Kefir Consumption and Health Effects Based on Human Clinical Trials: An Overview of Literature

TL;DR

This paper reviews clinical studies on kefir, a fermented milk drink, and its potential health benefits, but notes that evidence is limited due to inconsistent study designs.

Contribution

The paper provides a systematic overview of human clinical trials on kefir, highlighting the need for standardized research to confirm its health effects.

Findings

Kefir may benefit gut microbiota, metabolism, and immune function.

Study results are limited by variability in kefir composition and trial design.

More standardized clinical trials are needed to confirm kefir's health effects.

Abstract

Kefir is a traditional fermented milk beverage characterized by a complex community of lactic acid bacteria, acetic acid bacteria and yeasts that contributes to its unique sensory and nutritional properties. Regular consumption of kefir has been associated with a wide range of potential health benefits. This review aimed to evaluate the available clinical evidence on kefir consumption and its impact on human health. A literature search of the databases PubMed, Web of Science, and Scopus was conducted up to 30 August 2025. Eligible studies were human clinical trials investigating kefir as a fermented milk beverage without the addition of defined probiotic strains, prebiotics, or synbiotics. A total of 28 clinical studies were identified and included diverse study designs, such as crossover trials, parallel-group randomized controlled trials, multi-arm trials, early-phase exploratory studies, and pilot studies. Kefir intake showed potential benefits for gut microbiota modulation, metabolic parameters, inflammatory markers, immune function, and gastrointestinal health. However, interpretation of these findings is limited due to substantial heterogeneity in kefir preparation, microbial composition, dosage, intervention duration, study populations, and outcome measures. Consequently, although kefir may offer multiple health benefits, the overall strength of evidence remains limited. Larger, well-designed clinical trials with standardized kefir interventions are needed to better define kefir’s efficacy in specific populations and health conditions.

Full text

1. Introduction

Kefir is a traditional fermented milk beverage that originates from the Caucasus region and Eastern Europe, and was used in ancient times. The origins of kefir predate written records and has been consumed for centuries as part of the daily diet and valued for its perceived health-promoting properties. Owing to its long history of consumption, kefir has attracted increasing scientific interest as a functional food, prompting clinical investigations into its potential health effects [1,2].

Kefir is traditionally produced by fermenting milk with kefir grains, which consist of a complex microbial community of lactic acid bacteria (including Lactobacillus kefiranofaciens, Lactobacillus helveticus, Lentilactobacillus kefiri, Lentilactobacillus parakefiri, Limosilactobacillus fermentum, Levilactobacillus brevis, Lacticaseibacillus paracasei, Lactiplantibacillus plantarum, Lactococcus lactis), acetic acid bacteria (including Acetobacter spp.), and yeasts (including Kluyveromyces marxianus, Kluyveromyces lactis var. lactis, Debaryomyces hansenii, Dekkera anomala, Saccharomyces cerevisiae, Torulaspora delbrueckii, Pichia fermentans) [3,4,5,6,7,8].

The relative abundance and species composition of these microorganisms vary depending on the geographic origin of the grains, fermentation conditions, and substrate. Within the kefir grains, microorganisms are embedded in a polysaccharide matrix known as kefiran, which supports microbial stability and symbiosis. Through their metabolic activity, kefir microorganisms produce a wide array of bioactive metabolites, including organic acids (such as lactic and acetic acids), ethanol, carbon dioxide, bioactive peptides, exopolysaccharides, vitamins, and volatile compounds, which collectively contribute to kefir’s sensory properties and its potential health effects [3,4,5,6]. This unique microbial composition gives kefir a distinctive microbial richness and metabolic activity that differentiate it from other fermented dairy products. The fermentation process not only enhances the beverage’s digestibility and nutrient bioavailability but also generates a variety of bioactive compounds and peptides with potential health-promoting properties [6,9]. In addition to traditional grain-fermented kefir, commercially available kefir products are often produced using defined starter cultures designed to replicate the microbial profile of kefir grains, although they may differ in microbial diversity and metabolite production [7,10,11,12]. The term “water kefir” is used for a beverage produced with water kefir grains in a sugar-based solution. However, milk kefir and water kefir are distinct fermentation systems, characterized by different microbial consortia, substrates, and metabolite profiles [13,14]. Due to these fundamental differences, they should not be considered interchangeable when interpreting health-related findings.

