Why your kefir is not as good for you as you think: It’s hailed – alongside kombucha – for its gut benefits. But now scientists reveal why the supermarket bottles may not be as beneficial as thought

They sit alongside the yogurts and juices in supermarket fridges, promising ‘billions of good bacteria’ and better ‘gut health’.

Sales of fermented drinks have boomed as scientists uncover more about the trillions of bacteria and microbes that live in our digestive system, known as the microbiome.

Two in particular – kefir, a sour, slightly fizzy milk drink, and kombucha, made from fermented tea – have become hugely popular. Both have been consumed for centuries and, when made traditionally, can contain a teeming mix of bacteria and yeasts.

It is these microbes that are behind claims the drinks can improve gut health and even have wider effects on inflammation and the immune system. Comedian and actor Alan Davies, 60, for instance, has said drinking kefir helped clear up his eczema.

Yet experts speaking to The Mail on Sunday warn there can be an important difference between traditional brews and the bottles now produced on an industrial scale for supermarket shelves.

Many contain a far narrower range of bacteria and yeasts – raising questions over whether products marketed as ‘gut healthy’ can really offer the same benefits as traditional versions.

Kefir is traditionally made using ‘kefir grains’ – jelly-like clumps containing a naturally occurring mixture of bacteria and yeast – which are added to fresh milk and left to ferment.

Many commercial versions instead use a carefully selected mix of cultures, allowing manufacturers to produce millions of bottles with the same taste and texture. Live yeasts can also be left out as they may continue fermenting after bottling, altering the product while it sits on the shelf.

‘Traditional kefir contains a much wider mix of bacteria and yeasts,’ says Dr Luciana Torquati, a senior lecturer in nutrition at the University of Exeter. ‘As they ferment the milk, some produce substances which may themselves have health benefits.

‘For example, some produce acetic acid, which may help control blood sugar and support gut health. Others produce B vitamins and vitamin K, which we need for things like energy and normal blood clotting.

‘If you take out some of those bacteria and yeasts, as happens with some commercial kefirs, you may lose some of the potentially beneficial substances they produce.’

That does not mean that commercial kefir is bad for you, but it may not contain everything that you’d get from a traditionally fermented kefir.

It’s a similar story with kombucha. Traditionally, the drink is made by adding what’s known as a ‘SCOBY’ (symbiotic culture of bacteria and yeast) – a rubbery-looking mass – to sweetened tea and allowing it to ferment.

‘The composition is affected by the temperature and humidity where it’s kept, whether the final product is pasteurised or not, and the initial ingredients,’ says Dr Torquati. ‘Unlike a factory product, no two batches will be exactly alike. That’s the beauty of these drinks – they’re alive.’

Supermarket kombucha, by contrast, can be pasteurised and its production tightly controlled to ensure consistency and keep alcohol levels below legal limits. This can leave it with fewer live microbes and organic acids than traditional versions. Some products also contain additional ingredients, including sugar.

But there is another problem for shoppers trying to work out exactly what they are buying: In Britain, neither kefir nor kombucha has a specific legal definition. There is no minimum number or particular combination of microbes a product must contain to use either name. Nor are manufacturers required to list every species of bacteria and yeast in the bottle.

‘Major brands don’t list all the species,’ says Dr Torquati. ‘They just say “14 different species” or “ten billion kefir cultures,” but they don’t actually list which ones. It’s not really clear for consumers which ones are real kefir.’

She recommends choosing refrigerated products with short expiry dates and simple ingredient lists, while avoiding 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.