What it means
The gut microbiome is the community of roughly 30–40 trillion bacteria, archaea, fungi and viruses in the intestine, mostly the colon, along with their genes and metabolites. Its most important products are short-chain fatty acids (butyrate, propionate, acetate) from fermenting fibre, which feed the gut lining, regulate immunity and reduce inflammation. Diversity — the number and evenness of species — is the most used summary measure and falls with age, antibiotics and a low-fibre diet.
The gut barrier matters too: a permeable lining lets bacterial fragments (LPS) into the blood, a recognised source of inflammaging.
Why it matters for longevity
Centenarians have distinctive, diverse microbiomes rich in butyrate producers; frailty, obesity, type 2 diabetes and inflammatory disease all show characteristic shifts. The microbiome also decides how a person responds to foods (the glucose work behind the CGM entry), to metformin and acarbose, and whether they can make urolithin A. It is modifiable within days — which is both the opportunity and the reason single tests mean little.
What the evidence says
Established: fibre intake raises short-chain fatty acids and diversity; diversity predicts better metabolic markers in cohorts (American Gut, PREDICT); faecal microbiota transplant cures recurrent C. difficile infection in over 90% of cases (randomised trials). Wilmanski 2021 (9,000 adults) found that microbiomes became more unique with age in healthy agers and that this uniqueness predicted survival.
Not established: that any commercial stool test can tell an individual what to eat; that probiotics improve health in healthy adults (meta-analyses show strain-specific effects on specific conditions, little else); that "leaky gut" is a diagnosis. Stool tests report composition with large day-to-day variation and no validated reference range.
How to measure or use it
- Commercial stool sequencing shows composition and diversity; useful for curiosity, not for decisions. The clinically useful proxies are hs-CRP, HbA1c and symptoms.
- What moves it with evidence: 30+ different plants a week, 25–40 g of fibre a day, fermented foods (Wastyk 2021 showed increased diversity and lower inflammatory markers in ten weeks), fewer ultra-processed foods, no unnecessary antibiotics.
- Probiotics: a specific strain for a specific indication, with a trial behind it; not a general supplement.
Related
- Inflammation and immunity — the gut barrier and inflammation
- Metabolic health — microbiome and glucose response
- The immune and inflammation protocol — the gut in a plan
Frequently asked questions
Should I get my microbiome tested?
Only out of interest. Composition varies day to day, there is no validated reference range, and the actionable advice is the same for nearly everyone: more plant diversity and fibre.
Do probiotics work?
Specific strains for specific conditions, yes — antibiotic-associated diarrhoea, some IBS subtypes. As a general health supplement, the evidence is weak.
How fast does diet change the microbiome?
Measurable shifts within three to four days of a major dietary change; lasting change needs lasting habits.
Want this applied to your own numbers?
Fifteen minutes of intake, your labs, a human-reviewed protocol within about 24 hours, three documents, a free coach call and check-ins through the cycle. €399, follow-up included.
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