According to the International Scientific Association for Probiotics and Prebiotics (ISAPP), kefir—as a fermented milk beverage—does not meet the definition of a probiotic [15,16]. Marco et al. [16] emphasize that fermented foods and probiotics are related but distinct categories. Fermented foods are defined as foods made through desired microbial growth and enzymatic transformations, and their primary purpose is to improve shelf life, safety, sensory properties, or nutritional value. Importantly, while these foods contain live microorganisms at the time of consumption, the presence of live microbes alone is not sufficient to qualify them as probiotics. To meet the definition of a probiotic, microorganisms must be well-defined at the strain level, demonstrate proven health benefits in controlled clinical studies, and be present in the product at an effective dose at the time of consumption [15,17].

Fermented foods generally do not meet these criteria because their microbial composition is highly variable, often not characterized at the strain level, and typically not standardized across batches or producers. In addition, most fermented foods have not undergone strain-specific clinical trials required to substantiate a probiotic health claim. For these reasons, Marco et al. [16] clearly distinguish fermented foods from probiotics, even though some microbes present in fermented foods may individually qualify as probiotics when isolated, identified, and clinically validated. However, some microbial strains isolated from kefir, such as Lentilactobacillus kefiri LKF01, Lactococcus lactis subsp. cremoris YRC3780, Bifidobacterium longum BL986, Limosilactobacillus fermentum LF26, Lactobacillus helveticus LH43, Streptococcus thermophilus ST30 and others, have shown health benefits in clinical studies [18,19,20,21,22]. While these studies showed beneficial effects, these benefits cannot be extrapolated to all kefir beverages, since the microbial composition varies considerably among products, the concentrations of specific microorganisms differ widely, and not all kefir preparations contain the specific strains that are effective.

Regular consumption of kefir has been associated with a wide spectrum of benefits, including improved gut health, modulation of intestinal microbiota, enhanced immune function, and protective effects against metabolic, cardiovascular, and gastrointestinal disorders [23,24]. Kefir also exhibits antimicrobial, anti-inflammatory, antioxidant, and anticarcinogenic activities, and is therefore an important part of nutritional science and functional food research [25,26]. While many of these effects have been demonstrated in animal and in vitro studies, clinical studies continue to explore kefir’s role in human health and disease prevention [27,28,29]. Kefir is also associated with high acceptance, particularly regarding its health benefits [30,31].

While prior reviews have summarized evidence from randomized controlled trials, most have focused on general outcomes in healthy adults and specific populations such as athletes, patients with metabolic syndrome, inflammatory bowel disease, osteoporosis, fibromyalgia, functional constipation, and periodontal disease [27,28,29,32,33,34]. These reviews report limited and inconsistent efficacy data, often highlighting a high risk of bias, small sample sizes and a lack of population-specific analyses. Importantly, meaningful conclusions are further impaired by substantial heterogeneity in kefir interventions, including differences in preparation methods, microbial composition, and—critically—dosage and duration of consumption. The reviewed studies included a wide range of kefir products, from traditional kefir, produced using kefir grains (known as pitcher-fermented kefir), to commercially manufactured or probiotic-enriched formulations in which defined probiotic strains were added before or after fermentation. These products were administered in widely varying amounts, making direct comparison challenging. Outcomes assessed also vary considerably, from gut microbiota composition and inflammatory markers to metabolic, cognitive, and appetite-related parameters. Several recently published randomized controlled trials have not yet been incorporated into these existing reviews. Although many in vitro and animal studies have reported beneficial effects of kefir, these findings cannot be directly extrapolated to humans. Since dietary recommendations and health-related conclusions should primarily rely on human data, this review focuses exclusively on clinical trials in humans to provide a more clinically meaningful evaluation of kefir consumption.

