Kefir

Kefir: A Comprehensive Reference

1. Identity and Nomenclature

Common name: Kefir (also spelled kephir, kefyr). Type: Fermented probiotic beverage; classified as a functional food and dietary supplement. Codex definition: The Codex Alimentarius Standard for Fermented Milks (CXS 243-2003) defines kefir by its starter culture and microbial composition: the starter must be prepared from kefir grains and must include Lactobacillus kefiri, species of the genera Leuconostoc, Lactococcus, and Acetobacter, as well as both lactose-fermenting yeasts (Kluyveromyces marxianus) and non-lactose-fermenting yeasts (Saccharomyces unisporus, S. cerevisiae, S. exiguus). In other words, Codex requires that something sold as "kefir" be made with a culture originating from kefir grains and containing that mix of lactic acid bacteria and yeasts in symbiosis.

Etymology: Kefir is likely of central Eurasian origin and has a significant history in the Caucasus Mountains. The word kefir is believed to have originated from the Turkish word keyif, meaning "pleasure."

Natural source: Kefir is fermented milk resulting from the metabolism of a complex microbiota in symbiosis. The grain is primarily composed of bacterially-produced kefiran, which contains within it a complex consortium of bacteria and yeast that work in symbiosis to ferment the milk. Kefir grains will ferment the milk from most mammals and will continue to grow in such milk. Typical animal milks used include cow, goat, and sheep, each with varying organoleptic (flavor, aroma, and texture) and nutritional qualities.

Common Forms and Preparations

Kefir can be produced by fermenting milk with commercial freeze-dried kefir starter cultures, traditional kefir grains, and the product that remains after the removal of kefir grains.

• Grain-based (traditional/artisanal) kefir: Traditional kefir was made in goatskin bags that were hung near a doorway; the bags would be knocked by anyone passing through to keep the milk and kefir grains well mixed.
• Powdered starter cultures: Unlike commercial kefir developed in Russia, most commercially prepared kefir on the market today is made with direct-set powdered kefir cultures. These powdered cultures contain considerably fewer probiotic strains than kefir grains, but they produce a more consistent commercial product.
• Freeze-dried preparations: Kefir can be made using freeze-dried cultures commonly available in powder form from health food stores. A portion of the resulting kefir can be saved to be used a number of times to propagate further fermentations but ultimately does not form grains.
• Non-dairy / water kefir: Dairy kefir is not suitable for lactose-intolerant, vegan, and dairy-product-allergic users. An alternative method of reaping the health benefits of kefir is through its alteration to non-dairy substrates.

The conditions of kefir production can alter the community composition of microorganisms in the kefir grains. Chemical and probiotic composition of the final product may be affected by fermentation techniques, duration and temperature, milk variants, grain origin, grain-to-milk ratio, and post-fermentation cooling duration.

2. Traditional and Historical Use

The true origin of kefir grains is lost to time, shrouded in legend and mystery. They originated centuries ago among the nomadic tribes living on the northern slopes of the Caucasus Mountains, the region dividing Europe and Asia. The kefir grains and methods for making kefir were kept secret by people in the Caucasus mountains for many generations. Owning kefir grains was equated with wealth in this region. The people believed that the benefits of kefir would somehow be diminished if the secrets of making kefir were shared outside their region.

Kefir is a very popular drink in the North Caucasus and is a part of traditional Circassian cuisine to this day. In the 19th century, for example, Turkestan promoted kefir and other fermented foods as nourishment for those with tuberculosis and other diseases.

The word kefir, which is of North Caucasian origin, became an international word, having originally spread to Russia and Central European and Eastern European countries at least by 1884. The kefir grains were taken to the Moscow Dairy, and in September 1908, the first bottles of kefir drink were offered for sale in Moscow. People mostly consumed it for its alleged medicinal value. Commercial manufacturing of kefir on a large scale began in Russia in the 1930s.

Kefir spread from the former Soviet Union to the rest of Europe, Canada, Japan, and the United States by the early 21st century.

