Kefir originates from the Caucasus Mountains, where it was traditionally made by fermenting milk in animal-skin bags using "kefir grains" — a distinctive, gelatinous symbiotic culture of bacteria and yeast passed down and shared between households for generations, rather than manufactured. Its name is believed to derive from a Turkish word related to "good feeling."
Kefir's modern popularity has expanded well beyond its Caucasus origins into a global health-food staple, containing a notably more diverse microbial community than yogurt — often cited as containing dozens of distinct bacterial and yeast strains.
Kefir has genuinely well-established evidence for one specific benefit: improving lactose digestion and tolerance in people with lactose maldigestion, a finding described across multiple clinical studies as reasonably solid. This makes sense mechanistically, since kefir's fermentation process breaks down much of the lactose in milk before consumption.
For broader gut microbiome and metabolic claims, the evidence is considerably less settled. A rigorous recent review states plainly that "there is currently very limited scientific evidence" supporting effects on the human gut microbiome. A randomized controlled trial in people with metabolic syndrome found significant improvements within the kefir group — but no significant difference compared to an unfermented milk control group. Given solid evidence for lactose digestion but explicitly limited evidence for broader claims, "Limited-Moderate" is the honest, split rating.
Milk (or sweetened water for a dairy-free variant) fermented with kefir grains over roughly 24 hours at room temperature, producing a tangy, effervescent beverage.