Gut Microbiome

The trillions of bacteria in the human gut digest complex carbohydrates into short-chain fatty acids (about 10% of a person's calories), synthesise vitamin K and several B vitamins, tune the immune system, suppress pathogens and regulate gut-to-brain signalling molecules such as serotonin.

What the evidence supports

• Diversity is the only robust marker. A simple lack of biodiversity is the clearest red flag for poor gut health. There is no reference "right" microbiome: two very different communities can serve their hosts equally well.
• Diet diversity drives microbial diversity. American Gut Project (2018, citizen science): people eating 30 plant types a week had significantly more diverse microbiomes than those eating fewer than 10. Observational, self-reported.
• Fermented foods diversify the microbiome. Stanford (2021, Cell): kimchi, sauerkraut and similar foods raised diversity.

What it does not support

• Personalised, bug-by-bug optimisation. Direct-to-consumer (D2C) stool tests report which species are present and sell tailored diets, probiotics and supplements to fill "gaps". With no reference microbiome there is nothing to optimise toward, and the claim that adjusting particular species perceptibly improves health has scant published basis. Even Zoe's peer-reviewed work on its own customers finds tailored diets have at best marginal effects.
• The tests themselves. NIST (Servetas et al., Communications Biology, Feb 2026) sent 21 identical homogenised stool samples, three each, to seven D2C firms: results differed between firms and, in one case, between a single firm's replicates; firms used different sequencing methods and different definitions of "healthy". Jonathan Eisen (UC Davis): "Most of the claims are bogus."

Open questions

• Whether raising diversity changes clinical outcomes (the studies above measure diversity, not disease).
• Whether any D2C personalisation will survive once analytical standards exist for the tests.

Related

• Can You Hack Your Gut Microbiome (Economist) — source page with the study details.
• Food Fads — D2C microbiome testing as part of the wellness-spend wave.
• Electrolytes You Probably Don't Need (Economist) — the same "marketing ahead of evidence" pattern in hydration products.