3. Microbial Composition and Key Constituents

The Kefir Grain

Kefir grains range in size from 1 to 4 cm in length and look like small cauliflower florets in shape (irregular and lobed-shaped) and color (from white to light yellow). This gelatinous and slimy structure is comprised of a natural matrix of exopolysaccharides (EPS) kefiran and proteins in which lactic acid bacteria (LAB), yeasts, and acetic acid bacteria (AAB) co-exist in symbiotic connection.

Kefir grains are white to yellow-white, gelatinous, and variable in size (varying from 0.3–3.5 cm in diameter) and are composed of a microbial symbiotic mixture of lactic acid bacteria (10⁸ CFU/g), yeast (10⁶–10⁷ CFU/g), and acetic acid bacteria (10⁵ CFU/g) that stick to a polysaccharide matrix.

Up to 50 different bacterial and yeast species have been identified in artisanal kefirs.

Predominant Bacterial Species

The most predominantly found bacterial species in kefir grains are Lactobacillus kefiranofaciens, Lacticaseibacillus paracasei, Lactiplantibacillus plantarum, Lactobacillus acidophilus, and Lactobacillus delbrueckii subsp. bulgaricus.

Bacterial species found in high abundance in most artisanal kefirs include Lactobacillus kefiranofaciens, Lentilactobacillus kefiri, Lactobacillus ultunensis, Lactobacillus apis, Lactobacillus gigeriorum, Gluconobacter morbifer, Acetobacter orleanensis, Acetobacter pasteurianus, Acidocella aluminiidurans, and Lactobacillus helveticus.

Predominant Yeast Species

On the other hand, Saccharomyces cerevisiae, S. unisporus, Candida kefyr, and Kluyveromyces marxianus ssp. marxianus are the predominant yeast species present in kefir.

The microbial composition of kefir and kefir grains is believed to vary depending on geographic, climatic, and cultural conditions as well as the diversity of local species of wild yeasts and bacteria.

Kefiran — The Key Exopolysaccharide

The grain's structure is composed of proteins from milk (casein) and a complex exopolysaccharide called kefiran. Kefiran is a heteropolysaccharide made of roughly equal glucose and galactose, and it serves as the "glue" that holds the community together in a three-dimensional matrix. Lactobacillus kefiranofaciens, a prominent grain bacterium, is known to synthesize kefiran, contributing to the grain's growth and texture.

Nutritional and Bioactive Compounds

The nutritional values of kefir are due to its rich chemical composition, including minerals, sugars, carbohydrates, proteins, peptides, vitamins, and fats. Aside from such chemical makeup, it is the fermentation process that further enhances kefir's nutritional value owing to secondary bioactive ingredients such as catechin, vanillin, ferulic acid, and salicylic acid.

Kefir is enriched in vitamins B1, B2, B5, and C, minerals, and essential amino acids that are of value for improving fitness, the healing process, and homeostasis. Macro-elements enriched in kefir include calcium, magnesium, potassium, and sodium, which aid in the utilization of carbohydrates, fats, and proteins for cell growth, maintenance, and energy. Kefir also contains micro-elements, including iron, zinc, and copper, which are of special value in cellular metabolism and blood production.

The typical composition of kefir contains 80–90% moisture, 0.2% lipid, 3.0% protein, 6.0% sugar, and 0.7% ash, and approximately 1.0% lactic acid and alcohol.

These health-promoting characteristics are attributed to the abundance of probiotics in kefir grains and the wide range of bioactive substances produced during fermentation, such as bioactive peptides, exopolysaccharides, organic acids, vitamins, and minerals that enhance the overall nutritional profile of the beverage.

In summary, the bioactive compounds more commonly reported were exopolysaccharides, including kefiran, bioactive peptides, and organic acids, especially lactic acid.

Neurological Bioactive Compounds

Kefir contains bioactive compounds such as B vitamins, choline, and folic acid, which are essential for neuronal health and cognitive function.

