Guide · Gut and immunity · by Alexandra Lasconi
Fermented foods: what the studies show, food by food
Short answer. Fermented foods are made by microbes changing the food: yoghurt, kefir, sourdough bread, sauerkraut, brine pickles, kimchi, kombucha. The best human trial found that eating them daily raised gut microbial diversity and lowered inflammatory markers. Evidence for each single food is smaller, and baked or pasteurised foods keep the changes but not the live microbes.
Fermented is not always aliveSourdough is baked. Pasteurised sauerkraut is heated. Both are still fermented foods.
Variety beats one 'super' foodThe trial that showed changes used several different foods every day.
Start low, build upSome extra gas in the first week is common. One portion a day, then two or three.
Evidence labels: Strong consistent controlled trials or an authorised EU claim · Moderate controlled trials or large studies, with limits · Early small, pilot, laboratory or single studies.
01What counts as fermented
Live, fermented then heated, and not fermented at all.
02The key human trial
Six portions a day for ten weeks.
03Food by food
Yoghurt, kefir, sourdough, sauerkraut, kimchi, kombucha.
04How to start
A three-week build-up in grams.
05Who should be careful
Histamine, immunity, salt, SIBO.
06Two home recipes
Sauerkraut and milk kefir, in grams.
01 · What counts as fermented
An international expert panel defines fermented foods as foods made through desired microbial growth and enzyme changes to the food (see referenced study [1]). Three groups follow from that:
Fermented and alive
Live yoghurt, kefir, unpasteurised sauerkraut and brine pickles, kimchi, most kombucha, unpasteurised miso.
Fermented, then heated
Sourdough bread, pasteurised sauerkraut and pickles, cooked tempeh, soy sauce. The microbes are gone; the acids, broken-down fibres and other products stay.
Not fermented
Vinegar pickles, yeasted bread made in a few hours, 'yoghurt-style' desserts thickened with starch. See the pickles guide.
Go deeper: fermented versus probiotic
The word probiotic is reserved for defined live microbes shown in a study to provide a health benefit. Most fermented foods contain microbes that have not been characterised to that level, so the panel asks that they are called fermented foods containing live cultures, not probiotics (see referenced study [1]).
02 · The key human trial
Researchers at Stanford randomised 36 healthy adults to 10 weeks of either a high-fibre diet or a high-fermented-food diet, building up over the first weeks. The fermented group went from almost none to about six portions a day of yoghurt, kefir, cottage cheese, sauerkraut, kimchi, fermented vegetable brine and kombucha. Studies found the fermented-food diet steadily increased gut microbial diversity and lowered inflammatory markers, including interleukin-6, while the high-fibre diet did not change diversity (see referenced study [2]). Moderate
Read it for what it is: a small, careful trial in healthy people, measuring blood markers and gut bacteria, not illness. It is the best human evidence we have that eating fermented foods every day changes something measurable.
03 · Food by food
- Live yoghurt StrongLive yoghurt cultures in yoghurt improve digestion of lactose in yoghurt in individuals with lactose maldigestion. This is an authorised EU health claim, based on 13 of 14 human studies, and it requires at least 108 live cultures per serving (see referenced study [3]). Practical portion: 150 g plain yoghurt with live cultures.
- Kefir EarlyA 2023 systematic review of randomised trials of milk kefir found possible effects on mouth bacteria, as an add-on to H. pylori treatment and on some metabolic markers, but trials were small and of variable quality, and the authors asked for better studies before any recommendation (see referenced study [4]). Kefir carries a wider mix of bacteria and yeasts than yoghurt. Practical portion: 200 ml plain kefir.
- Sourdough bread (pâine cu maia) ModerateBaking kills the microbes, so the value is in what fermentation changed. A mild drop in dough acidity, as in sourdough, broke down about 70% of the phytate in wholemeal flour against about 40% without it; phytate binds iron, zinc and magnesium (see referenced study [5]). A meta-analysis of 18 trials found sourdough bread raised blood glucose less at 60 and 120 minutes than industrial bread, though the certainty was low (see referenced study [6]), and a wider review concluded that the benefit depends heavily on the flour, the starter and the fermentation time (see referenced study [7]). Long fermentation also lowers FODMAPs, yet in a pilot trial in people with IBS the sourdough bread was not better tolerated than yeast bread (see referenced study [8]). Choose wholemeal or rye sourdough with a long fermentation: flour, water, salt and starter only.
- Sauerkraut (varză murată) and brine pickles EarlyIn a pilot trial in people with IBS, both pasteurised and unpasteurised sauerkraut improved symptoms over 6 weeks (see referenced study [9]). Practical portion: 30–50 g, counted for salt. Full detail in the pickles guide.
- Kimchi EarlyPart of the Stanford diet above, with the same salt caution as pickles. Practical portion: 30–50 g.
- Kombucha EarlyA 2019 systematic review found only one study of kombucha in people; most claims come from laboratory and animal work (see referenced study [10]). Check the sugar per 100 ml, and note it can contain a little alcohol.
- Miso, tempeh and borșTraditional and worth eating, but we found no human trials on them as single foods. Miso is salty; tempeh is cooked before eating.
