Consumers and Health

Kefir: a functional food?

July 3, 2025

1357

Kefir, a traditional fermented dairy beverage, has garnered significant scientific interest as a functional food with potential therapeutic properties. This selective narrative review synthesises evidence from a targeted selection of clinical studies in humans to evaluate kefir’s nutritional composition, production methods, and health benefits.

Whilst numerous studies on kefir exist in the scientific literature, this review presents a focused analysis based on specific high-impact publications and recent systematic reviews, rather than attempting comprehensive coverage of all available evidence.

The fermented product, characterised by its complex microbial consortium of bacteria and yeasts, represents a promising dietary intervention for various health conditions affecting modern populations.

Methodology

This narrative review employed a selective approach to literature synthesis. The review process included:

incorporation of references specifically provided for inclusion;

targeted searches in web-based databases focusing on high-impact journals (prioritising Scopus CiteScore ≥ 8 where available);

emphasis on randomised controlled trials (RCTs) and systematic reviews published in English; and

inclusion of seminal studies establishing fundamental knowledge about kefir.

The search strategy was not systematic or exhaustive, focusing instead on identifying key studies that illustrate kefir’s primary health effects. Literature selection was guided by relevance to clinical outcomes in human populations, with animal studies referenced only to provide mechanistic context.

This approach, whilst not comprehensive, aimed to provide a coherent narrative of kefir’s therapeutic potential based on selected high-quality evidence.

History and local traditions of kefir

Kefir originates from the Caucasus Mountains, where it has been consumed for centuries as a traditional beverage with purported health-promoting properties. The word ‘kefir’, of North Caucasian origin, has become internationally recognised, reflecting the beverage’s global dissemination from its regional roots (Farag et al., 2020). Archaeological evidence suggests that fermented dairy products similar to kefir have been present in human diets for millennia, with recent findings in Bronze Age sites dating back 3,600 years. Traditional production methods involved the careful preservation and transmission of kefir grains through generations, with these microbial consortia considered precious cultural assets in their regions of origin.

In Eastern European and Central Asian cultures, kefir consumption extends beyond mere nutrition, forming part of daily dietary rituals. The beverage is traditionally consumed at various times throughout the day, often accompanying regional pastries and incorporated into cold soups. Local traditions emphasise the importance of maintaining viable kefir grains through regular feeding with fresh milk, creating a sustainable system of continuous fermentation that has persisted through centuries. These traditional practices have established foundational knowledge that continues to inform modern scientific understanding of kefir’s production and properties.

Production methods

Artisanal production

Traditional artisanal kefir production employs a straightforward yet precise methodology. Kefir grains, resembling small cauliflower florets ranging from white to creamy yellow in colour, are inoculated into pasteurised milk at ratios typically between 1-10% (w/v). The fermentation proceeds at ambient temperatures (20-25°C) for 18-24 hours, during which the complex symbiotic culture of bacteria and yeasts transforms the milk substrate (Bourrie et al., 2016). The grain-to-milk ratio significantly influences the final product’s characteristics, with lower ratios (1%) yielding a viscous, mildly acidic beverage, whilst higher ratios (10%) produce a more acidic, effervescent product with lower viscosity (Garrote et al., 1998).

The artisanal process requires careful handling of kefir grains, which increase their biomass by approximately 5-7% during each fermentation cycle. Following fermentation, the grains are separated through sieving and can be immediately reused or preserved in fresh milk under refrigeration for 1-7 days. This sustainable production method enables continuous kefir production whilst maintaining the viability and diversity of the microbial consortium. Agitation during fermentation influences microbial composition, favouring homofermentative lactococci and yeast development, whilst temperatures above 30°C stimulate thermophilic lactic acid bacteria growth at the expense of mesophilic species and yeasts (Rattray & O’Connell, 2011).