To address these gaps, the present review adopts a population-, intervention-, and outcome-specific approach, aiming to provide a more nuanced and critical synthesis of the evidence, while explicitly acknowledging the limitations imposed by the lack of standardized kefir dosing and intervention protocols.

2. Search Strategy and Study Selection

A literature review was performed to assess the health effects of milk-based kefir consumption. Electronic databases (PubMed, Web of Science, and Scopus) were searched using the search strategy listed in Table 1. Google Scholar was used as a bibliographic search engine to identify additional relevant studies. Further studies were identified by screening the reference lists of relevant articles.

Studies were included if they investigated commercial or industrially produced kefir as a fermented milk product obtained through microbial fermentation without added probiotic strains and additional lyophilisation, and reported human health outcomes that were published in peer-reviewed English-language journals.

Multi-arm trials were included when one study arm investigated kefir consumption as a fermented milk product, even if additional comparator arms included probiotic supplements or other interventions; only outcomes related to kefir consumption were considered in the qualitative synthesis.

3. Results

3.1. Overview of the Included Clinical Trials on Kefir Consumption

Based on the predefined inclusion criteria, a total of 28 studies evaluating the health effects of kefir consumption were identified [35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62]. Studies were conducted primarily in Turkey (13), Iran (4), and the United States (6), with individual studies from Slovenia, Brazil, Ireland, Taiwan, and Canada.

These studies included diverse populations, ranging from healthy adults to individuals with specific metabolic, gastrointestinal, or immunological conditions, and encompassed a variety of interventions in terms of kefir type, dosage, and duration.

Data extracted for each study included author(s), year, study design, population, kefir dosage and duration, outcomes, and main findings. The study designs (e.g., randomized controlled trials, double-blind, single-blind, and open-label studies) were named according to standard clinical research methodology [63,64]. Parallel-group randomized controlled trials allocate participants to either an intervention or a control group for the entire study period, allowing direct comparison between groups. In crossover trials, participants received both the intervention and the control in sequential periods separated by a washout phase, which reduces inter-individual variability but may be influenced by potential carryover effects. Open-label and single-blind designs carry a higher risk of expectation bias compared to double-blind trials. These methodological differences should be considered when interpreting and comparing outcomes across studies.

For clarity and ease of comparison, the studies are presented in Table 2 in descending chronological order and, within each year, alphabetically by the first author. Table 2 summarizes key study characteristics, including design, population, kefir intervention details, outcomes measured, and main findings, allowing readers to readily assess patterns, consistencies, and variability across the evidence base. Due to the distinctive sensory properties of kefir, true double-blinding was not feasible and is noted as a limitation of the studies.

The clinical studies investigating kefir consumption used diverse study designs. Eight studies were crossover trials [40,41,43,45,52,54,57,61,62], while eleven were two-arm parallel-group randomized controlled trials (RCTs) [35,37,42,44,46,47,56,58,59,60,66]. Several studies used multi-arm designs in which kefir was compared with probiotic supplements, unfermented dairy products, or standard care, allowing differentiation between the effects of kefir as a complex fermented milk beverage matrix and those of defined probiotic formulations. Six publications described three-arm parallel-group RCTs [39,48,49,50,51,57]. One four-arm study [53] and two single-arm studies [38,55] were also included. Four publications originated from two clinical trials [42,46,49,51].

