are addictive." Osmotic laxatives, particularly polyethylene glycol, are not habit-forming and have strong evidence including for long-term use. This unfounded fear leads people to endure treatable symptoms for years. Stimulant laxatives are better suited to short-term or rescue use.

Also frequently missed: pelvic floor dysfunction (dyssynergic defecation), which does not respond to more fibre or more laxatives. It responds to biofeedback therapy, and it is a common cause of constipation that has failed everything else.

13. What The Evidence Does Not Support

• Consumer microbiome tests for health decisions
• Probiotics for general gut health in healthy adults
• Glutamine for "leaky gut" in healthy people
• "Detox" and "gut cleanse" protocols
• Colon cleansing / colonic irrigation (no benefit; real risk)
• Routine food-sensitivity IgG testing (not validated; leads to unnecessary restriction)
• Treating IBD with supplements instead of medical therapy — the most dangerous item on this list

14. Practical Summary

For general gut health: eat more fibre and a wider variety of plants. That is the intervention with the best evidence, and it is not a supplement.

For constipation: psyllium, titrated up gradually with adequate fluid; polyethylene glycol if needed. Do not fear it.

For IBS: soluble fibre, the low-FODMAP diet under dietitian supervision (with reintroduction), gut-directed behavioural therapy, and peppermint oil for pain. Check a calprotectin first.

For reflux: lose weight if you carry excess, don't eat late, elevate the head of the bed. Get alarm features investigated.

If you have blood in your stool, are losing weight, or are anaemic: none of the above. See a clinician.

15. Open Questions

• Is there a definable "healthy" microbiome, or only healthier and less healthy patterns?
• Is altered intestinal permeability a cause of disease, a consequence, or a bystander?
• Can probiotic strains be engineered to durably colonise an adult gut — and would that help?
• Will microbiome-based therapeutics extend beyond C. difficile to other conditions?
• Why does the gut–brain axis respond so well to psychological therapy, and can that be improved upon?

16. Sources (verify-to-source pass required before publication)

• Rome IV criteria for disorders of gut–brain interaction
• ACG Clinical Guideline: Management of Irritable Bowel Syndrome
• ACG Clinical Guidelines: Ulcerative Colitis; Crohn's Disease in Adults
• ECCO guidelines (Crohn's disease and ulcerative colitis)
• ACG Clinical Guideline for the Diagnosis and Management of GERD
• NICE diagnostics guidance on faecal calprotectin testing
• Meta-analyses of dietary fibre and cardiovascular / all-cause mortality
• Cochrane reviews of probiotics for antibiotic-associated diarrhoea
• Randomised trial of high-dose glutamine in critical illness (mortality signal)
• Faecal microbiota transplantation trials in recurrent C. difficile infection