Gut and microbiome: Evidence-Based Supplement Guide

The gut digests food and absorbs nutrients, and most of the body's bacteria live in the colon (PMID 27541692). A revised estimate found the number of bacteria in the body is about the same as the number of human cells, not the widely quoted ten to one (PMID 27541692). The microbiome is the community of microbes living in and on the body, together with their genes. In a 17-week randomised study of 36 healthy adults, a diet high in fermented foods steadily increased microbiome diversity and decreased inflammatory markers (PMID 34256014). A high-fibre diet left diversity stable, and immune responses to it varied with each person's starting microbiome (PMID 34256014). Prebiotics and probiotics are different things. An international expert consensus defines a prebiotic as a substrate that host microbes selectively use, conferring a health benefit (PMID 28611480). Probiotics are live microbes, and in one study, taking them after antibiotics delayed the return of the person's own gut microbiome (PMID 30193113).

1. **If you want to try one dietary change, fermented foods have trial evidence here.** They increased microbiome diversity in a small randomised study (PMID 34256014). 2. **Expect individual responses to fibre.** Responses to a high-fibre diet depended on each person's starting microbiome (PMID 34256014). 3. **Check what a product means by prebiotic.** The expert definition requires selective use by microbes and a documented health benefit (PMID 28611480). 4. **Talk to a clinician before taking probiotics after a course of antibiotics**, since in one study they delayed the gut microbiome's return (PMID 30193113).

The fermented food and fibre study ran for 17 weeks, and microbiome diversity rose steadily on the fermented food diet (PMID 34256014). In the antibiotic study, the microbiome's return was delayed and still incomplete with probiotics (PMID 30193113).

- **The diet study was small**, with 18 people in each group (PMID 34256014). - **The probiotic finding comes from one study in mice and people** that used a probiotic mixture (PMID 30193113). - **A definition is not evidence that a product works.** The consensus statement sets out terms, not trial results (PMID 28611480). - **The bacteria count is an estimate for a reference adult man** (PMID 27541692). - **This guide has not been reviewed by a clinician.** `reviewStatus` above says so.

- **The fermented food study measured diversity and inflammatory markers, not symptoms**, so it does not show that any symptom improves (PMID 34256014). - **Do not stop or change antibiotics because of anything here.** Antibiotic decisions belong with your prescriber. - **Probiotic and prebiotic products vary.** Ask a pharmacist or clinician before using them alongside medicines.

No ingredient is linked from this guide. It covers how the gut microbiome responds to diet, and it does not point to a product.

| PMID | Title | | --- | --- | | 27541692 | "Revised Estimates for the Number of Human and Bacteria Cells in the Body" | | 34256014 | "Gut-microbiota-targeted diets modulate human immune status" | | 28611480 | "Expert consensus document: The International Scientific Association for Probiotics and Prebiotics (ISAPP) consensus statement on the definition and scope of prebiotics" | | 30193113 | "Post-Antibiotic Gut Mucosal Microbiome Reconstitution Is Impaired by Probiotics and Improved by Autologous FMT" |