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.
But such research is challenging to interpret. It not only has focused largely on rodents but also has used kefir products made with limited bacterial starter cultures to pinpoint which culture may be responsible for the potential benefits. The effects that grocery-store kefir might have on cancer patients haven't been studied in rigorous clinical trials.
Studies examining kefir's effects on cholesterol have also used variants of the drink. A number of European studies have shown that kefir can reduce cholesterol levels -- as long as the drink is enriched with phytosterols or stanols, plant-based chemicals known to lower levels of LDL, or "bad," cholesterol on their own. Studies on traditional kefir are fewer in number and have produced mixed results. A small 2003 study conducted in Japan showed that drinking 300 milliliters of milk fermented with some of the strains found in kefir daily for four weeks lowered cholesterol slightly in half the participants. In a small 2002 study conducted in Canada, drinking traditional kefir daily for the same period of time had no effect on cholesterol.
A few studies have suggested that fermented milk similar to kefir can reduce blood pressure -- in rats. Human evidence is scarcer and conflicting. A Dutch study published in the journal Hypertension this year concluded that drinking fermented milk for eight weeks was not an effective way to lower blood pressure. A Finnish study published in the American Journal of Clinical Nutrition in 2003 showed that drinking fermented milk for 21 weeks did appear to lower systolic and diastolic blood pressure slightly.
If there's one thing kefir may be most likely to do, it's aid in digestive health. Like yogurt, kefir contains lactase, the enzyme that breaks down lactose, the dominant sugar in milk. In a 2003 study of 15 adults unable to digest lactose, Ohio State University researchers found that drinking kefir reduced lactose maldigestion symptoms -- including bloating, stomach pain and gas -- by 70%. French researchers produced similar findings; they've also shown that kefir can speed recovery from diarrhea in infants.
If the taste doesn't offend, there's certainly no harm in drinking kefir for its nutrients.