04 · How to start
| Week | Daily fermented foods |
|---|
| 1 | One portion: 150 g live yoghurt or 200 ml kefir. |
|---|
| 2 | Two portions: the above, plus 30 g sauerkraut or brine pickles with lunch. |
|---|
| 3 and after | Two to three portions, rotating at least four different foods across the week. Sourdough counts as your bread, not as a portion. |
|---|
- Plain, unsweetened versions. Add fruit yourself.
- Pair them with fibre: vegetables, legumes, oats, wholegrains. Fibre is what gut bacteria live on.
- If you get more gas or bloating, hold at the current step for another week.
05 · Who should be careful
- Histamine sensitivity: aged and fermented foods are high in histamine. If they give you flushing, headaches or a blocked nose, talk to your doctor.
- Weakened immune system (for example chemotherapy or transplant medicines): ask your doctor before eating foods with live cultures.
- High blood pressure or kidney disease: sauerkraut, pickles, kimchi and miso carry a lot of salt.
- SIBO or IBD: read the SIBO or IBD guide first.
- Pregnancy: choose pasteurised-milk yoghurt and kefir, and skip kombucha.
06 · Two home recipes
Sauerkraut (varză murată, shredded)
Makes about 1 litre · 30 minutes · ferments 1–4 weeks
Ingredients
- 1,000 g white cabbage, cored and finely shredded
- 20 g coarse salt (2% of the cabbage)
- Optional: 5 g caraway seeds, 2 bay leaves, 5 g black peppercorns
Method
- Mix the cabbage and salt in a large bowl and squeeze with clean hands for 5–10 minutes until it releases liquid.
- Pack it into a clean jar, pressing hard after each handful, until the liquid rises above the cabbage.
- Keep the cabbage under the liquid with a weight. Cover loosely.
- Leave at 18–22 °C. Taste from day 7. Most people like it at 2–3 weeks.
- Close and refrigerate. Keeps several months.
Milk kefir from grains
Makes 500 ml · 5 minutes · ferments 18–24 hours
Ingredients
- 500 ml milk (pasteurised)
- 10 g kefir grains
Method
- Put the grains in a clean glass jar and add the milk.
- Cover with a cloth or loose lid. Leave at 20–24 °C for 18–24 hours, until thick and tangy.
- Strain through a plastic sieve. Drink the kefir or keep it in the fridge for up to 3 days.
- Add the grains straight to fresh milk for the next batch.
Myths and facts
| Myth | Fact |
|---|
| Sourdough bread is probiotic. | Baking kills the microbes. Its value is in the lower phytate, the acids and the slower glucose rise. |
| One spoon of kombucha a day fixes your gut. | Kombucha has the least human evidence of any food here. |
| If it is pasteurised, it is useless. | In the sauerkraut trial, pasteurised and unpasteurised worked equally well for symptoms. |
What the evidence does not show. No trial shows that fermented foods prevent colds, cure gut conditions or replace treatment. The main trial was small and in healthy adults. For single foods other than yoghurt and lactose, the trials are small or missing. What the evidence supports is a habit: several fermented foods, most days, alongside plenty of fibre.
Questions clients ask
Is sourdough bread probiotic?
No. Baking kills the microbes. It is a fermented food with lower phytate, and in many trials a smaller blood glucose rise than yeasted bread.
How many portions a day?
Start with one. The Stanford trial built up to about six a day over several weeks; two to three is a realistic target for most people.
Is pasteurised sauerkraut still worth eating?
In a pilot trial in people with IBS, pasteurised and unpasteurised sauerkraut improved symptoms equally. It is still a fermented food, just without live cultures.
Kefir or yoghurt?
Both are good. Yoghurt has the authorised lactose-digestion claim; kefir carries a wider range of microbes. Rotating both adds variety.
Who should avoid fermented foods?
People with histamine sensitivity, a weakened immune system, or salt restrictions should check with their doctor first. Read the SIBO or IBD guide if either applies.
Is kombucha worth it?
Enjoy it if you like it, but it has the least human evidence of the common fermented foods. Check the sugar.
Sources
- Marco ML et al. The ISAPP consensus statement on fermented foods. Nat Rev Gastroenterol Hepatol (2021).
- Wastyk HC et al. Gut-microbiota-targeted diets modulate human immune status. Cell (2021).
- EFSA NDA Panel. Health claims related to live yoghurt cultures and improved lactose digestion. EFSA Journal (2010).
- The effects of kefir consumption on human health: a systematic review of randomized controlled trials. Nutr Rev (2023).
- Leenhardt F et al. Moderate decrease of pH by sourdough fermentation is sufficient to reduce phytate content of whole wheat flour through endogenous phytase activity. J Agric Food Chem (2005).
- Rolim ME et al. Consumption of sourdough bread and changes in the glycemic control and satiety: a systematic review. Crit Rev Food Sci Nutr (2024).
- Ribet L et al. Nutritional benefits of sourdoughs: a systematic review. Adv Nutr (2023).
- Laatikainen R et al. Pilot study: comparison of sourdough wheat bread and yeast-fermented wheat bread in individuals with wheat sensitivity and irritable bowel syndrome. Nutrients (2017).
- Nielsen ES et al. Lacto-fermented sauerkraut improves symptoms in IBS patients independent of product pasteurisation: a pilot study. Food Funct (2018).
- Kapp JM, Sumner W. Kombucha: a systematic review of the empirical evidence of human health benefit. Ann Epidemiol (2019).
This page is nutrition coaching information, not medical advice. It does not recommend any product.