These studies examined a broad spectrum of health outcomes. Metabolic parameters—such as lipid profile, glucose, insulin, blood pressure, body composition, metabolic syndrome, and NAFLD—were among the most frequently investigated parameters and showed that kefir intake was associated with improved metabolic and cardiovascular parameters, including HDL-C, LDL-C, ApoA1, triglycerides, glycemic index, blood pressure, fat-free mass, and reduced cardiovascular risk, with effects often dependent on microbial composition and specific strains [35,40,42,43,44,46,49,51,52,62]. Several studies focused on gut microbiota composition and gastrointestinal function, assessing microbial diversity, short-chain fatty acid production, constipation, diarrhea, antibiotic-associated diarrhea, IBD symptoms, and lactose digestion and 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 tolerance [37,38,47,55,59,61]. Other investigations addressed immune and inflammatory markers and found that kefir consumption was associated with reductions in pro-inflammatory cytokines, CRP, TNF-α, and IL-6, improvements in periodontal health, and decreased side effects of Helicobacter pylori therapy [39,40,42,45,46,56,57,58,60]. Additional outcomes included cognitive and mood-related measures such as memory, mood, and stress regulation. Studies reported that kefir supplementation improved hippocampal-dependent memory and mood regulation [41,45]. Bone metabolism parameters (bone mineral density, serum calcium, parathyroid hormone, β-CTX) were also examined, and several investigations found beneficial effects on these markers [66]. Oral health parameters were also investigated, with kefir consumption leading to reduced Streptococcus mutans and Lactobacillus counts, and showing effects comparable to sodium fluoride rinses in tooth caries prevention [48,54,57]. Athletic performance was assessed, and it was found that kefir intake improved endurance, VO_2_ max, finishing speed, post-exercise recovery and reduced inflammation [37,50,53].

3.2. Details of Kefir Products Used in the Included Clinical Trials

Kefir is a fermented dairy product produced either with traditional kefir grains or defined commercial starter cultures. This definition is in line with international standards, including the Turkish Food Codex Fermented Dairy Products Directive (No: 2009/25) and the Codex Standard for Fermented Milks (CODEX STAN 243-2003) [67], which recognize kefir as a product fermented by lactic acid bacteria and yeasts derived from kefir grains [47,68]. Traditional grains contain a complex symbiotic community of bacteria and yeasts that produce a diverse range of bioactive metabolites and contribute to the characteristic flavor, texture, and potential health effects. Commercial starter cultures are designed to mimic these microbes for standardized, consistent production, but generally have lower microbial diversity [7,10,11,12]. This distinction is important when interpreting clinical trials, as differences in production can influence kefir’s functional properties. Table 3 summarizes the kefir products used in the 28 clinical trials included in this review, noting the production method for the kefir beverage (grains vs. starter cultures), microbial composition, and key characteristics. In some trials, information was not reported and is indicated as “details not reported.”

Table 3 presents the differences in kefir production that may have influenced study outcomes and functional properties. Across the 28 included clinical studies, kefir was produced using traditional grains in 8 studies [38,44,45,53,54,55,58,60] and commercial starter cultures in 12 studies [35,36,38,40,41,42,46,48,49,50,51,57]. Where Gupta et al. 2024 [38] reported the use of both kefir grains and starter culture, perhaps meaning an exopolysaccharide matrix with kefir microbiota. In 9 studies, the production method was not reported [37,39,43,47,52,56,59,61,62]. Most commercial kefirs provided 100–140 kcal per 237–250 mL serving, with larger servings or phase-specific formulations reaching 334–1000 kcal. Macronutrient content varied, with fat ranging from 2 to 7.3 g/250 mL, carbohydrates 4–15 g/250 mL, and protein 3–9 g/250 mL; milk-based controls generally had slightly higher protein. Calcium content was reported in a few studies, around 0.3 g/250 mL.

Lactic acid bacteria—including Lactobacillus, Lacticaseibacillus, Lactococcus, Leuconostoc, and Streptococcus genera—were consistently present across studies that reported microbial composition, while Bifidobacteria were reported in only three studies [35,40,41]. Yeast genera such as Saccharomyces, Kluyveromyces, Kazachstania, Rhodosporidium, and Pichia were reported in studies that provided a more comprehensive characterization of the microbial composition of kefir grains or starter cultures, rather than limiting analyses to lactobacilli [36,40,41,42,45,46,54,55,57,59,61]. Total microbial counts of kefir microbiotas mainly ranged between 10^8^ and 10^10^ cfu/mL. Several studies did not report microbial composition [37,38,39,44,49,50,51,52,56,58,60,62].

