Fermented foods
Pooled trials found fermented foods eased bowel symptoms in healthy adults and irritable bowel syndrome, and probiotic fermented dairy cut respiratory infections 19% against placebo. Results for C-reactive protein, blood sugar and blood pressure differ by food and review. Kefir trials found no weight change. Fermented foods can trigger dangerous blood pressure rises with MAOI antidepressants.
- Status
- A food, not a regulated health product. The prescribing information for the MAOI antidepressant phenelzine lists sauerkraut, yogurt and most cheeses among foods to avoid during treatment and for two weeks after [1].
- Also called
- yogurt, yoghurt, kefir, kimchi, sauerkraut, kombucha, miso, tempeh, natto, fermented milk, cultured dairy, pickles, live cultures
- Studied for
- gut health, digestion, bloating, constipation, irritable bowel syndrome, gut microbiome, immunity, colds, inflammation, blood sugar, weight loss, blood pressure, longevity
Strong several reviews agree · Moderate one review or consistent trials · Limited one or a few small studies · Untested no study in people, listed with what it rests on
Shown to help6
- ModerateShown to increase bowel movement frequency and ease gut symptoms in healthy adults, in one meta-analysis of 19 trials rated mostly low in certainty.
- ModerateShown to raise the share of people with irritable bowel syndrome reporting symptom relief by 22%, in one meta-analysis. Pain and bloating scores did not change significantly.
- ModerateShown to lower respiratory infections by about 19% with probiotic fermented dairy against placebo, in one meta-analysis of 22 trials across ages.
- ModerateShown to lower fasting glucose slightly with kimchi and kefir, in two meta-analyses. Probiotic yogurt did not differ from ordinary yogurt.
- LimitedOne 17-week trial of 36 adults found a high fermented food diet steadily raised gut microbiome diversity, which a high-fiber diet did not.
- LimitedLinked to lower death from any cause and heart disease for cheese and fermented milks in cohorts. Miso and bread showed no link.
Reported risks3
- LimitedThe phenelzine label warns that foods high in tyramine, including fermented foods such as sauerkraut, aged cheese and yogurt, can cause hypertensive crises, sometimes fatal.How often is not known
- LimitedLinked to about 50% higher stomach cancer risk in a meta-analysis of 60 studies of pickled vegetables, mostly from Korea, China and Japan.How often is not known
- LimitedCase reports: a US public health investigation of two severe illnesses, one fatal, in people drinking kombucha daily, and a report of severe liver injury.How often is not known
What it was studied for, and what was found
Shown to help6
Bowel habits and gut symptoms in healthy adults
Shown to increase bowel movement frequency and ease gut symptoms in healthy adults, in one meta-analysis of 19 trials rated mostly low in certainty.
Compared with control groups the abstract does not describe [2]
The numbers and sources
- Effect
- Fermented foods raised bowel movement frequency (MD 0.60, 95% CI 0.04 to 1.16), softened stool on the Bristol scale (MD 0.25, 95% CI 0.03 to 0.47), eased gastrointestinal symptoms (SMD -0.60, 95% CI -1.05 to -0.15) and shortened intestinal transit time (MD -13.65, 95% CI -21.88 to -5.43) [2].
- Population
- 4,328 healthy adults in 25 studies, 19 randomized trials pooled [2].
- Sources
- [2]
- Caveats
- Heterogeneity was high (I² 72% to 95%) and the certainty of evidence was rated mostly low [2]. The trials used different foods.
Irritable bowel syndrome
Shown to raise the share of people with irritable bowel syndrome reporting symptom relief by 22%, in one meta-analysis. Pain and bloating scores did not change significantly.
Compared with control groups the abstract does not describe [3]
The numbers and sources
- Effect
- Symptom relief was more frequent with fermented foods (RR 1.22, 95% CI 1.04 to 1.42, I² 0%, 12 trials), and global symptom scores fell slightly (SMD -0.15, 95% CI -0.29 to -0.02). Abdominal pain and bloating scores did not improve significantly. Fermented milk alone showed RR 1.19 (95% CI 1.01 to 1.39) [3].
- Population
- 1,264 people with irritable bowel syndrome in 16 randomized trials [3].
- Sources
- [3]
- Caveats
- The authors advise caution given limits of the evidence [3]. Many products were fermented milks with added probiotic strains.
Respiratory infections
Shown to lower respiratory infections by about 19% with probiotic fermented dairy against placebo, in one meta-analysis of 22 trials across ages.
