They now occupy the chilled supermarket shelves alongside yogurts and juices, promoted with promises of “billions of good bacteria” and improved “gut health”.

Fermented drinks have surged in popularity as scientists learn more about the trillions of bacteria and other microbes living in the digestive system — collectively known as the microbiome.

Kefir, a tart and lightly sparkling milk drink, and kombucha, which is made from fermented tea, are two of the biggest successes. Both have been consumed for centuries and, when prepared traditionally, may contain a diverse blend of bacteria and yeasts.

Those microbes have fuelled claims that fermented drinks can support gut health and may even influence inflammation and the immune system. Comedian and actor Alan Davies, 60, has said that drinking kefir helped clear his eczema.

However, experts speaking to The Mail on Sunday caution that traditional brews can differ significantly from the products manufactured on an industrial scale for supermarket shelves.

Many mass-produced drinks contain a much narrower selection of bacteria and yeasts, prompting questions about whether products labelled as “gut healthy” can deliver the same potential benefits as traditionally made versions.

Traditional kefir is made with “kefir grains” — jelly-like clusters containing a naturally occurring combination of bacteria and yeast. These are added to fresh milk, which is then left to ferment.

Commercial kefirs often use a carefully chosen blend of cultures instead, enabling manufacturers to produce millions of bottles with a consistent taste and texture. Live yeasts may also be excluded because they can continue fermenting after bottling, changing the drink while it remains on the shelf.

“Traditional kefir contains a far wider variety of bacteria and yeasts,” says Dr Luciana Torquati, a senior lecturer in nutrition at the University of Exeter. “As they ferment the milk, some of these microbes produce substances that may themselves have health benefits.”

“For example, some produce acetic acid, which may help regulate blood sugar and support gut health. Others produce B vitamins and vitamin K, which are needed for functions including energy production and normal blood clotting.”

“When some of those bacteria and yeasts are removed, as they are in certain commercial kefirs, some of the potentially beneficial substances they produce may be lost.”

That does not make commercial kefir unhealthy, but it could mean the drink does not provide everything found in a traditionally fermented version.

Kombucha faces a similar issue. Traditional versions are made by adding a SCOBY — a symbiotic culture of bacteria and yeast — to sweetened tea and allowing the mixture to ferment.

“Its composition is influenced by factors such as the temperature and humidity where it is kept, whether the finished drink is pasteurised and the ingredients used at the start,” says Dr Torquati. “Unlike a factory-made product, no two batches will be exactly the same. That is the beauty of these drinks — they are alive.”

Supermarket kombucha, meanwhile, may be pasteurised and produced under tightly controlled conditions to ensure a consistent product and keep alcohol levels below legal limits. As a result, it can contain fewer live microbes and organic acids than traditional kombucha. Some brands also add ingredients such as sugar.

For shoppers, identifying exactly what is in the bottle is made harder by the lack of a specific legal definition for either kefir or kombucha in Britain. There is no required minimum number or particular combination of microbes for a product to use either name, and manufacturers do not have to list every bacterial and yeast species present.

“Major brands do not list every species,” says Dr Torquati. “They may say ‘14 different species’ or ‘ten billion kefir cultures’, without identifying them. It is therefore difficult for consumers to know which products are genuine kefir.”

Her advice is to choose refrigerated products with short expiry dates and straightforward ingredient lists, while avoiding those containing artificial sweeteners.

Other experts say there is a bigger question: how beneficial are these drinks in the first place?

Kieran Tuohy, a professor of energy metabolism and the microbiome at the University of Leeds, says while there’s strong evidence from lab and animal studies that they have health benefits, ‘we need more controlled human trials before we can say with confidence that any particular type – traditional or mass-produced – is better’.

For most people, he argues, there is also a danger of focusing too much on precisely how a product has been fermented. Swapping biscuits, crisps or chocolate for kefir is still likely to be a meaningful improvement to their diet.

Dr Sharad Ramchandra Kamble, a microbiologist and member of Applied Microbiology International, agrees there is not yet good evidence that the greater variety of microbes found in traditional kefir makes it healthier.

Biotiful Gut Health, the UK market leader, acknowledged that traditional grain-based kefir has a broader range of microbes but said its kefir was designed to provide billions of live cultures per serving while maintaining a consistent taste and texture.