4. Discussion

In the review, 28 clinical trials were analyzed [35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62] that investigated the effects of kefir consumption on a broad range of health outcomes in humans. 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, and focused on the influence of kefir on gastrointestinal health, metabolic, cardiovascular health, oral health outcome as well as 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 the clinical efficacy.

4.1. Effects of Kefir Consumption on Gastrointestinal Health via Gut Microbiome Modulation

This section synthesizes findings from the clinical trials included in Table 2 and Table 3 of this review that specifically investigated the effects of kefir consumption on gastrointestinal health and gut microbiome modulation. Across these studies, kefir intake was associated with broadly consistent outcomes, including reductions in gastrointestinal symptoms [47,55,58], improved lactose tolerance [61], alleviation of constipation-related complaints [55] and modulation of the gut microbiota. Reported microbial changes included increased abundances of beneficial taxa such as Lactobacillus, Bifidobacterium adolescentis, Akkermansia muciniphila, and Faecalibacterium prausnitzii, which are commonly linked to anti-inflammatory effects and improved metabolic parameters [37,38,47]. The observed increases in taxa such as Lactobacillus, Bifidobacterium, Akkermansia muciniphila, and Faecalibacterium prausnitzii are important, as these microorganisms are well-known producers of short-chain fatty acids (SCFAs), particularly acetate and butyrate, which contribute to epithelial barrier integrity and immunomodulation. In the included clinical trials, improvements in gastrointestinal symptoms were also noted along with microbial shifts [37,47,55] and, in some studies, reductions in inflammatory markers such as CRP and TNF-α were observed [42,46]. Although SCFA concentrations were not measured in all clinical trials, the increase in SCFA-producing taxa and reduction in inflammatory parameters indicate that microbiota modulation may partly explain the beneficial metabolic and anti-inflammatory effects of kefir observed in these studies.

These findings are supported by previous reviews [10,70,71,72,73] which describe kefir as a complex fermented beverage containing a diverse consortium of lactic acid bacteria, acetic acid bacteria and yeasts, and a wide range of microbial metabolites and bioactive compounds, including exopolysaccharides, bioactive peptides, and organic acids; these collectively contribute to kefir’s capacity to modulate the intestinal microbiota, exert antimicrobial effects, enhance microbial diversity, and support the growth of beneficial taxa, thereby plausibly underlying the gastrointestinal benefits observed in the included clinical trials.

However, not all studies demonstrated consistent microbiota changes. For example, some trials reported no significant differences in overall microbial diversity, gut health or inflammatory markers following kefir consumption [38,59,60]. These findings suggest that the effects of kefir on gut microbiota may depend on baseline microbial composition, population characteristics, and intervention duration.

Despite these promising associations, it remains challenging to draw firm conclusions about kefir’s effects on a “healthy” human microbiome. The concept of a healthy microbiome is not universally defined, as individual gut microbiota compositions are highly personalized. However, certain core taxa are shared across humans [74,75,76]. Recently, the International Scientific Association for Probiotics and Prebiotics (ISAPP) published a new consensus document stating that gut health is defined as “a state of normal gastrointestinal function without active gastrointestinal disease and gut-related symptoms that affect quality of life” [77]. Another problem is high heterogeneity in study design, including kefir production, kefir composition and dosage, intervention duration, and participant characteristics, which further complicates interpretation. Although kefir consumption appears to support microbiota modulation and related metabolic and inflammatory outcomes in humans, further standardized, well-characterized clinical studies are required to clarify its precise effects and to improve frameworks for defining microbiome health [10,70].

Most studies assessing gut microbiota relied on 16S rRNA gene sequencing or culture-based approaches, as reflected by reporting of alpha-diversity indices such as Shannon and Chao1 [37,38,47]. Few studies incorporated functional analyses, and none of the reviewed trials applied comprehensive shotgun metagenomic or untargeted metabolomic approaches. This methodological variability may contribute to differences in reported findings and limits direct comparison of functional outcomes across studies.