Industrial production

Industrial kefir production addresses scalability challenges through various methodological adaptations. The ‘Russian method’ enables larger-scale production through serial fermentation, utilising the percolate from initial grain fermentation as a mother culture for subsequent batches. Industrial processes typically involve milk standardisation to 8% dry matter, heat treatment at 90-95°C for 5-10 minutes, cooling to 18-24°C, and inoculation with 2-8% kefir cultures. The fermentation duration remains similar to artisanal methods (18-24 hours), followed by mechanical separation and packaging (Koroleva, 1988).

Modern industrial approaches increasingly utilise freeze-dried starter cultures or backslopping techniques to ensure product consistency and reproducibility. These methods eliminate the need for grain maintenance whilst providing greater control over fermentation parameters. Industrial producers often exclude certain microorganisms, particularly yeasts responsible for ethanol production, to meet regulatory requirements and consumer preferences. This selective approach, whilst ensuring product standardisation, may compromise some of the beneficial properties associated with traditional whole-grain fermentation. Recent innovations in industrial production focus on maintaining microbial diversity whilst achieving the consistency required for commercial distribution.

Nutritional properties

Macronutrients and micronutrients

Kefir’s nutritional profile reflects both its milk substrate and the transformative effects of fermentation. Typical kefir contains approximately 90% moisture, 3.0% protein, 0.7% ash, 0.2% lipid, and varying levels of residual carbohydrates, with exact composition dependent on milk type, fermentation conditions, and grain characteristics (Otles & Cagindi, 2003). The fermentation process enhances protein digestibility through proteolytic activity, generating bioactive peptides with potential health benefits. These peptides, resulting from microbial enzymatic action on milk proteins, may contribute to kefir’s reported antihypertensive and immunomodulatory effects.

The beverage provides significant quantities of B-complex vitamins, with fermentation potentially increasing bioavailability of certain micronutrients. Minerals like calcium, phosphorus, and magnesium remain largely retained from the milk substrate, while fermentation may enhance their absorption through the production of organic acids that improve mineral solubility. The microbial metabolism during fermentation also generates various bioactive compounds, including exopolysaccharides such as kefiran, organic acids (primarily lactic and acetic acid), and small quantities of ethanol and carbon dioxide, contributing to kefir’s distinctive nutritional and sensory properties.

Lactose content and evolution during fermentation

A critical nutritional transformation during kefir fermentation involves lactose metabolism. The diverse microbial consortium, including lactose-fermenting yeasts such as Kluyveromyces marxianus and various lactic acid bacteria possessing β-galactosidase activity, reduces lactose content by approximately 20-30% compared to the initial milk substrate (Rattray & O’Connell, 2011).

This enzymatic breakdown converts lactose into glucose and galactose, which are subsequently metabolised into lactic acid and other fermentation products.

The reduced lactose content, combined with the presence of active β-galactosidase enzymes and the delayed gastric emptying characteristic of fermented products, contributes to enhanced digestibility (Hertzler and Clancy, 2003). This makes kefir a viable dairy option for individuals who typically experience adverse symptoms from conventional milk products.

Health benefits from human studies

Influence on gut microbiota

Clinical investigations have revealed kefir’s significant impact on gut microbiota composition. Bellikci-Koyu et al. (2019) conducted a parallel-group randomised controlled trial in patients with metabolic syndrome, demonstrating that 12-week kefir consumption (180 mL/day) induced notable shifts in microbial populations. The intervention increased the relative abundance of Actinobacteria, whilst reducing potentially pathogenic Enterobacteriaceae. These taxonomic changes were accompanied by increased production of short-chain fatty acids, particularly butyrate, which serves as the primary energy source for colonocytes and supports intestinal barrier function.

Recent metagenomic analyses have provided deeper insights into kefir’s microbiota-modulating effects. The fermented beverage appears to enhance microbial diversity, a marker of gut health, through the introduction of viable microorganisms capable of surviving gastric transit. Studies indicate that kefir consumption can increase beneficial genera such as Lactobacillus and Bifidobacterium whilst competitively inhibiting potentially harmful bacteria through various mechanisms, including bacteriocin production and substrate competition. These microbial shifts may contribute to kefir’s broader health effects through modulation of the gut-brain axis and systemic inflammatory responses.

