A bowl of chicken, miso and rice soup

The health benefits of fermenting

Popular across cultures for centuries, fermenting has made a fashionable comeback as a provider of 'good' bacteria that contributes to a healthy digestive system. Want to know what the fuss is all about? Nutritionist Jo Lewin gives us the lowdown.

Historically the fermentation technique was used as a way of preserving foods and drinks long before the days of refrigeration. During the process of fermentation, microorganisms such as bacteria, yeast or fungi convert organic compounds – such as sugars and starch –  into alcohol or acids. For example, starches and sugars in vegetables and fruits are converted to lactic acid and this lactic acid acts as a natural preservative. Fermentation can produce quite distinctive, strong, slightly sour flavours.


The consumption of foods and drinks that have undergone fermentation contain benefits to health that stretch beyond food preservation. The transformation of sugars and starches enhances the natural, beneficial bacteria in food. These bacteria, known as probiotics or ‘good’ bacteria are thought to help a multitude of health issues, specifically digestive health.

Discover more tips for digestive health and browse our gut-healthy recipes. Want to have a go at making your own ferments? Follow our guides for how to make kombucha, how to make kefir and our quick kimchi recipe. Also check out our health and nutrition page for more recipe inspiration, health benefits guides and advice on special diets.

Bacteria – good or bad?

The bacteria that live in our gut are essential. They help with digestion, absorption and assimilation of nutrients. Plus, they play a role in the function of our immune system. However there are ‘bad’ bacteria that also reside in the gut and the challenge is achieving the right balance between the two. When the balance is shifted in favour of the bad bacteria, symptoms may arise such as bloating, constipation or diarrhoea. This is termed ‘dysbiosis’ – the state in which the gut flora are out of balance.

Modern diets, high in refined sugars and busy, stressful lifestyles can contribute to dysbiosis by feeding the bad bacteria, enabling them to flourish. Eliminating refined, high sugar foods and including probiotic-rich fermented foods is thought to bring the gut back into balance and support the immune system.

DID YOU KNOW: a total of one trillion bacteria live together in our digestive system. Their total weight is about four pounds!

Probiotic powerhouses to include in your diet:

  • Kefir – A probiotic cultured drink, kefir contains multiple strains of bacteria and yeast. Kefir is rich in minerals and vitamins, particularly the B vitamins and vitamin K
  • Sauerkraut – Easy to make at home, this fermented cabbage dish has been around for centuries. It’s high in fibre, as well as vitamins A, C, K and various B vitamins. It’s also a good source of iron, manganese, copper, sodium, magnesium and calcium 
  • Miso – This traditional Japanese paste is made from fermented soybeans and grains consisting of millions of beneficial bacteria. It’s rich in essential minerals and a good source of various B vitamins, vitamins E, K and folic acid
  • Kimchi – Spicier than sauerkraut, kimchi is also a form of fermented cabbage and other vegetables. It contains vitamins A, B1, B2 and C and minerals such as iron, calcium and selenium
  • Lassi – Made from soured milk, lassi has been drunk as a pre-dinner yogurt drink for centuries. They are a popular way of achieving probiotic bacteria
  • Kombucha – A fizzy, fermented black tea. Yeast turns sugar into alcohol and carbon dioxide, and bacteria called acetobacter convert the alcohol into acetic acid, giving it a sour taste. Watch out for sugar in shop-bought kombucha, you’re better off making it at home
  • Tempehanother version of fermented soy beans, tempeh is a rich protein source so a good choice for vegetarians
  • Bread – Some breads, such as sourdough are made from dough that is fermented
  • Yogurt – Lactobacilli bacteria convert lactose sugar in milk into glucose and galactose, which break down further into lactic acid, giving yogurt its sour taste. Live bacteria remain in the yogurt and provide a valuable contribution to gut microflora

Did you know?

In Africa, millet is fermented for several days to produce a sour porridge called ogi, and in India rice and lentils are fermented for at least two days before making idli and dosas.

Nutritional Highlights

Fermented foods are rich in probiotic bacteria so by consuming fermented foods you are adding beneficial bacteria and enzymes to your overall intestinal flora, increasing the health of your gut microbiome and digestive system and enhancing the immune system.

