A tangy, sour, fermented milk drink may not sound like a likely candidate to move from health food stores to mainstream supermarkets, but that's exactly what kefir has done. The beverage is steadily gaining fans convinced of the health benefits -- proponents tout its purported ability to help cure cancer, reduce high cholesterol and treat high blood pressure -- yet the scientific studies to support the claims are still few.
Kefir's closest cousin is yogurt, also made by fermenting milk with bacteria. But kefir is fermented with more and different types of bacteria, in addition to yeast, which means the final product has more of the beneficial microorganisms, or "probiotics," that first made yogurt a popular health food. Probiotics can control the growth of harmful bacteria and aid digestion, and some even manufacture vitamins in the gut.
The drink is a good source of calcium, protein and potassium (and less desirably, in its fruit-flavored forms, sugar).
Whether the drink is any more immune-boosting than, say, spinach, or any other nutrient-dense food, remains to be seen. Claims that kefir can help cure cancer stem from findings that the drink, or some of its components, hindered tumor cells in test tubes and lab animals. In vitro, kefir has been shown to slow the growth of breast cancer cells. In mice injected with cancer cells, it has slowed the development of tumors and increased the activity of such immune system cells as so-called natural killer and T-helper cells. A 2007 Japanese study suggested the drink may do the same in humans: 19 adults who drank kefir daily for three weeks had unusually active natural killer cells.