Are fermented foods worth the hype?

Foods like kombucha, kimchi and sauerkraut have been getting a lot of attention because of their potential benefits for gut (and overall) health. But what is the evidence? Are these trendy foods worth the hype?

Fermented foods have soared in popularity over the last few years. A decade ago, if you'd mentioned kimchi, kombucha or kefir, not many Westerners would have known what you were talking about. Today, they are almost considered staples of a healthy diet. 

For those who haven’t tried them: kimchi is a Korean dish made from fermented vegetables; kombucha is a fizzy fermented tea from China; and kefir is a cultured milk drink from the North Caucasus region.

Other fermented foods include sauerkraut, apple cider vinegar, yoghurt, pickled vegetables, and soy products like miso and tempeh. You may even have jumped on the fermentation bandwagon during lockdown and made your own sourdough bread.

As well as tasting great (if a little tangy), these foods have been gaining a lot of attention because of their purported health benefits. Proponents say they are good for your gut health - and therefore your general health - and have wasted no time in dubbing them 'superfoods'. So are these ancient treats worth the recent hype?

What is fermentation?

Dr Kathryn O’Sullivan, a nutrition scientist for BIOMES and expert in fermented foods, points out that fermentation is one of the oldest techniques around for food preservation. The practice began around 14,000 years ago, and may have helped hunter-gatherers make the leap towards agricultural societies. 

"It is a natural process whereby microorganisms such as bacteria, yeast or fungi feed on sugars and starch in food, converting it to alcohol or acids," says O'Sullivan. "This provides an environment that preserves the food, encourages healthy bacteria to thrive, and also gives fermented foods their unique sour, tangy flavours."

The health benefits are due to these 'healthy' bacteria. Living inside our guts is a thriving ecosystem of bacteria (microbiome). Some of these microbial communities are harmful for health, while many others are beneficial.

"This microbiome exists in a delicate balance between beneficial and harmful bacteria, and plays a very important role in health by helping control digestion and benefiting the immune system," says Dr O'Sullivan. "Poor diet, age, illness and antibiotic use can disrupt the gut microbiome balance, resulting in dysbiosis. Dysbiosis has been linked to a host of gut symptoms like diarrhoea, cramps and constipation."

The possible health benefits

It's only recently that scientists have begun to unlock the ways that our gut bacteria affect our health. But it does appear probiotic foods (those containing beneficial bacteria) can help to rebalance the microbiome. This can improve digestive symptoms, contribute to healthy immune function and keep levels of 'bad' bacteria in check. 

"Studies suggest populations who consume fermented dairy foods like yoghurts and kefir may have reduced risk of cardiovascular disease and type 2 diabetes, and improved weight management," adds Dr O'Sullivan. "There is evidence suggesting that other fermented foods like kimchi may help protect against type 2 diabetes."

Jenna Hope, a nutrition consultant, adds that high levels of good bacteria in the gut are associated with a whole range of positive health outcomes.

"These include better sleep, improved dietary choices, brain function and general well-being," she says. "Some also suggest that fermented foods may be higher in antioxidants which help to remove free radicals in the body. There is a wide range of evidence supporting the association between fermented foods and improved gut health, but more research is required to be able to make additional claims around their health benefits."

Variety is the spice

While the excitement around fermented foods is not entirely misplaced, it's unlikely that your sauerkraut salad will work miracles. Many of the bolder health claims lack supporting evidence - or the evidence comes from small trials or animal studies.

Currently, manufacturers are not allowed to make health claims for probiotic foods (or to use 'fermented foods' and 'probiotic foods' interchangeably). This is largely because the levels of live bacteria are difficult to control and standardise. Outside of laboratory conditions, their strain, composition and stability are poorly understood.

Some fermented foods (like beer and wine) undergo steps in their production that render the bacteria inactive. In other cases, there may not be enough bacteria to have an effect, or the bacteria may not survive long enough to reach your gut. Simply put, fermented foods aren't medicines, and you can't guarantee they'll have the desired effect.

"Fermented foods are not created equal and will have different types and amounts of live bacteria. This is why variety is important in what you eat," points out Dr O'Sullivan.

How should we be consuming them?

Both O'Sullivan and Hope recommend introducing fermented foods into your diet gradually and consuming them in moderation.

"Some people who struggle with IBS may find consuming fermented foods increases symptoms and that therefore they're not suitable for everyone," says Hope. "There are currently no guidelines around how much to be consuming - however, one portion of fermented foods on a daily basis is a great place to start. Much like everything, they shouldn't be over-consumed."

O'Sullivan adds the additional caveat that some fermented foods, like pickles, are high in sodium, the constituent of salt which is linked to raised blood pressure. Others, like sauerkraut, are often preserved in vinegar, meaning they don't actually contain probiotics. In both cases, it makes sense to check the label.

"Adding fermented food to the diet is relatively easy," she says. "They can be an acquired taste, so fermented dairy foods like yoghurts or kefir may be the easiest fermented food to start with. Try pickled vegetables like kimchi as side dishes, in sandwiches or in wraps; go Asian when you eat out and, if you are feeling adventurous, try fermented fish."

With fermented foods, as with all foods, the key word is balance. They may well have some benefits for gut health - but they aren't a panacea, and they might not have the same effect on everybody. As is often the case with 'superfoods', it's best to enjoy them as part of a balanced diet while steering clear of the hype.

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