Kefir and kefir grain cultures

What it is

Kefir is a fermented milk drink made when milk is fermented by lactic acid bacteria and yeasts that live together inside a 'kefir grain', a matrix of protein and polysaccharide (mostly a sugar polymer called kefiran). Commercial kefir is rarely made with grains; manufacturers usually use defined starter cultures isolated from kefir, and studies find these products have a very different microbial make-up from traditional grain kefir.

What the research says

The evidence is limited. A 2023 systematic review of 16 randomised trials found kefir may help as an add-on for reducing a caries-linked mouth bacterium (Streptococcus mutans) and in Helicobacter pylori eradication therapy, with very limited evidence for cholesterol and blood pressure; most trials had a high risk of bias. A placebo-controlled trial in young children on antibiotics found kefir did not prevent antibiotic-associated diarrhoea. Kefir appears safe for healthy people, but a yeast found in kefir and other dairy (Kluyveromyces marxianus, formerly Candida kefyr) has caused invasive infections in transplant patients. No trials test dried 'wild kefir culture' capsules, and analyses show commercial kefir cultures do not resemble grain kefir, so trial results for kefir drinks cannot be assumed to apply to them.

Evidence grade: Limited evidence.

What goes wrong

A single-author case report with a review of seven published case reports described invasive Candida kefyr (Kluyveromyces marxianus) infections, including fungaemia, in transplant recipients; most of the reported patients were neutropenic and consumed dairy products. (Source 1)

• Case series, Very low certainty.
• Size: 1 new case report plus 7 published case reports (single-author literature review)
• Who: Solid organ or haematopoietic stem cell transplant recipients.
• How long: not applicable.
• Result: Fungaemia, keratitis, catheter-related bloodstream infections; median age 61.0 across the reviewed case reports.
• Funding: not stated.

Limit of this finding: The evidence is a handful of individual case reports collected by one author. It shows this dairy-associated yeast can cause serious infection in severely immunocompromised people; it does not show how often this happens or that kefir specifically was the source in each case.

C. kefyr should be considered a potential opportunistic pathogen in transplant recipients, particularly in older or neutropenic patients with regular dairy consumption.

The systematic review noted that safety was assessed in only 5 of 18 publications. (Source 2)

• Systematic review, Low certainty.
• Size: 18 publications.
• Who: Human RCTs of kefir.
• How long: various.
• Result: Safety reporting sparse; kefir appeared safe for generally healthy people.
• Funding: not stated in abstract.

Safety was only assessed in 5 of the 18 included publications, and 12 of the studies had an overall high risk for bias.

What the evidence supports

A systematic review of randomised trials found kefir may have potential as an add-on for reducing oral Streptococcus mutans and in H. pylori eradication therapy, but most studies were at high risk of bias. (Source 2)

• Systematic review, Low certainty.
• Size: 16 studies reported in 18 publications.
• Who: Human randomised controlled trials of fermented-milk kefir on any health outcome.
• How long: Searches from inception to July 31, 2021.
• Result: Narrative synthesis only; no pooled effect size. 12 of 16 studies at overall high risk of bias.
• Funding: not stated in abstract.

fermented-milk kefir may have potential as a complementary therapy in reducing oral Streptococcus mutans, thereby reducing dental caries risk, and in Helicobacter pylori eradication therapy

What the evidence does not support

In a double-blind placebo-controlled trial in 125 children aged 1-5 taking antibiotics, commercial kefir did not reduce antibiotic-associated diarrhoea. (Source 3)

• Randomized trial, Low certainty.
• Size: 125 children.
• Who: Children aged 1 to 5 years in primary care in the Washington, DC area receiving antibiotics.
• How long: 14-day follow-up.
• Result: Diarrhoea 18% kefir vs 21.9% placebo; relative risk 0.82 (95% CI 0.54-1.43); the abstract also reports no differences in secondary outcomes.
• Funding: not verified: an industry-funding statement (Lifeway Foods, Inc.) was recorded earlier but could not be confirmed against the abstract or full text.

