Dr Michael Mosley: Is fermented food really good for your gut microbiome? © Getty Images

Dr Michael Mosley: Is fermented food really good for your gut microbiome?

Gut-boosting foods are popping up on supermarket shelves, but are they more than just a trendy side dish?

Although fermented foods like kimchi and sauerkraut have been around for hundreds of years, and were traditionally just a way to help preserve vegetables over the long winter months, in recent times they have become super trendy. I like making and eating my own fermented foods, but what is the evidence that they do you any good?

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The current excitement is based largely on the impact that eating fermented foods has on your gut microbiome – the trillions of microbes that live in your gut and which have a profound impact on our health.

One way to influence your microbiome is to eat foods that are rich in prebiotics and probiotics. Prebiotics are, broadly speaking, fibre and other nutrients that our microbiome likes to feast on and which confer health benefits. Probiotics, on the other hands, are living microbes in food which, when eaten, bolster the armies of ‘good’ bacteria living in your gut.

Fermented foods, like sauerkraut and kimchi, are rich in both prebiotics and probiotics, or at least they are if you make them yourself, which I do. The bottles of fermented vegetables you buy in supermarkets are often pasteurised, so any living bacteria are now long dead.

A jar of sparkly kimchi © Jason Raish
© Jason Raish

Kimchi is a Korean dish, made of cabbage, seasoned with chilli, garlic and ginger, and then fermented. It is rich in vitamin K and riboflavin (B2) and also contains plenty of Lactobacillus bacteria, which can survive the acid bath of the stomach and make their way down to your large intestine.

In a small study carried out a few years ago by Korean researchers (‘Beneficial effects of fresh and fermented kimchi in prediabetic individuals’), 21 patients with prediabetes were asked to either eat fresh (1-day-old) or fermented (10-day-old) kimchi for eight weeks. Then, after a four-week washout, they switched to the other form of kimchi for the next eight weeks.

Eating either type of kimchi led to a significant reduction in weight and waist size, but it was only when the patients were eating the fermented stuff that they saw significant improvements in blood pressure and insulin sensitivity, suggesting the Lactobacillus were doing something.

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Like kimchi, sauerkraut is based on fermenting cabbage. Despite being associated with German cuisine, it almost certainly originated in China more than 2,000 years ago. Sauerkraut is packed with vitamins and minerals, including iron, potassium, copper and manganese. It is also low in calories and rich in fibre, which your microbiome will enjoy.

That said, I couldn’t find any specific human studies showing that eating sauerkraut leads to particular benefits. The alleged benefits are mainly based on animal studies.

Nonetheless I am a huge fan of fermented cabbage, and eat a good dollop of either homemade kimchi or sauerkraut most days. One word of warning: if you decide to do the same, ease your way in. If your guts are not used to fermented food then they may react badly to a sudden invasion of a vast army of foreign microbes with bloating, stomach cramps or wind.

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