In recent years, the health food industry has been experiencing a real boom in fermented beverages. Supermarket shelves are full of various bottles of kombucha, craft kefir and ayran, which marketers position as a universal elixir for digestive health. We are promised an instant improvement in immunity, perfect digestion and detoxification.

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?