4. Mechanisms of Action

Gut Microbiota Modulation

The exopolysaccharides of kefir represent potential bioactive compounds for microbiota modulation with beneficial effects both at the gastrointestinal and extra-intestinal levels. Toscano et al. demonstrated a randomized trial in 20 Italian healthy people whose diets were supplemented with L. kefiri for one month. The results showed significant reduction in several bacterial genera directly involved in the onset of pro-inflammatory response and gastrointestinal diseases, indicating the ability of L. kefiri to modulate the gut microbiota composition.

Anti-Inflammatory and Immunomodulatory Activity

Kefir can inhibit the activity of proinflammatory cytokines. Kefir bioactive compounds presented antimicrobial, anticancer, and immune-modulatory activities corroborated by meta-analysis. The mechanisms of their action were diverse, indicating that they can act by different signaling pathways.

Antimicrobial Activity

The antimicrobial activity of kefir was mainly attributed to exopolysaccharides (EPSs), especially kefiran, and organic acids, especially lactic acid. However, bioactive peptides with antimicrobial activity have also been identified. Several lactobacilli and yeast strains from kefir exhibited antipathogenic activities, inhibiting the growth of Salmonella typhimurium and enteropathogenic Escherichia coli O157:H7.

Antioxidant Mechanisms

The intervention with antioxidant molecules is crucial since they interact with free radicals, ending the chain chemical reaction and reducing the attack on proteins and DNA that would cause cell damage.

Anticancer Mechanisms (Preclinical)

Research on kefir has revealed that its anticancer potential is mediated by multiple bioactive components, including peptides and polysaccharides such as EPS and kefiran. EPS, particularly kefiran, modulate apoptotic pathways by downregulating B-cell lymphoma 2 (Bcl-2) and upregulating Bcl2-associated X protein (Bax), cytochrome c, and caspases 3, 8, and 9. Bioactive peptides demonstrate selective cytotoxicity toward cancer cells, such as estrogen-sensitive MCF7-E3 breast cancer cells, without affecting normal human epithelial cells. These compounds may act indirectly through immune system activation and antioxidative mechanisms, inhibiting tumor growth, DNA damage, and pro-carcinogen activation.

Antihypertensive Mechanisms

Researchers identified 35 peptides with ACE inhibitory activity in kefir, reaffirming its potential in reducing blood pressure.

Bone and Calcium Metabolism

Kefir is a product made by kefir grains that degrade milk proteins into various peptides with health-promoting effects, including antithrombotic, antimicrobial, and calcium-absorption enhancing bioactivities. Kefir-derived peptides exhibit osteoprotective potential in various animal models of osteoporosis, in which several antioxidative and ACE-inhibitory peptides have been shown to promote osteoblast differentiation and mineralization. Emerging evidence supports the role of kefir-derived probiotics and exopolysaccharides (kefiran) in mitigating bone loss.

5. Scientific Evidence by Health Area

5.1 Gastrointestinal Health and Gut Microbiota

In a review of 28 clinical trials, the studies included diverse populations ranging from healthy individuals to patients with metabolic syndrome, non-alcoholic fatty liver disease (NAFLD), inflammatory bowel disease (IBD), osteoporosis, chronic functional constipation, and athletes, focusing on gastrointestinal health, metabolic and cardiovascular health, oral health, and lactose intolerance. Overall, kefir consumption was associated with various beneficial effects; however, the observed outcomes were dose-dependent, duration-dependent, and the population profile was critical in determining clinical efficacy.

Several studies focused on gut microbiota composition and gastrointestinal function found that kefir increased beneficial bacteria such as Lactobacillus, Bifidobacterium, Akkermansia, and Faecalibacterium, improved stool frequency and consistency, reduced constipation and antibiotic-associated diarrhea, and enhanced lactose digestion.

Evidence strength: Moderate. Multiple clinical trials show consistent direction of effect, but studies are often small and heterogeneous in design, kefir type, and population.