Support for nutritional balance

Clinical evidence suggests kefir consumption may support metabolic homeostasis through multiple mechanisms. A systematic review and meta-analysis by Ostadrahimi et al. (2021) evaluated kefir’s effects on glycaemic control, revealing significant improvements in fasting insulin levels and HOMA-IR indices. The fermented beverage’s impact on glucose metabolism appears mediated through several pathways, including modulation of intestinal microbiota, production of short-chain fatty acids, and potential effects on incretin hormone secretion.

Kefir’s influence on lipid metabolism remains an area of active investigation. Whilst some studies report improvements in lipid profiles, including increased apolipoprotein A1 concentrations and reduced LDL cholesterol in specific populations, results have been inconsistent. St-Onge et al. (2002) found no significant changes in serum cholesterol parameters in mildly hypercholesterolaemic men consuming kefir for four weeks, though faecal short-chain fatty acid production increased. These variable outcomes may reflect differences in study populations, kefir preparations, and baseline metabolic status, highlighting the need for personalised approaches to probiotic interventions.

Effects on intestinal permeability

Emerging evidence indicates kefir’s potential to modulate intestinal barrier function, a critical factor in preventing metabolic endotoxaemia and systemic inflammation. Peptides, bioactive compounds, and microbial metabolites present in kefir can influence tight junction proteins, thereby regulating intestinal permeability (Rosa et al., 2021). The production of short-chain fatty acids, particularly butyrate, supports epithelial cell energy metabolism and maintains the structural integrity of the intestinal barrier.

In vitro studies utilising intestinal epithelial cell models have demonstrated that kefir components can enhance transepithelial electrical resistance, a marker of barrier function. Water kefir preparations have shown similar effects, with both pasteurised and non-pasteurised products improving barrier integrity through mechanisms involving nuclear factor-κB modulation and cytokine regulation. These findings suggest that kefir’s beneficial effects extend beyond viable probiotics to include postbiotic compounds generated during fermentation, supporting potential therapeutic applications in conditions characterised by increased intestinal permeability.

Regulation of immune response

Clinical and mechanistic studies have elucidated kefir’s immunomodulatory properties. The fermented beverage appears to shift immune responses from pro-inflammatory Th1 profiles towards anti-inflammatory Th2 responses, whilst increasing secretory IgA production. Bellikci-Koyu et al. (2019) reported significant reductions in inflammatory markers TNF-α and IFN-γ following 12-week kefir consumption in patients with metabolic syndrome, suggesting potential benefits in chronic inflammatory conditions.

Kefir’s immune effects may be mediated through multiple mechanisms, including direct interaction of microbial components with intestinal immune cells, production of immunomodulatory metabolites, and indirect effects via gut microbiota modulation. Specific lactic acid bacteria strains isolated from kefir have demonstrated the ability to induce regulatory T-cell differentiation and IL-10 production, potentially contributing to immune tolerance and anti-inflammatory effects. These properties position kefir as a potential dietary intervention for immune-related disorders, though further clinical trials are needed to establish therapeutic protocols.

Major clinical outcomes and mechanisms

Systematic reviews of randomised controlled trials have identified several areas where kefir demonstrates clinical efficacy. Kairey et al. (2023) synthesised evidence from 16 studies, highlighting kefir’s potential as a complementary therapy in reducing oral Streptococcus mutans and supporting Helicobacter pylori eradication. The fermented beverage may also aid in managing adult dyslipidaemia and hypertension, though evidence quality varies across outcomes. A notable clinical trial examining kefir’s effects on intestinal permeability in female football players demonstrated improvements in gut barrier function and athletic performance markers (Öneş et al., 2025).

