What Your Microbiome Actually Is

The gut microbiome is one of the more recently understood organ-scale systems in the body — and it's also one where a popular statistic has been circulating in a corrected form for years without most people hearing the correction.

The Actual Numbers, and the Myth They Replaced

A rigorous re-analysis of the underlying cell-counting data estimated roughly 38 trillion bacteria in a reference adult body — an enormous number[1]. What the same paper also did, less famously, was correct a long-repeated claim: the popular "you have 10 times more bacteria than human cells" statistic is not accurate. The revised estimate puts the ratio close to 1:1 — roughly 38 trillion bacterial cells against roughly 30 trillion human cells — which is still a striking number, just not the 10:1 ratio that's been repeated in health content for over a decade based on an older, less rigorous estimate.

What This Community Actually Does

This community of microorganisms — collectively the microbiome — is now understood to influence immune system development and function, metabolic health, and, through the gut-brain axis covered in Section 4, mood and cognitive function. The diversity of this community (how many different species are present, not just the total count) is generally considered more informative of gut health than the raw bacterial count, which is part of why this guide's practical advice centres on diversity — of fermented foods, of plant fibre — rather than on any single number to maximise.