5.2 Lactose Intolerance

The lactic acid bacteria in fermented dairy foods — like kefir and yogurt — turn the lactose into lactic acid, so these foods are much lower in lactose than milk. They also contain enzymes that can help break down the lactose even further. That is why kefir is generally well tolerated by people with lactose intolerance, at least compared with regular milk.

Evidence strength: Moderate for improved lactose digestion vs. unfermented milk; supported by clinical trials included in the 28-study review above.

5.3 Metabolic Health, Cardiovascular Parameters, and Glycemic Control

Kefir intake was linked to improvements in lipid profiles, reductions in blood pressure, and enhanced insulin sensitivity. Several RCTs investigated the effects of kefir consumption on metabolic and cardiovascular health and found that kefir consumption is associated with beneficial effects, particularly in individuals with overweight, metabolic syndrome, or non-alcoholic fatty liver disease (NAFLD).

Reported outcomes included decreases in LDL-cholesterol, triglycerides, homocysteine, fasting glucose, insulin, HOMA-IR, and inflammatory markers, alongside increases in HDL-cholesterol and apolipoprotein A1 (ApoA1). These improvements in lipid profile, insulin sensitivity, and inflammatory markers may be partially explained by kefir-induced alterations in the gut microbiota, including increased abundance of SCFA-producing taxa.

One specific RCT on metabolic syndrome examined 12 weeks of kefir administration: The 12 weeks of kefir administration decreased serum tumor necrosis factor-α, interleukin 6, interleukin 10, interferon-gamma, and homocysteine concentrations significantly (P < 0.05). Kefir intake decreased blood pressure, fasting glycemia, LDLc, non-HDLc, triglycerides, and oxidized LDL, and increased HDLc levels in women.

A double-blind RCT in 67 elderly subjects (fortified kefir 240 cc/day for 8 weeks vs. regular kefir): After eight weeks, significant differences were seen in atherogenic and Castelli's risk index I between the fortified and regular groups. No significant differences were found in Castelli's risk index II, HDL-C, total cholesterol, triglycerides, non-HDL-C, TG-cholesterol index, and fasting blood sugar. The investigation demonstrated that fortified kefir with probiotics did not significantly affect lipid profiles overall, though it could significantly affect some indices including Castelli's risk index I and the atherogenic index. More studies are required to confirm the findings.

A 2023 meta-analysis of 6 RCTs (314 subjects) examined cardiometabolic risk factors: Cardiometabolic risk indices extracted included insulin and insulin resistance (HOMA-IR), total cholesterol, triglycerides, HDL-C, LDL-C, fasting blood sugar, HbA1c, and body weight. In total, six RCTs (314 subjects) were selected for the meta-analysis.

A 2025 meta-analysis of 7 RCTs (385 subjects) on blood pressure found: Kefir consumption had no significant impact on systolic blood pressure (WMD: −1.76 mmHg; 95% CI: −5.21, 1.69; P = 0.317), diastolic blood pressure (WMD: −1.19 mmHg; 95% CI: −3.40, 1.03; P = 0.295) or CRP levels (WMD: −0.17 mg/L; 95% CI: −0.84, 0.49; P = 0.609) compared to controls. However, subgroup analysis indicated that CRP levels significantly decreased with longer durations of kefir consumption (≥ 8 weeks).

Evidence strength: Mixed to moderate. Several positive RCTs exist, particularly in metabolic syndrome patients, but meta-analyses do not confirm statistically significant reductions in blood pressure across all populations. Results appear dose- and duration-dependent. Animal studies are also inconsistent: Traditional kefir was found to reduce body weight gain, plasma cholesterol levels, and hepatic triglycerides in mice, whereas commercial kefir showed no beneficial effect.