Compared with placebo [4]
The numbers and sources
- Effect
- Probiotic fermented dairy lowered respiratory tract infections overall (RR 0.81, 95% CI 0.74 to 0.89), in children (RR 0.82), adults (RR 0.81, 95% CI 0.66 to 1.00) and older people (RR 0.78). Upper respiratory infections fell (RR 0.83), the common cold fell (RR 0.68, 95% CI 0.49 to 0.96), and lower respiratory infections fell marginally (RR 0.78, 95% CI 0.60 to 1.01) [4].
- Population
- 10,190 participants in 22 randomized trials [4].
- Sources
- [4]
- Caveats
- The products were dairy foods with added probiotic strains, so this may not apply to other fermented foods. The adult estimate's interval reaches 1.00.
Blood sugar
Shown to lower fasting glucose slightly with kimchi and kefir, in two meta-analyses. Probiotic yogurt did not differ from ordinary yogurt.
Compared with control groups the abstract does not describe [7][9]; conventional yogurt [10]
The numbers and sources
- Effect
- Kefir lowered fasting glucose (MD -8.46 mg/dL) and HOMA-IR (MD -1.71) [7]. Fermented kimchi lowered fasting glucose (WMD -1.93 mg/dL, 95% CI -3.82 to -0.03) [9]. Probiotic yogurt did not change HbA1c, fasting glucose, fasting insulin or HOMA-IR against mostly conventional yogurt in people with type 2 diabetes or obesity [10].
- Population
- 24 studies of kefir [7]. 205 participants in 5 kimchi trials [9]. 472 people with type 2 diabetes or obesity in 9 trials [10].
- Sources
- [7], [9], [10]
- Caveats
- Small trials of single foods. The yogurt review compared two fermented foods with each other, not fermented food with none [10].
Gut microbiome diversity
One 17-week trial of 36 adults found a high fermented food diet steadily raised gut microbiome diversity, which a high-fiber diet did not.
Compared with a high-fiber diet, with changes measured over time in each group [8]
The numbers and sources
- Effect
- The high fermented food diet steadily increased microbiota diversity. The high-fiber diet left community diversity stable. The primary outcome, a cytokine response score, did not change [8].
- Population
- 36 healthy adults, 18 in each group [8].
- Sources
- [8]
- Caveats
- One small trial. Microbiome diversity is not itself a health outcome.
Death from any cause, in cohorts
Linked to lower death from any cause and heart disease for cheese and fermented milks in cohorts. Miso and bread showed no link.
Compared with the lowest intake categories [12]
The numbers and sources
- Effect
- Comparing highest with lowest intake, cheese and fermented milks including yogurt were linked to lower death from any cause and from cardiovascular disease, and fermented milk to lower cancer death. Miso and bread were not linked to death [12].
- Population
- More than 3 million participants in 50 cohort studies [12].
- Sources
- [12]
- Caveats
- Observational. People who eat yogurt and cheese may differ in other habits, so confounding is possible. The abstract gives no effect sizes.
Mixed results3
Inflammation and C-reactive protein
Mixed results: one meta-analysis found fermented dairy lowered C-reactive protein slightly, while two meta-analyses of kefir trials found no change.
Compared with control groups the abstract does not describe [5]; people who did not drink kefir [6]; control groups [7]; a high-fiber diet, with changes measured over time in each group [8]
The numbers and sources
- Effect
- Across 14 trials of yogurt, fermented milk and kefir, C-reactive protein fell (SMD -0.21, 95% CI -0.40 to -0.02), mainly in people with metabolic disease, and TNF-alpha and interleukin-6 did not change [5]. Kefir did not change C-reactive protein (WMD -0.17 mg/L, 95% CI -0.84 to 0.49), though it fell in trials of 8 weeks or longer [6]. A second kefir review found no significant effect on inflammatory markers [7]. In a 17-week trial, a high fermented food diet steadily raised gut microbiome diversity and lowered inflammatory markers [8].
- Population
- 14 trials [5]. 385 adults in 7 trials [6]. 24 studies [7]. 36 healthy adults, 18 in each diet group [8].
- Sources
- [5], [6], [7], [8]
- Caveats
- Few small trials with different foods. The abstract of [8] does not name C-reactive protein among the markers that fell.
Type 2 diabetes, in cohorts
Mixed results: in cohorts, cheese was linked to slightly less type 2 diabetes and yogurt to a 22% lower risk whose interval included no difference.
Compared with people eating 50 g of cheese or 200 g of yogurt a day less [11]
The numbers and sources
- Effect
- Per 50 g of cheese a day, type 2 diabetes was lower (RR 0.92, 95% CI 0.86 to 0.99). Per 200 g of yogurt a day, RR was 0.78 (95% CI 0.60 to 1.02), with high heterogeneity (I² 70%) and an inverse curve that flattened at higher intakes [11].
