Fermented foods
Foods carrying live microbial cultures, linked to gut diversity.
Fermented foods are made by the deliberate action of microorganisms: yoghurt, kefir, sauerkraut, kimchi, miso, tempeh, traditional cheeses, sourdough. The fermentation changes flavour and texture, often improves keeping qualities, and in some cases changes the nutritional content.
Not all of them deliver live microbes to you. Beer, bread and most shop bought sauerkraut are fermented and then heated or pasteurised, which kills the organisms, so the food is fermented without being a source of live cultures.
The evidence took a step forward with a randomised trial from Stanford. Over ten weeks, a diet high in fermented foods increased gut microbiome diversity and reduced several markers of inflammation, while a high fibre arm did not produce the same immune changes in that timeframe.
That is a genuinely interesting result and one study rather than settled science. It was small, short, and in healthy adults, and it does not establish clinical outcomes.
It is nonetheless a better supported route than most of the probiotic supplement aisle, because it delivers a wide range of organisms in a food matrix rather than a few strains in a capsule, at a cost closer to groceries than to supplements.
Two cautions are worth naming. Commercial versions of drinks such as kombucha often carry a lot of sugar, and fermented foods are high in histamine and in FODMAPs, which some people with IBS tolerate poorly.
In practice, choose unpasteurised versions where you can, add them gradually, and treat them as a pleasant addition to a high fibre diet rather than a replacement for one.