Future research concerning kefir consumption, gastrointestinal health and intestinal microbiota modulation would benefit from the inclusion of shotgun metagenomic and metabolomic approaches [78,79], which could provide deeper insight into functional changes in the gut microbiome as well as the microbial metabolic activity, rather than relying solely on taxonomic shifts [80]. Such approaches would allow assessment of microbial pathways, metabolite production, and host–microbe interactions that may underlie the observed clinical effects of kefir. However, the application of these advanced techniques is currently limited by high costs, analytical complexity, and the need for specialized infrastructure [81,82], which may partly explain their limited use in clinical nutrition studies.

4.2. Effects of Kefir Consumption on Metabolic and Cardiovascular Health

Several RCTs included in this review [35,40,42,45,46,49,51,62] investigated the effects of kefir consumption on metabolic and cardiovascular health and found that kefir consumption is associated with beneficial effects on metabolic and cardiovascular health, particularly in individuals with overweight, metabolic syndrome, or non-alcoholic fatty liver disease (NAFLD). Kefir intake was linked to improvements in lipid profiles, reductions in blood pressure, and enhanced insulin sensitivity. 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.

These findings are consistent with evidence from systematic reviews and meta-analyses, which report that kefir consumption can improve glycemic control, reduce insulin resistance, and lower inflammatory markers such as TNF-α and IL-6 across diverse adult populations [28,33,71]. These reviews also suggested that kefir is generally safe for consumption in healthy individuals and may confer cardiometabolic benefits; however, effects on lipid profiles, gut microbiota composition, and metabolic outcomes remain inconsistent across studies, highlighting the need for further well-designed human trials [7,32,70,83].

On the other hand, several studies reported no significant improvements in lipid profiles, glycemic parameters, or anthropometric outcomes compared with control groups [35,42,62]. These inconsistencies highlight the heterogeneity of study designs, kefir composition, and participant characteristics.

Previously published animal model studies have also demonstrated that traditional kefir has the potential to improve metabolic dysfunction associated with obesity. Traditional kefir was found to reduce body weight gain, plasma cholesterol levels, and hepatic triglycerides in mice. In contrast, commercial kefir showed no beneficial effect [84]. Additionally, kefir-derived exopolysaccharides have shown promising effects in experimental rats’ models of type 2 diabetes mellitus, suggesting potential nutraceutical applications [85].

Several other preclinical studies in animal models have shown beneficial effects of kefir consumption, showing reductions in oxidative stress, inflammation, and improvements in metabolic and cardiovascular parameters. In aged, stressed, hypertensive, NASH, and myocardial infarction animal models, kefir restored antioxidant enzyme activities, reduced lipid peroxidation, decreased pro-inflammatory cytokines (IL-6, TNF-α, TGF-β1), and limited fibrosis in liver, kidney, and heart tissues [86,87,88,89,90]. Systematic reviews of animal model studies also confirm that kefir consistently modulates immune and oxidative pathways, supporting its classification as a functional food with multi-system benefits for metabolic health promotion [73,91,92]. Although animal studies cannot be directly extrapolated to humans, their concordance with the clinical evidence reviewed here supports the conclusion that kefir consumption exerts beneficial effects on metabolic and cardiovascular health, while highlighting the need for further standardized human studies.

4.3. Other Effects of Kefir Consumption on Health

Among the RCTs included in this review, kefir consumption was associated with additional health benefits beyond gastrointestinal and cardiometabolic outcomes. One study [37] found that professional athletes consuming kefir showed an increase in Akkermansia muciniphila and Faecalibacterium prausnitzii, microbial taxa linked to improved energy metabolism and anti-inflammatory effects. Another study [53] found reduced inflammation after endurance training after kefir intake. Several studies [39,48,54,57] investigated oral health benefits and found reductions in Streptococcus mutans counts and improved periodontal indices. Additionally, one study [61] showed that kefir improved lactose digestion, likely mediated by β-galactosidase activity. These effects are likely driven by the live microbiota of kefir, particularly Lactobacillus spp. and yeasts, which can temporarily colonize the oral cavity, compete with pathogenic bacteria, and support lactose hydrolysis.