- Population
- 17 cohort studies [11].
- Sources
- [11]
- Caveats
- Observational, open to confounding and reverse causation.
Blood pressure
Mixed results: kefir did not lower blood pressure in a meta-analysis of 7 trials, while kimchi lowered it about 3.5 mmHg after excluding outlying trials.
Compared with people who did not drink kefir [6]; control groups the abstract does not describe [9]
The numbers and sources
- Effect
- Kefir did not change systolic (WMD -1.76 mmHg, 95% CI -5.21 to 1.69) or diastolic pressure [6]. After excluding heterogeneous studies, kimchi lowered systolic pressure by 3.48 mmHg (95% CI -5.95 to -1.01) and diastolic by 2.68 mmHg [9].
- Population
- 385 adults in 7 kefir trials [6]. 205 participants in 5 kimchi trials [9].
- Sources
- [6], [9]
- Caveats
- The kimchi result depends on excluding some studies [9]. Both reviews pooled few small trials.
Not shown to help1
Body weight
Not shown to change body weight or body measurements in a meta-analysis of kefir trials. Kimchi cohorts linked higher intake to normal body weight.
Compared with control groups [7]; people eating less kimchi [9]
The numbers and sources
- Effect
- Kefir had no significant effect on anthropometric measures [7]. Prospective cohorts linked higher kimchi intake with a greater likelihood of normal body weight [9].
- Population
- 24 kefir studies [7]. 42,455 participants in 4 kimchi cohorts [9].
- Sources
- [7], [9]
- Caveats
- The kefir abstract gives no size for the weight result. The kimchi finding is observational.
Amounts studied
The gut symptom and irritable bowel trials used a range of fermented foods, mostly fermented milks [2][3]. The infection trials used probiotic fermented dairy products [4]. Kefir trials ran at least 2 weeks [6], and the kefir review grouped results by dose [7]. The microbiome trial ran 17 weeks, with one group eating a high fermented food diet [8]. The cohort analyses compared intakes per 50 g of cheese and 200 g of yogurt a day [11], or the highest against the lowest intake [12].
Risks
Reported risks3
Dangerous blood pressure rises with MAOI antidepressants
The phenelzine label warns that foods high in tyramine, including fermented foods such as sauerkraut, aged cheese and yogurt, can cause hypertensive crises, sometimes fatal.
The numbers and sources
- How common
- How often is not known: the sources give no rate.
- Effect
- The label states that hypertensive crises during phenelzine treatment may be caused by foods high in tyramine or dopamine, and tells patients to avoid high-protein food that has undergone aging, fermentation, pickling, smoking or bacterial contamination. It lists sauerkraut, cheese other than cottage and cream cheese, yogurt, pickled herring, dry sausage and yeast extract, during treatment and for two weeks after. It describes these crises as sometimes fatal [1].
- Population
- People taking phenelzine, an MAOI antidepressant [1].
- Sources
- [1]
- Caveats
- A label warning. The label gives no rate, and tyramine content varies widely between foods and batches.
Stomach cancer with pickled vegetables
Linked to about 50% higher stomach cancer risk in a meta-analysis of 60 studies of pickled vegetables, mostly from Korea, China and Japan.
The numbers and sources
- How common
- How often is not known: the sources give no rate.
- Effect
- Pickled vegetable or food intake was linked to stomach cancer (OR 1.52, 95% CI 1.37 to 1.68), 1.32 in cohort studies and 1.56 in case-control studies. The link was 1.89 in Korean, 1.86 in Chinese and 1.16 in Japanese studies, and not significant elsewhere (OR 1.14, 95% CI 0.96 to 1.35) [13].
- Population
- 50 case-control and 10 prospective studies [13].
- Sources
- [13]
- Caveats
- Observational, with high heterogeneity. The pickled foods studied were not all fermented.
Severe illness reported after kombucha
Case reports: a US public health investigation of two severe illnesses, one fatal, in people drinking kombucha daily, and a report of severe liver injury.
The numbers and sources
- How common
- How often is not known: the sources give no rate.
- Effect
- In 1995, two people in Iowa who had drunk kombucha daily for about 2 months developed unexplained severe illness, and one died. State health officials, the CDC and the FDA investigated [14]. A 2023 report describes severe acute liver injury in a previously healthy woman after drinking kombucha [15].
- Population
- Two people in Iowa [14]. One woman [15].
- Sources
- [14], [15]
- Caveats
- Case reports cannot prove cause or give a rate. The 1995 report describes the link as possible [14].
