Fermented Foods

What it is

Fermented foods are foods "made through desired microbial growth and enzymatic conversions of food components" [1]. Familiar examples are sauerkraut, kimchi, kefir, yogurt, cheese, kombucha and miso, which commonly contain living microbes [2].

Sauerkraut is cabbage fermented by lactic acid bacteria [3]. In one trial, the fresh sauerkraut had a pH of about 3.4 and was dominated by the bacterium Lacticaseibacillus paracasei, while the pasteurized version (heated at 75 °C for 5 minutes) contained no living bacteria [3]. Not everything labelled fermented keeps live microbes: cured meats and some vinegars and soy sauces are made with chemical processes or heat [1]. Fermented foods are also not the same as probiotics, which need defined strains at defined doses [1].

Our Kimchi page covers the Korean fermented cabbage dish in detail, and our Yogurt and Kefir page covers fermented dairy.

How it might work

The popular idea is that live lactic acid bacteria reach the gut and help it work better. The evidence so far is more mixed. In a sauerkraut trial in people with irritable bowel syndrome (IBS), pasteurized sauerkraut with no living bacteria helped about as much as unpasteurized, so the authors suggested the benefit may come more from prebiotic compounds in the cabbage, which feed gut bacteria, than from the live bacteria [4]. In healthy adults, only pasteurized sauerkraut raised blood levels of short-chain fatty acids, which are compounds made by gut bacteria that fuel the gut lining [3]. A 2025 review also describes a kimchi trial in which 210 g a day raised levels of butyrate-producing gut bacteria and lowered stool acidity [2]. These are proposed mechanisms, not proven ones.

What the research says

Digestion and bowel regularity. A 2025 systematic review of 25 studies (4,328 participants; 19 randomised trials pooled) found that eating fermented foods increased the frequency of bowel movements, improved stool consistency, reduced gastrointestinal symptoms and shortened gut transit time [2]. The certainty of the evidence was mostly low, results varied a lot between trials, and most trials tested fermented dairy such as fermented milk and yogurt; a smaller number tested plant foods such as fermented cabbage, cucumber, rice and soy milk [2].

Irritable bowel syndrome. In a randomised pilot trial of 34 people with IBS, eating sauerkraut every day for 6 weeks lowered symptom scores in both groups: by about 57 points with unpasteurized sauerkraut and about 39 points with pasteurized, with no difference between them [4]. The trial was small, and everyone ate sauerkraut, so it cannot separate sauerkraut's effect from a placebo response [4].

Gut bacteria. In a 17-week trial of 36 healthy adults (18 per group), a diet high in fermented foods steadily increased gut microbiome diversity and lowered inflammatory markers, while a high-fibre diet did not raise diversity [5]. A larger trial gave 87 healthy adults 100 g of sauerkraut a day for 4 weeks, fresh or pasteurized. It found that gut bacteria were fairly resilient: only single species shifted, and overall diversity did not change significantly [3]. People tolerated it well, with 95% satisfied after the fresh and 90% after the pasteurized phase, and stool frequency, stool form and stool acidity did not change [3]. Blood levels of short-chain fatty acids rose only after pasteurized sauerkraut, and fresh sauerkraut lowered stool butyrate [3].

Inflammation. Results beyond the Stanford trial are mixed. In 59 adults with excess weight on a calorie-restricted diet, adding 200 mL of green tea kombucha a day for 10 weeks did not increase weight loss, but it lowered a body-fat-accumulation index and IL-6 was higher in the control group than in the kombucha group [6]. The same trial group found one marker of oxidative stress (hydrogen peroxide) was lower with kombucha while other markers did not change [7]. In 24 healthy adults, a 4-week kombucha supplement produced no significant change in blood inflammation markers across the group [8].

