Kefir is milk fermented with kefir grains, a mix of lactic bacteria and yeasts, giving a tart drink thinner than yogurt; 2.5% is a standard fat level in Russia and Eastern Europe. A 250 ml glass has about 135 kcal, 7.5 g of protein and 6.5 g of fat, and gives about 300 mg of calcium; it is high in vitamin B12 and riboflavin. Fermentation turns part of the lactose into lactic acid, so it has less sugar than milk.