Imagine yogurt in a drinkable form, and with bubbles. If you can picture that, you've got a decent idea what kefir is.
Similar to yogurt, kefir (pronounced ka-FEER) is made by fermenting milk. The difference between the two lies in what's used for fermentation. To make yogurt, you simply use bacteria, while kefir calls for both bacteria and yeast.
In both yogurt and kefir, bacteria ferments the lactose found in milk into lactic acid, giving these products a tangy flavor. Additionally, the yeast in kefir transforms lactose into carbon dioxide (with a small amount of alcohol). This gives kefir its characteristic carbonation, earning it the nickname "the champagne of milk."
The different preparations result in different consistencies. While yogurt is a creamy semi-solid, kefir is a bubbly drink.
Kefir offers a number of established and potential health benefits. It has three times the probiotic content — the "healthy" bacteria that block harmful bacteria from living in your gut — that yogurt has.
Kefir is also a good source of calcium, B vitamins and phosphorous. It's a low-fat source of protein, too. Some health experts also claim digestive benefits for kefir, but while a few studies have shown it aids in lactose digestion, the research is limited.