Trillions of microbes, mostly in the colon. They ferment fiber, make some vitamins (like K2) and interact with the immune system.
What's known
Diet shapes the microbiome within days. Fiber-rich diets support fiber-fermenting bacteria.
What's not
There's no agreed definition of a 'healthy microbiome', and most consumer microbiome tests can't tell you what to change.
Evidence & sources
Diet can change the composition of the gut microbiome within days. Evidence: moderate. Known: Controlled feeding studies show rapid shifts when diets change dramatically. Unknown: Long-term health effects of these shifts are not well established. Source: David et al., Nature, Diet rapidly and reproducibly alters the human gut microbiome (2014).