The mechanisms underlying kefir’s health benefits appear multifaceted, involving direct microbial effects, bioactive peptide actions, and modulation of host physiology. Antimicrobial compounds produced during fermentation, including organic acids, bacteriocins, and hydrogen peroxide, contribute to pathogen inhibition. Simultaneously, exopolysaccharides like kefiran may exert prebiotic effects, selectively promoting beneficial bacteria growth. The synergistic action of these components, rather than single isolated factors, likely accounts for kefir’s therapeutic potential, supporting the concept of the fermented beverage as a complex functional food rather than a simple probiotic delivery vehicle.

Consumer recommendations

For optimal health benefits, consumers should prioritise products containing ‘live active cultures‘, with plain varieties preferred over flavoured options to minimise added sugar intake (EFSA, 2022). Daily consumption of 150-200 mL appears sufficient to confer health benefits based on clinical trial protocols, though individual responses may vary.

Lactose tolerance considerations

Lactose-intolerant individuals can generally tolerate kefir well, supported by substantial clinical evidence. The landmark study by Hertzler and Clancy (2003) demonstrated that kefir consumption in adults with clinically diagnosed lactose maldigestion resulted in significantly reduced breath hydrogen excretion compared to milk (87±37 ppm·h versus 224±39 ppm·h, p<0.001), indicating improved lactose digestion. Participants reported that both plain and flavoured kefir reduced flatulence severity by 54-71% compared to milk consumption, with negligible abdominal pain or diarrhoea symptoms across all treatments.

The mechanisms underlying kefir’s enhanced tolerability amongst lactose-intolerant populations are multifaceted. Beyond the 20-30% reduction in lactose content during fermentation, viable β-galactosidase-producing bacteria in kefir continue to aid lactose digestion within the intestinal tract. As Hertzler noted, ‘some of the bacterial cells give up their lives in the intestinal tract, release their enzymes and digest the lactose‘, providing additional enzymatic activity beyond that present in the beverage itself. Furthermore, kefir’s diverse microbial consortium, including Kluyveromyces marxianus and various lactobacilli, may colonise the intestines temporarily, offering prolonged lactose-digesting benefits compared to conventional yogurt. The delayed gastric emptying characteristic of fermented products also contributes to improved tolerance by allowing more time for enzymatic lactose breakdown.

Product selection and labelling considerations

Identifying products with genuine ‘live active cultures’ presents challenges, particularly within the European regulatory framework. Unlike regions such as the United States, where manufacturers can freely use terms like ‘contains live and active cultures’ (requiring a minimum of 10⁸ CFU/g at manufacture), the European Union maintains strict restrictions on probiotic-related terminology. The European Ombudsman has recently upheld the European Commission’s position — still devoid of binding legal effect (Dongo, 2025) — that the mere use of the term ‘probiotic’ constitutes a health claim requiring prior authorisation under Regulation (EC) No 1924/2006. See:

The only EU-authorised health claim related to live cultures applies specifically to yogurt, stating that ‘live cultures in yogurt improve lactose digestion of the product in individuals who have difficulty digesting lactose‘, applicable only when products contain a minimum of 10⁸ CFU/g of Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus. Consequently, European kefir products typically cannot display ‘probiotic’ or similar terminology, instead listing specific bacterial strains in the ingredients list using Latin nomenclature. This regulatory landscape creates consumer confusion, as products containing well-researched probiotic strains cannot communicate their potential benefits, while products with limited scientific backing appear identical on shelves.

Nonetheless, several EU Member States — including Italy, Spain, and Denmark — have proactively introduced national guidelines (Dongo, 2023) that permit the use of the term ‘probiotic’ on food supplement labels, provided specific criteria are met, such as a minimum content of 10⁷ to 10⁹ viable cells per daily dose. This fragmented approach, however, remains unharmonised at the EU level and is generally limited to food supplements, excluding fermented foods such as kefir. Consumers seeking kefir with viable cultures should examine ingredient lists for specific bacterial strains, check for refrigeration requirements (indicating live cultures), and preferably select products from manufacturers providing CFU counts through third-party channels such as websites or healthcare professionals.