  • Digestion and absorption

As some of the sugars and starches in food have been broken down through the process, fermented foods are easier to digest. For example, fermentation breaks down the lactose in milk to simpler sugars – glucose and galactose – which, if you are lactose intolerant, can make products such as yogurt and cheese potentially easier to digest.

  • Synthesis and availability of nutrients

Fermentation can also increase the availability of vitamins and minerals for our bodies to absorb. Additionally, by boosting the beneficial bacteria in your gut, you are promoting their ability to manufacture B vitamins and synthesise vitamin K.

  • Immune functions

A large proportion of the immune system is housed in the gut. By consuming probiotic-rich foods, you are supporting the mucosa (gut lining) as a natural barrier, making the immune system more robust. A lack of beneficial bacteria allows disease causing microbes to grow causing inflammation in the gut wall. If you have recently taken a course of antibiotics, probiotic foods are particularly helpful.

Read more about how to prevent a cold and which natural cold remedies actually work.

  • Phytic Acid

Some natural compounds that interfere with the absorption of nutrients can be removed by fermentation. Phytic acid, for example, which is found in legumes and seeds, binds minerals such as iron and zinc, reducing their absorption when eaten. However, phytic acid can be broken down during fermentation so the minerals become available.

  • Mood and behaviour

The gut and brain are linked, through the hypothalamic-pituitary-adrenal (HPA) axis. Technically called the enteric nervous system, the gut is lined with neurons that can influence our emotions and feelings. Serotonin – a neurotransmitter involved in mood – is made in the gut and research further suggests that as probiotic bacteria contribute to a healthy gut, they are also linked to a healthy mind.


The bacteria in our gut are not something we think about, yet research suggests they fulfil a number of vital functions. The links between fermented foods, gut bacteria and how they may affect our physical and mental health warrants considerable attention. For more information, take a look at the The Human Microbiome Project.

How to select and store

Keep fermented foods in the fridge and beware of buying those straight from the shelf at a supermarket. If it’s not in the fridge, it’s been heat treated and pasteurisation destroys the naturally occurring probiotics. Watch your choice of yogurts and kombucha too – those packed with sugar, high fructose corn syrup or artificial sweeteners are not going to support the gut as much as natural, live options.

Taking a probiotic supplement has become popular. But beware – they can be a waste of time and money as some bacteria do not survive transit, manufacturing practices and heat damage if not stored properly. Strains of lactobacillus and bifidobacterium bacteria are the most commonly used as they can survive the passage through the digestive system to the gut, including the highly acidic conditions of the stomach.

What’s the difference between pickled and fermented veg?

There’s often confusion over the difference between pickled foods and fermented ones. Pickles, like the ones you buy in the supermarket, are preserved in an acidic liquid, typically vinegar. Although vinegar is a product of fermentation the pickles themselves are not fermented, and so do not offer the same health benefits of fermented vegetables.

Fermenting at home

Save your pennies and make your own fermented foods. It’s easy and they are a great backup when there are no fresh veggies in the fridge or you don’t have much time to cook. Choose vegetables that are fresh, local and organic, as your ferment will be only as good as the ingredients you use. You can ferment any vegetable but some work better than others. Cabbage is easy, as are radishes, carrots, turnips, apples and beetroot. The fermentation process creates a distinctive sour flavour but experiment to discover what you like.

Include prebiotic-rich foods too such as onions, asparagus, leeks and artichokes. These fibre-rich foods feed the good bacteria in the gut.

Recipe suggestions

Miso chicken & rice soup
Miso-marinated salmon
Saucy miso mushrooms with udon noodles
More miso recipes
Sticky tempeh, mango & lime salad

Got any tips for fermenting your own vegetables? Share them below…

This article was updated on 1st October 2018 by Kerry Torrens.

Kerry Torrens is a qualified Nutritionist (MBANT) with a post graduate diploma in Personalised Nutrition & Nutritional Therapy. She is a member of the British Association for Nutrition and Lifestyle Medicine (BANT) and a member of the Guild of Food Writers. Over the last 15 years she has been a contributing author to a number of nutritional and cookery publications including BBC Good Food.

Jo Lewin is a registered nutritionist (RNutr) with the Association for Nutrition with a specialism in public health. Follow her on Twitter @nutri_jo.


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