Fermented foods like yogurt, kefir, kimchi, sauerkraut, miso, and kombucha contain live microbes and the compounds they produce. In a 2021 Stanford study, people who added fermented foods to their diet for 10 weeks showed increased gut-microbiome diversity and lower levels of several inflammation markers. Interestingly, a high-fiber diet alone did not increase diversity as much over the same period.
The takeaway isn't that fiber doesn't matter — it very much does — but that fermented foods add something distinct. Choose ones with live and active cultures (for things like sauerkraut, the refrigerated versions rather than shelf-stable pasteurized ones), and start small if your gut isn't used to them.