Home production using authentic kefir grains offers advantages in terms of microbial diversity and bioactive compound content, though it requires careful attention to hygiene and fermentation conditions. Commercial products, whilst potentially less diverse microbiologically, provide consistency and convenience. Individuals new to kefir consumption should begin with smaller portions (50-100 mL) to allow digestive adaptation, potentially minimising initial gastrointestinal symptoms such as bloating or gas.

Outlook for innovation in kefir-based foods and beverages

The future of kefir innovation encompasses both traditional dairy applications and novel plant-based alternatives. Research into water kefir, utilising sugar solutions with fruit components, has revealed distinct microbial communities capable of fermenting non-dairy substrates whilst maintaining probiotic properties. These developments address growing consumer demand for lactose-free, vegan-friendly fermented beverages. Industrial applications are exploring kefir-derived ingredients, including isolated strains, bioactive peptides, and exopolysaccharides, for incorporation into functional foods beyond traditional beverages.

Technological advances in microbiome analysis and metabolomics are facilitating deeper understanding of kefir’s complex ecosystem, enabling targeted strain selection and fermentation optimisation. Future innovations may include personalised kefir formulations based on individual microbiome profiles, enhanced delivery systems for bioactive compounds, and novel applications in areas such as sports nutrition and cognitive health (Dongo, Della Penna, 2025). The integration of traditional fermentation wisdom with modern biotechnology promises to expand kefir’s therapeutic potential whilst maintaining its status as a natural, minimally processed functional food.

Limitations

This narrative review has several important limitations that should be acknowledged. First, the selective nature of the literature search means that relevant studies may have been overlooked, particularly those published in languages other than English, in lower-impact journals, or in regional databases not accessed during the search process. The review lacks the systematic methodology of a true systematic review, including pre-specified inclusion/exclusion criteria, comprehensive database searching, and formal quality assessment of included studies.

Second, the heterogeneity in kefir preparations across studies presents a significant challenge for synthesis. Studies utilised different kefir grains, fermentation conditions, dosages, and intervention durations, making direct comparisons difficult. The microbial composition of kefir varies considerably based on geographical origin, production methods, and substrate used, yet many studies provide insufficient characterisation of their specific kefir products.

Third, many of the included clinical trials had small sample sizes and short intervention periods, limiting the generalisability of findings. The review identified that 12 of 16 studies in the Kairey et al. (2023) systematic review had a high risk of bias, indicating quality concerns in the primary literature. Additionally, publication bias may favour positive results, whilst negative or null findings remain unpublished.

Finally, this review cannot claim to represent the entire body of evidence on kefir’s health effects. Recent studies published after the search period, ongoing clinical trials, and research in emerging areas such as mental health, cognitive function, and sports performance may provide additional insights not captured here. The focus on human clinical trials also excludes valuable mechanistic insights from well-designed animal studies that could inform the understanding of kefir’s biological effects.

Interim conclusions

This selective narrative review of clinical studies suggests that kefir possesses potential as a functional food with multiple health benefits. The evidence examined indicates effects on gut microbiota, intestinal barrier function, immune regulation, and metabolic parameters that support its inclusion in dietary strategies for health promotion. However, the selective nature of this review and the limitations identified in the primary literature necessitate caution in making definitive claims. Whilst the studies reviewed provide encouraging results, particularly for lactose tolerance and certain metabolic outcomes, further large-scale, well-designed clinical trials are essential to establish definitive therapeutic protocols and clarify conflicting findings. The heterogeneity in kefir preparations and study methodologies highlights the need for standardisation in future research. Despite these limitations, the convergence of traditional knowledge and modern scientific validation, as represented in the selected studies, positions kefir as a promising example of how fermented foods may contribute to human health in an era of increasing chronic disease prevalence. Future systematic reviews employing rigorous methodology will be crucial for comprehensively evaluating kefir’s therapeutic potential.

Dario Dongo

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