Live bacteria, real research, and one label distinction worth checking for.
Fermentation uses live microorganisms (bacteria or yeast) to transform food — yogurt and kefir from milk, sauerkraut and kimchi from cabbage, miso and natto from soybeans, kombucha from tea. Beyond flavour and preservation, this process can introduce beneficial live bacteria into your gut.
Research on fermented foods and gut microbiome diversity has generally been encouraging, including studies finding measurable shifts in microbiome composition and some inflammatory markers among people who regularly consume a variety of fermented foods.
Some commercially produced fermented foods are pasteurised after fermentation, which kills the live bacteria — a genuine distinction worth checking for on labels if the live-culture benefit specifically is what you're after (look for "contains live cultures" or similar labelling).
A reasonable starting point: adding one fermented food you actually enjoy — yogurt, kimchi, sauerkraut — a few times a week captures most of the practical benefit without needing to eat all of them.