Fermented Foods: Gut Health Benefits, a UK List and Who Should Be Careful

What are fermented foods?

Fermented foods are foods changed by microbes. Bacteria or yeasts feed on the sugars in the raw food and turn them into acids, gases or alcohol, which preserve it and change its taste and texture. Milk becomes yoghurt or kefir, cabbage becomes sauerkraut or kimchi, soya becomes miso or tempeh, and sweet tea becomes kombucha.

An international panel of scientists (ISAPP) defines fermented foods as “foods made through desired microbial growth and enzymatic conversions of food components”. That means a food can be fermented without containing live microbes when you eat it: bread, most beer and wine, tempeh and shelf-stable sauerkraut have been baked, filtered, cooked or pasteurised.

Fermented foods list: which have live cultures?

The fermented foods most likely to contain live cultures are live yoghurt, kefir, chilled unpasteurised sauerkraut and kimchi, unpasteurised miso and raw kombucha. Tempeh, sourdough bread and most jarred pickles don’t, because they are cooked, baked or pasteurised. Vinegar pickles such as gherkins are usually not fermented at all.

Our best fermented foods for gut health section compares the evidence food by food, and our kombucha benefits guide covers its sugar and alcohol in detail.

What is the gut microbiota?

The gut microbiota is the community of bacteria, fungi and viruses that live in your digestive tract, mostly in the large intestine. A typical adult carries around 38 trillion bacteria, roughly as many as their own body cells. They break down fibre you can’t digest, make short-chain fatty acids and help train the immune system.

“Microbiome” is often used for the same thing, though strictly it means the microbes plus their genes. Everyone’s mix is different, and it changes with diet, age, medicines (especially antibiotics), illness and travel. A more diverse microbiota is generally linked to better health in studies, but nobody has yet defined a single “healthy” gut microbiota, and stool tests sold online can’t tell you what to eat.

What feeds the gut microbiota?

Fibre feeds it most directly. Gut bacteria ferment fibre in the large intestine, which is why the NHS advice of 30g of fibre a day matters as much as any fermented food. Most UK adults eat about 20g. Our high fibre foods list shows grams per portion, and psyllium husk is a fibre supplement if food alone falls short.

Are fermented foods good for gut health?

Fermented foods look good for gut health, but the evidence is still small. A 2021 Stanford trial found a high fermented food diet made the gut microbiota more diverse and lowered 19 inflammatory proteins in 36 healthy adults. A 2025 review of 19 trials linked fermented foods to more regular, softer stools, mostly on low-certainty evidence.

Two cautions. The Stanford trial was small and short, and the authors said it still needs testing in people with health conditions, older adults and pregnant women. And many fermented food studies are run or funded by makers of dairy and probiotic products, so look for independent trials before believing a specific brand’s claims.

For a wider plan, see our gut health guide, which covers fibre, plant variety and sleep, and the anti-inflammatory diet, which uses yoghurt and kefir in its 7-day plan. The gut-brain link is covered in food and mood.

How much fermented food should you eat?

There is no official UK amount. A sensible start is one portion a day, such as a small pot of live yoghurt, a small glass of kefir or a tablespoon of sauerkraut or kimchi with a meal, built up over 2 to 4 weeks. The Stanford trial reached about 6 servings a day, which is more than most people need to start.

• Start small: a sudden increase can cause wind and bloating, which usually settles as your gut adjusts.
• Vary them: different foods carry different microbes. Rotate dairy and vegetable ferments.
• Keep them live: add miso, kimchi or sauerkraut at the end of cooking, off the heat.
• Read the label: pick plain yoghurt and kefir, and check salt on sauerkraut and kimchi (over 1.5g per 100g is high).
• Keep the fibre up: fermented foods work alongside fibre, not instead of it. See foods that help constipation.

Fermented foods vs probiotic supplements

Fermented foods contain a changing mix of microbes that are usually not named or counted. A probiotic supplement contains specific strains at a stated dose, and only some strains have trial evidence, each for a particular use. The NHS says there is some evidence probiotics may help some conditions, including IBS, and suggests trying one for at least 4 weeks.

In the UK, “probiotic” on a food label counts as a health claim, which is why many fermented foods say “live cultures” instead. Our probiotics guide explains which strains have evidence. If you are bloated after meals rather than looking for live cultures, digestive enzymes work in a different way, and supplements for digestion compares the options.

Who should be careful with fermented foods?

Most adults can eat fermented foods safely. People with a weakened immune system should check with their doctor before eating unpasteurised ferments or taking probiotics. In pregnancy, avoid unpasteurised milk products. People with histamine intolerance, those taking MAOI antidepressants and anyone watching their salt intake should also take care.

• Weakened immune system: the NHS advises speaking to a doctor before taking probiotics if you have a weakened immune system or a long-term condition. This includes chemotherapy, organ transplants and some autoimmune treatments.
• Pregnancy: the NHS lists unpasteurised milk, cream and yoghurt among foods to avoid in pregnancy because of listeria. Pasteurised live yoghurt is fine. Check homemade or unpasteurised kefir, and avoid unpasteurised kombucha, which can also contain a little alcohol.
• Histamine intolerance: fermented foods such as sauerkraut, aged cheese, miso and kombucha can be high in histamine. If you get flushing, headaches, a blocked nose or tummy upset after them, keep a food diary and talk to a GP or dietitian.
• MAOI antidepressants: if you take an MAOI such as phenelzine, your patient leaflet will list tyramine-rich foods to avoid, which include some aged and fermented foods such as aged cheese, miso and other fermented soya products.
• High blood pressure: miso, kimchi and sauerkraut can add a lot of salt. Use small amounts.
• Babies and young children: stick to pasteurised products, and never give homemade unpasteurised ferments to babies.

See a GP if gut symptoms last for weeks, or if you have blood in your poo, unexplained weight loss or a change in bowel habit that doesn’t settle. Our guide to the signs of poor gut health lists the red flags, and our IBS guide covers the NICE approach. For acid and heartburn, see acid reflux.