There were no differences in the rates of diarrhea per group, with 18% in the active group and 21.9% in the placebo group (relative risk, 0.82; 95% confidence interval, 0.54-1.43).

Commercial kefirs made with defined cultures had a completely different microbial composition from traditional grain kefir and higher residual lactose, so they may not have the same effects. (Source 4)

• Lab study in cells, Low certainty.
• Size: commercial and traditional kefir samples.
• Who: Food products (laboratory analysis)
• How long: not applicable.
• Result: Traditional kefir dominated by Lactobacillus kefiranofaciens; commercial kefir dominated by Lactococcus lactis.
• Funding: not stated in abstract.

In summary, the analyzed commercial kefirs do not resemble the microbial community and metabolite characteristics of traditional grain-based kefir.

Where the evidence is mixed

The same review judged evidence for kefir in adult dyslipidaemia and hypertension to be very limited, and said high-quality trials are needed before any recommendations. (Source 2)

• Systematic review, Very low certainty.
• Size: 16 studies.
• Who: Human randomised controlled trials.
• How long: various.
• Result: Not pooled.
• Funding: not stated in abstract.

Kefir may further aid treatment of adult dyslipidemia and hypertension, although evidence was very limited.

In a small open-label, single-arm ICU safety study with no control group, kefir did not increase gut microbial alpha-diversity, though a composite gut-health index (GMWI) improved in the 13 patients with paired stool samples. (Source 5)

• Case series, Very low certainty.
• Size: 54 patients enrolled; 26 paired stool samples from only 13 patients.
• Who: Critically ill adults with an intact gastrointestinal tract.
• How long: ICU stay.
• Result: No increase in alpha-diversity; Gut Microbiome Wellness Index improved (P = 0.034, one-sided Wilcoxon)
• Funding: independent (Zoll Foundation, Mayo Clinic Critical Care Research Committee)

Limit of this finding: This was an open-label phase I safety and feasibility study with a single arm and no control group, and the microbiome comparison rests on paired samples from just 13 patients. The GMWI result was tested one-sided. It shows kefir was tolerated in this setting; it cannot show that kefir caused the index change.

Analysis of the 26 paired samples revealed no increase in gut microbial α-diversity between the two timepoints. However, there was a significant improvement in the Gut Microbiome Wellness Index (GMWI) by the second timepoint (P = 0.034, one-sided Wilcoxon signed-rank test)

Where the research disagrees

Whether commercial or dried kefir cultures are equivalent to traditional grain kefir used in many studies

• Bourrie, Willing and Cotter (2016 review), narrative review: "Commercial, industrial-scale production rarely utilizes kefir grains for fermentation, but rather uses starter cultures of microbes that have been isolated from kefir or kefir grains in order to provide more consistent products." (Source 6)
• Nejati et al. (2022), laboratory comparison of products: "Thus, they may deliver different functional effects to the consumers, which remain to be examined in future studies." (Source 4)

How much

• Reference intake: No reference intake exists; kefir is a food, not a nutrient. The systematic review said high-quality trials are needed before any recommendations can be made. (Source 2)
• Upper limit: No upper limit has been set by any body for kefir. The 2023 systematic review described it as appearing safe for generally healthy people. (Source 2)
• Studied: In an ICU safety study, patients were given kefir in escalating doses: 60 mL, then 120 mL after 12 hours, then 240 mL daily. (Source 7)

A common belief, and what the research shows

The belief: Kefir is a proven gut-health fix and a 'wild kefir culture' capsule gives the same benefits as the drink.

What the research shows: Trials of kefir itself are few and mostly at high risk of bias ("12 of the studies had an overall high risk for bias"), and commercial cultures "do not resemble the microbial community and metabolite characteristics of traditional grain-based kefir." No trials of dried kefir-culture supplements were found.

