In the summer heat, the desire to quench our thirst with something cool and refreshing often leads us to the shelves with kvass, kombucha or kefir. Traditionally, it is believed that such fermented drinks are almost an elixir of health for our digestion. Marketers convince us of the presence of "live bacteria", and folk medicine has been promoting kvass for years as the best way to improve the functioning of the gastrointestinal tract. But is this really so?