On the other hand, kefir did not demonstrate clear benefits in all clinical contexts. For example, some studies investigating chemotherapy-related outcomes or antibiotic-associated diarrhea reported no significant advantage compared with control interventions [58,59,60].

These findings demonstrate that the functional effects of kefir extend beyond gut microbiota modulation to include oral health, lactose digestion, and microbial contributions to systemic metabolic and anti-inflammatory outcomes. Consistent with these findings, other studies on fermented dairy products containing live beneficial microbes have also demonstrated reductions in oral pathogen load, improvements in dental health, and enhanced lactose tolerance in both adults and children [93,94,95,96].

4.4. Population-Specific Observations Due to Kefir Consumption

The studies included in this review cover a broad range of populations, which inevitably affects how their findings can be interpreted. Some trials were focused on individuals with metabolic conditions, such as non-alcoholic fatty liver disease, metabolic syndrome, elevated body mass index and dyslipidemia [35,40,42,44,45,46,49,51,52,62]. Other studies involved more clinically complex groups, for example, patients with Crohn’s disease or other inflammatory bowel diseases [47] and other various gastrointestinal diseases [55,56,58,59,61] or even critically ill patients [38,60]. At the same time, some interventions were tested on dental patients [39,48,57] and physically active or athletic cohorts [37,50,53].

These populations not only differ considerably in their metabolic status, inflammatory load, diet and physical activity, but also in gut microbiota composition [97,98,99]. It is therefore not surprising that the magnitude, and sometimes even the direction of the effects vary between studies. Rather than indicating inconsistency, this likely reflects population-specific mechanisms. The diversity of the included groups shows how widely kefir is being studied and highlights the need for caution when comparing results across such distinct settings [28,92]. Despite the heterogeneity of the study populations included in this review, meaningful improvements in metabolic, inflammatory, gastrointestinal, or microbiota-related outcomes were found following kefir consumption. This suggests that the potential benefits of kefir intake are not limited to a single clinical context.

4.5. Terminology and Mislabelling

A considerable number of authors have used terminology such as “probiotic kefir,” “probiotic microorganisms in kefir,” or “probiotic fermented milk” inappropriately [22,35,37,39,41,42,43,44,45,46,47,49,50,51,53,54,55,56,57,59,60,62,66,100,101]. According to the International Scientific Association for Probiotics and Prebiotics (ISAPP), kefir is classified as a fermented food and not as a probiotic product unless it contains well-defined, characterized strains at demonstrated health-benefit doses [15,16].

Although kefir consumption clearly leads to a broad range of clinically demonstrated health effects, researchers should avoid labeling kefir itself as “probiotic” or implying probiotic health claims without strain-specific evidence. Kefir grains contain a diverse and variable consortium of bacteria and yeasts, and the microbial composition differs substantially from standardized probiotic formulations. Therefore, the term “probiotic kefir” is scientifically inaccurate and potentially misleading. A similar discussion has been raised for other fermented foods, such as kimchi, where the question of whether it deserves probiotic status has been debated in the literature [102]. However, according to the International Scientific Association for Probiotics and Prebiotics (ISAPP), fermented foods should not be classified as probiotics unless they contain well-defined, strain-characterized microorganisms shown to confer a health benefit at an effective dose. In line with this position, kefir is more appropriately described as a fermented milk beverage that may exert probiotic-like effects rather than as a probiotic product.