Claims tested and not found
- That kefir lowers blood pressure. A meta-analysis of 7 trials found no significant change in systolic or diastolic pressure [6].
- That probiotic yogurt controls blood sugar better than ordinary yogurt. Nine trials in people with type 2 diabetes or obesity found no difference in HbA1c, fasting glucose or insulin resistance [10].
Related
Sources
- Parke-Davis Div of Pfizer Inc. NARDIL (phenelzine sulfate) tablets. Prescribing information. DailyMed. 2026. https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=513a41d0-37d4-4355-8a6d-a2c643bce6fa
- Mukherjee A, Farsi DN, Garcia-Gutierrez E, et al. Impact of fermented foods consumption on gastrointestinal wellbeing in healthy adults: a systematic review and meta-analysis. Frontiers in Nutrition. 2025. https://doi.org/10.3389/fnut.2025.1668889
- Ding L, Duan J, Yang T, et al. Efficacy of fermented foods in irritable bowel syndrome: a systematic review and meta-analysis of randomized controlled trials. Frontiers in Nutrition. 2025. https://doi.org/10.3389/fnut.2024.1494118
- Rashidi K, Razi B, Darand M, et al. Effect of probiotic fermented dairy products on incidence of respiratory tract infections: a systematic review and meta-analysis of randomized clinical trials. Nutrition Journal. 2021. https://doi.org/10.1186/s12937-021-00718-0
- Zhang X, Zhang X, Luo Q, et al. Effects of fermented dairy products on inflammatory biomarkers: A meta-analysis. Nutrition, Metabolism and Cardiovascular Diseases. 2023. https://doi.org/10.1016/j.numecd.2022.12.014
- Rashidbeygi E, Samarin MM, Sheikhhossein F, et al. The Effect of Kefir Consumption on Blood Pressure and C-Reactive Protein: A Systematic Review and Meta-Analysis of Randomised Controlled Trials. Endocrinology, Diabetes & Metabolism. 2025. https://doi.org/10.1002/edm2.70124
- Hamsho M, Hawari R, Yeşil Z, et al. Effect of different kefir dosages on inflammation status, metabolic profile, and anthropometric measurements in adults: A systematic review and meta-analysis. Nutrition, Metabolism and Cardiovascular Diseases. 2026. https://doi.org/10.1016/j.numecd.2025.104364
- Wastyk HC, Fragiadakis GK, Perelman D, et al. Gut-microbiota-targeted diets modulate human immune status. Cell. 2021. https://doi.org/10.1016/j.cell.2021.06.019
- Ahn S, Darooghegi Mofrad M, Nosal BM, et al. Effects of Fermented Kimchi Consumption on Anthropometric and Blood Cardiometabolic Indicators: A Systematic Review and Meta-Analysis of Intervention Studies and Prospective Cohort Studies. Nutrition Reviews. 2025. https://doi.org/10.1093/nutrit/nuae167
- Barengolts E, Smith ED, Reutrakul S, et al. The Effect of Probiotic Yogurt on Glycemic Control in Type 2 Diabetes or Obesity: A Meta-Analysis of Nine Randomized Controlled Trials. Nutrients. 2019. https://doi.org/10.3390/nu11030671
- Aune D, Norat T, Romundstad P, Vatten LJ. Dairy products and the risk of type 2 diabetes: a systematic review and dose-response meta-analysis of cohort studies. The American Journal of Clinical Nutrition. 2013. https://doi.org/10.3945/ajcn.113.059030
- Matalas A, Panagiotakos D, Fardet A, et al. Fermented foods consumption, all-cause, and cause-specific mortality: a meta-analysis of prospective cohort studies. Frontiers in Nutrition. 2026. https://doi.org/10.3389/fnut.2026.1714437
- Ren JS, Kamangar F, Forman D, Islami F. Pickled food and risk of gastric cancer: a systematic review and meta-analysis of English and Chinese literature. Cancer Epidemiology, Biomarkers & Prevention. 2012. https://doi.org/10.1158/1055-9965.epi-12-0202
- Centers for Disease Control and Prevention. Unexplained severe illness possibly associated with consumption of Kombucha tea, Iowa, 1995. MMWR Morbidity and Mortality Weekly Report. 1995. https://pubmed.ncbi.nlm.nih.gov/7476846/
- Sannapaneni S, Philip S, Desai A, et al. Kombucha-Induced Massive Hepatic Necrosis: A Case Report and a Review of Literature. Gastro Hep Advances. 2023. https://doi.org/10.1016/j.gastha.2022.09.014