Cancer risk. In a case-control study of Polish-born women in the US (131 breast cancer cases, 284 controls), women who ate the most cabbage and sauerkraut as adolescents had about 64% lower odds of breast cancer (odds ratio 0.36), and high raw or short-cooked intake in adulthood was also linked to lower odds (0.37) [9]. The study could not separate sauerkraut from cabbage in general and relied on memory of past diet [9]. The other side is pickled vegetables: a meta-analysis of 60 mostly case-control studies linked pickled vegetables to about 50% higher stomach cancer odds overall (odds ratio 1.52), and 32% in the prospective cohorts (1.32), with high heterogeneity and the strongest signal in Korea and China [10]. A Japanese and Korean meta-analysis found 28% higher odds with high pickled vegetable intake while high fresh vegetable intake was linked to 38% lower odds [11]. These pooled studies cover traditional salted pickles broadly and did not analyse sauerkraut on its own [10].

Brain health. A systematic review of 29 mostly observational studies in older people found that fermented-food diets, along with low-to-moderate alcohol, coffee and soy, were associated with lower risk of dementia and Alzheimer's disease [12]. A review of 21 studies of foods fermented with Lactobacillus or Bifidobacterium reported benefits for memory and executive function in most studies, but concluded the evidence was "neither convincing nor sufficient" to show cause and effect [13].

Blood sugar, blood pressure and cholesterol. For fermented vegetables this evidence comes from kimchi. A meta-analysis of 5 trials (205 participants) found fasting blood glucose was about 2 mg/dL lower, and after excluding studies that added heterogeneity, triglycerides and blood pressure were also lower [14]. See the Kimchi page for details.

Doses used in studies

Sauerkraut: 100 g a day for 4 weeks in healthy adults [3], and a daily supplement to meals for 6 weeks in IBS patients [4]. Kimchi: 210 g a day for 28 days and for 12 weeks in healthy adults, as described in a 2025 review [2]. Kombucha: 200 mL of green tea kombucha a day for 10 weeks [6]. The high-fermented-food diet in the Stanford trial lasted 17 weeks in total [5]. These are the amounts tested, not recommendations.

Safety and interactions

Side effects. Sauerkraut at 100 g a day was well tolerated for 4 weeks in healthy adults [3]. When properly made, fermented foods are rarely linked to stomach upsets or food poisoning, but low-acid fermented foods contaminated with pathogens such as Listeria, Salmonella or Clostridium botulinum can be a risk [1].

Salt. Salt is a built-in ingredient of fermented foods such as kimchi and soy sauce and should be eaten in moderation [1]. The pooled studies linking pickled vegetables to higher stomach cancer risk are the main caution here [10]. The sodium content of sauerkraut was not reported in the studies we reviewed.

Histamine and tyramine. Some fermented foods, including fermented vegetables, contain histamine, tyramine and other biogenic amines, which can cause effects from mild to severe, such as migraines, in people whose bodies cannot break them down [1]. The studies reviewed did not test interactions with medicines, so people with histamine intolerance, or who take antidepressants of the MAOI type, may want to ask a doctor or pharmacist first.

Pregnancy and breastfeeding. We found no trials of sauerkraut in pregnancy. India is the only country with a guideline specifically encouraging pregnant women to eat fermented foods [1].

Kidney, liver, bleeding disorders and weakened immunity. No data on these groups were found in the studies reviewed.

Caveats and open questions

"Fermented foods" covers very different products, from live-culture sauerkraut and kimchi to pasteurized jars, kombucha, yogurt and miso, so results for one do not carry over to another [1]. The strongest pooled evidence leans heavily on fermented dairy rather than cabbage [2]. Sauerkraut itself has only two small-to-moderate trials, one in IBS (34 people) and one in healthy adults (87 people), plus one case-control study [4][3][9]. Many measured outcomes are surrogate markers such as microbiome composition and blood inflammation markers, rather than diagnosed disease [3][5].

The kombucha trial in 24 adults was funded by a kombucha maker, and several authors of the digestion review report industry funding from food companies [8][2]. Registered trials of sauerkraut in IBS and of fermented-food diets in ulcerative colitis and rectal cancer may help settle open questions.