5.4 Bone Health and Osteoporosis

In a controlled, parallel, double-blind intervention study over 6 months, researchers investigated the effects of kefir-fermented milk (1,600 mg) supplemented with calcium bicarbonate (CaCO₃, 1,500 mg) in 40 osteoporosis patients, and compared them with CaCO₃ alone without kefir supplements. Among patients treated with kefir-fermented milk, the relationships between baseline turnover and 6-month changes in DXA-determined BMD were significantly improved. The serum β C-terminal telopeptide of type I collagen (β-CTX) significantly decreased after three months of treatment. The formation marker serum osteocalcin turned from negative to positive after 6 months. PTH may promote bone remodeling after treatment with kefir for 6 months. In this pilot study, kefir-fermented milk therapy was associated with short-term changes in turnover and greater 6-month increases in hip BMD among osteoporotic patients.

Kefir holds significant promise in the management of osteoporosis due to its unique composition of bioactive components promoting bone health. While research is still in its early stages, evidence suggests kefir's potential as a natural approach to osteoporosis prevention and management.

Evidence strength: Preliminary. Only small pilot studies exist in humans; larger, adequately powered, blinded RCTs are needed before clinical conclusions can be drawn.

5.5 Immune Function

Kefir intake showed potential benefits for gut microbiota modulation, metabolic parameters, inflammatory markers, immune function, and gastrointestinal health. Kefir bioactive compounds presented antimicrobial, anticancer, and immune-modulatory activities corroborated by meta-analysis. However, clinical evidence is urgently needed to strengthen the practical applicability of these bioactive compounds.

Evidence strength: Mainly preclinical (in vitro and animal); limited direct human clinical trial data on immune endpoints.

5.6 Anticancer Activity

Kefir compounds may act indirectly through immune system activation and antioxidative mechanisms, inhibiting tumor growth, DNA damage, and pro-carcinogen activation. Moreover, polysaccharides such as kefiran may stimulate the host immune system via T-cell activation rather than exert direct cytotoxicity on tumor cells. In vivo, kefir-supplemented Wistar rats exhibited fewer DMH-induced colon tumors, and kefir reduced mammary gland tumor growth in hormone-dependent breast cancer models.

Evidence strength: Preclinical only. All anticancer findings to date are from cell-culture and animal studies. There are no completed human clinical trials establishing kefir as an anticancer agent.

5.7 Neurological Health and Cognitive Function

A systematic review conducted in January 2024 examined the effects of kefir in both in vivo animal models and human patients with neurodegenerative conditions. The review was based on studies retrieved from BVS, Embase, PubMed/MEDLINE, Scopus, and Web of Science databases. Seven studies were included, involving invertebrates, murine models, and human participants.

In animal models, the primary outcomes were antioxidant effects, reduced beta-amyloid deposition, and attenuation of vascular damage and neurodegeneration. In human studies, kefir supplementation resulted in decreased levels of inflammatory cytokines, reactive oxygen species (ROS), and oxidative proteins, and was associated with improvements in memory.

A randomized, double-blinded, and placebo-controlled clinical trial (n = 60) conducted by Agahi et al. (2018) did not find significant improvement in cognitive and biochemical markers in patients with severe Alzheimer's disease (AD) with probiotic supplementation, which suggests that there are other factors to consider before starting a supplementary approach. Also, the composition and homogeneity of kefir microorganisms vary significantly depending on the region of production, which presents a significant limitation when comparing results of different studies on its benefits.

Evidence strength: Preliminary and mixed. The majority of evidence is animal- or in-vitro-based. Human trial results are limited and inconsistent, particularly for severe neurodegenerative disease.

5.8 Water Kefir and Non-Dairy Substrates

Water kefir increased beneficial short-chain fatty acid production at the microbial level, reduced detrimental proteolytic fermentation compounds, and increased Bifidobacterium genus abundance. The observed benefits are enhanced by pasteurization. Non-dairy kefir derivatives were also linked to specific bioactivity including anti-inflammatory, antiulcerogenic, antihyperglycemic, and antihyperlipidemic potential in rodent models.

Evidence strength: Primarily in vitro and animal; human clinical data on water kefir are very limited.