Questions and answers

What is it?

Kefir is a fermented milk drink. It is made by adding 'kefir grains', clumps of protein and sugar polymer that house a community of bacteria and yeasts, to milk. A 'kefir culture' in a supplement is a dried mix of microbes originally isolated from kefir. (Source 6)

What does it do in the body?

Laboratory and animal work has linked kefir to many effects, but human trials are few. A systematic review of randomised trials found possible benefit as an add-on for a caries-linked mouth bacterium and in H. pylori treatment, with very limited evidence for cholesterol and blood pressure. (Source 2)

Is it good or bad for you?

For generally healthy people kefir appears safe, but its benefits are not firmly established. In people with severely weakened immunity, such as transplant recipients with low white cell counts, a dairy yeast found in kefir has caused serious infections. (Source 2)

How do you get more of it?

The studied route is drinking fermented-milk kefir. Most commercial kefir is made with defined starter cultures rather than grains, and it differs microbially from traditional grain kefir; dried culture supplements have not been tested in trials we found. (Source 4)

If it is harmful, what reduces it?

Does not apply for most people: kefir is a food, not something the body carries that needs reducing. The one caution in the literature concerns immunocompromised people, in whom a kefir-associated yeast has caused infection. (Source 1)

Why might someone be low in it or missing it?

Does not apply in the usual sense. Kefir microbes are not a normal resident of the human gut that someone lacks; they come from eating the fermented food. Whether the microbes settle in the gut varies: in an ICU study, kefir species showed varying levels of engraftment. (Source 5)

Which whole foods contain it or feed it?

Kefir itself is the food: milk fermented with kefir grains or with starter cultures derived from them. Traditional grain kefir and commercial kefir differ in their dominant microbes and in how much lactose remains. (Source 4)

What happens if you do not have it?

Nothing is known to happen. Kefir is not an essential nutrient, and the review of trials found no health outcome for which it is established enough to make recommendations. (Source 2)

How can you test for it?

No clinical test exists for 'kefir status'. Research labs can detect kefir species in stool by sequencing, as in the ICU study, but this is a research method, not a validated consumer test. (Source 5)

We searched: Searched PubMed/PMC and kefir reviews for any clinical or consumer test of kefir microbes; found only research stool sequencing (ICU kefir study) and product-level qPCR/NGS analyses of kefir drinks.

References

1. Diagnostic Microbiology and Infectious Disease. Candida kefyr as an emerging cause of invasive fungal infection in transplant patients: Case report and literature review. 2025. DOI 10.1016/j.diagmicrobio.2025.117052. Read the source
2. Nutrition Reviews. The effects of kefir consumption on human health: a systematic review of randomized controlled trials. 2023. PMID 35913411, DOI 10.1093/nutrit/nuac054. Read the source
3. Archives of Pediatrics & Adolescent Medicine (JAMA Pediatrics). A randomized clinical trial measuring the influence of kefir on antibiotic-associated diarrhea: the measuring the influence of Kefir (MILK) Study. 2009. PMID 19652108, DOI 10.1001/archpediatrics.2009.119. Read the source
4. Applied Sciences (MDPI). Traditional Grain-Based vs. Commercial Milk Kefirs, How Different Are They?. 2022. DOI 10.3390/app12083838. Read the source
5. BMC Medicine. Safety, feasibility, and impact on the gut microbiome of kefir administration in critically ill adults. 2024. PMID 38378568, DOI 10.1186/s12916-024-03299-x. Read the source
6. Frontiers in Microbiology. The Microbiota and Health Promoting Characteristics of the Fermented Beverage Kefir. 2016. DOI 10.3389/fmicb.2016.00647. Read the source
7. BMC Medicine. Safety, feasibility, and impact on the gut microbiome of kefir administration in critically ill adults. 2024. PMID 38378568, DOI 10.1186/s12916-024-03299-x. Read the source