Kefir is a tangy, probiotic-rich drink made by fermenting cow’s or goat’s milk or water with a culture starter called kefir grains — small clusters of bacteria and yeast that resemble tiny cauliflower ...

You could say that kefir has it all. The fermented milk product—which is made from milk and “kefir grains” consisting of bacteria and yeast—is rich in probiotics, B vitamins, vitamin C, and calcium.