6. Body Systems Associated with Kefir

• Gastrointestinal system: Microbiota modulation, reduced constipation and diarrhea, improved lactose digestion, reduced antibiotic-associated diarrhea, IBD symptom reduction.
• Cardiovascular system: Improvements in lipid profiles, blood pressure, homocysteine, and inflammatory markers in some but not all RCTs.
• Endocrine/metabolic system: Improved insulin sensitivity, reduced fasting glucose and HbA1c, favorable changes in body composition parameters in metabolic syndrome populations.
• Skeletal system: Calcium absorption enhancement, bone mineral density changes in pilot osteoporosis studies.
• Immune system: Immunomodulation via cytokine regulation, mucosal IgA stimulation, and microbiota-immune axis effects.
• Neurological system: Preliminary evidence of antioxidant effects, reduced neuroinflammation, and improved memory markers in early human studies.
• Integumentary system: A case-control study was designed to explore the impact of regular consumption of kefir on cutaneous water homeostasis in healthy participants.

7. Dosage Forms and Dosages Reported in Studies

Kefir is most often used by adults in doses of 400–500 mL by mouth daily for up to 4 weeks. Lower doses of 100–180 mL daily have been used for up to 12 weeks.

In the 6-month double-blind controlled osteoporosis study, the dose used was kefir-fermented milk (1,600 mg) supplemented with calcium bicarbonate (CaCO₃, 1,500 mg).

In an 8-week double-blind RCT in elderly individuals (n = 67), participants received one bottle of fortified kefir (240 cc) or regular kefir daily.

Clinical studies encompassed a variety of interventions in terms of kefir type, dosage, and duration. Currently, the lack of standardized microbial enumeration and detailed reporting of nutritional composition limits the comparability of products and hinders the establishment of dose–response relationships.

Overall, kefir consumption was associated with various beneficial effects; however, the observed outcomes were dose-dependent, duration-dependent, and the population profile was critical in determining clinical efficacy.

8. Safety Considerations and Interactions

Alcohol Content

As a fermented product, kefir naturally contains trace amounts of alcohol, typically between 0.5% and 2%, which may be a concern for those avoiding alcohol or with alcohol intolerance.

Gastrointestinal Side Effects

Initial consumption of kefir may cause gastrointestinal symptoms in some individuals. A significant decrease in gastrointestinal symptoms was observed in a kefir intervention study after the intervention period (P < 0.05), suggesting that initial discomfort may resolve with continued use.

Immunocompromised Individuals

Kefir contains actively growing bacteria and yeast. There is some concern that people with a weakened immune system might be more likely to develop infections from these bacteria or yeast. Although evidence suggests that probiotics are generally safe for people with autoimmune conditions, some case reports have associated probiotic use with serious adverse effects, including an elevated risk of infection. In rare situations, probiotics have been linked to complications such as sepsis, a potentially life-threatening response to infection.

Drug Interactions

• Disulfiram (Antabuse): Taking kefir along with disulfiram (Antabuse) can cause a pounding headache, vomiting, flushing, and other unpleasant reactions. Alcohol should not be consumed when taking disulfiram.
• Immunosuppressants: Kefir contains live bacteria and yeast. The immune system usually controls bacteria and yeast in the body to prevent infections. Medications that decrease the immune system can increase the chances of getting sick from bacteria and yeast. Specific immunosuppressants noted to interact include azathioprine, cyclosporine, tacrolimus, sirolimus, mycophenolate, and corticosteroids.

Pregnancy and Lactation

There is not enough reliable information about the safety of taking kefir during pregnancy or breastfeeding.

Variability Between Products

Industrial and artisanal kefir may differ regarding functional potential (OR of 8.56, 95% CI: 2.27–32.21, P ≤ .001), according to the observed health effect, which can be associated with differences in the microbial composition between both types of kefir.

Due to heterogeneity in kefir composition, dosage, and study populations, kefir should not be considered a therapeutic substitute for established medical treatments. Healthcare professionals should interpret the available evidence cautiously and consider individual patient characteristics, including baseline metabolic status, dietary habits, and tolerance to fermented dairy products.

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