Research reveals 'fermented' foods may not be so healthy after all

Kefir, kombucha, kimchi. You might have heard the words but not been quite sure what they mean. They are, in fact, the latest foods claimed to do wonders for health — and they're apparently flying off supermarket shelves.

What kefir (a type of milk drink), kombucha (a tea drink, often served cold), and kimchi have in common is that they are all fermented.

Fermented food or drink is given its flavour or texture through micro organisms such as bacteria or yeast. Kombucha, for example, is tea left to brew with sugar, bacteria and yeast, while kefir is milk cultured with bacteria and yeast to thicken it and add a slight 'fizz'.

These types of 'trendy' foods have been around for centuries. Sauerkraut — the traditional German pickled cabbage — is a fermented food, as is balsamic vinegar and traditional sourdough.

In fact, 20 per cent of the food we eat is fermented to some degree, says Dr Megan Rossi, a dietitian and gut health researcher at King's College London. These include some cheeses such as aged parmesan, olives and traditional salami. Beer and wine are also fermented.

So what is it about fermentation that makes it so desirable?

First, it can improve the nutritional properties of food by breaking down nutrients, making them more 'bio-available' — easier for us to absorb.

In effect, the bacteria pre-digest the foods, and this can mean we take on more vitamins and minerals than we would in their unfermented state. You might, for example, absorb more nutrients, such as zinc or iron, from fermented soya such as tempeh than you would from the same amount of soya beans.

But the popularity of certain fermented foods really stems from our emerging understanding about how the mix of our gut bacteria affects health — including our immunity, weight and mood. Because of the way they are prepared, some fermented foods contain live 'good' bacteria and yeasts known as probiotics.

The health benefits a probiotic offers when eaten depends on its species and its strain, as well as the number of the micro-organisms present.

Some fermented foods — such as kefir, kimchi and sauerkraut — are what's known as synbiotics. This means that as well as containing probiotic 'good' bacteria, they also contain prebiotics. These aren't a type of bacteria themselves, but are carbohydrates we can't digest which act as food for the 'good' bacteria.

However, despite the trend for everything from skin cream to cola describing itself as 'fermented', not all fermented products are equal or healthy — salami, for instance, is high in fat and salt.

Kefir. Photo / Getty Images

Dr Rossi warns: "Don't think all fermented foods are probiotics. Fermented foods contain probiotics and can also include prebiotic ingredients — but just because a food is labelled fermented does not automatically mean that it contains probiotics.

"Look for the words 'live' or 'bio' on labels," she adds. And there can be a big difference between fermented products produced traditionally and large scale. If a product is pasteurised, this can destroy the live cultures, for example.