On the other hand, several other studies have examined kefir fortified with additional probiotic strains, and such products may indeed confer health benefits; [22,36,66,101,103,104,105,106]; however, it remains unclear whether the observed effects are attributable to the added probiotic strains, the fermented kefir matrix itself, or a combination of both. Due to this, they were not included in the evidence synthesis, which focused on the health benefits of kefir consumption. For example, the study included in this review [36] investigated kefir and kefir with added probiotics, and the latter showed superior outcomes in depression and appetite regulation compared to kefir alone, suggesting potential synergistic effects. Studies that reported non-health-related outcomes (e.g., microbiota composition alone) [70] were also outside the predefined eligibility criteria.

4.6. Differences in Using Kefir Grains vs. Starter Cultures

The clinical studies included in this review included heterogeneous kefir interventions that differ substantially in daily dosage, duration of intake, and type of kefir preparation. While some kefir products were distributed directly by manufacturers, others were fermented under controlled laboratory or household conditions. Also, some kefir products were produced via fermentation using kefir grains [38,44,45,53,54,55,58,60] while others were made from starter cultures [35,36,38,40,41,42,46,48,49,50,51,57] and for some studies no data was given [37,39,43,47,52,56,59,61,62].

As highlighted by Vieira et al. [71], artisanal kefirs using traditional kefir grains and industrial kefirs produced from starter cultures differ in their bioactive compound profiles (exopolysaccharides, including kefiran, bioactive peptides, and organic acids, especially lactic acid), which can influence their functional potential [107,108]. This methodological diversity represents an important source of variability across trials and complicates the direct comparison of outcomes. Rather than indicating contradictory evidence, this heterogeneity is likely a consequence of the differences in kefir composition, fermentation conditions, and host-related factors [6,109,110]. Only one study included in this review [40] compared pitched kefir using traditional microorganisms and commercial kefir, finding differences in health benefits, where pitched kefir was more effective for cardiometabolic effects and commercial kefir was more effective in increasing TNF-alpha.

Although the studies included in this review that reported using kefir grains generally described a broader microbial community, several starter cultures also contained mixed bacterial and yeast consortia, similar to those in traditional kefir [10,109], the distinction in effectiveness was not necessarily obvious. Consequently, the available clinical evidence does not support firm conclusions regarding systematic differences in microbial composition or health effects between grain-based and starter-culture kefirs. Therefore, there is a need for standardized microbial characterization in future trials.

4.7. Dose-Dependent and Duration-Dependent Effects of Kefir Consumption

Findings from this review indicate that the beneficial effects of kefir consumption depended on both the daily volume of kefir consumed and the duration of intake. Short-term interventions (≤4 weeks) with moderate volumes (200–250 mL/day) were sufficient to improve gut microbiota diversity, stool frequency, and bowel satisfaction [37,38,55]. Whereas medium-term studies (6–12 weeks) with higher volumes (300–600 mL/day) demonstrated improvements in metabolic parameters, inflammatory markers, and appetite regulation [44,46,49,51]. These findings suggest that although gut-related effects may manifest relatively low volumes over short periods, systemic metabolic and inflammatory outcomes generally require higher volumes and longer durations of kefir intake.

Similarly, clinical studies using defined probiotic strains have found that short-term interventions (2 to 4 weeks) were sufficient to improve gastrointestinal symptoms, stool frequency, reflecting local gut microbiota modulation and dynamics [111,112]. In contrast, systemic metabolic, immunomodulatory or inflammation outcomes required longer intervention periods to achieve measurable changes in markers such as lipid profiles, inflammatory cytokines, or short-chain fatty acid production in human subjects [111,113,114,115]. Although it remains essential to distinguish kefir as a complex fermented food and probiotics as defined microbial interventions with strain-specific evidence, the parallels in dosage and duration effects between kefir and probiotics emphasize the general principles of microbial interventions.

4.8. Practical Implications of Kefir Consumption

Kefir is also increasingly becoming recognized as a functional fermented milk beverage that can support human health on multiple levels [23,31], and based on the evidence from this review, the potential benefits include gastrointestinal health, metabolic regulation, and modulation of inflammatory processes. While general recommendations can be given for daily consumption (200–600 mL/day depending on duration and targeted outcome), more research is needed to determine the optimal dose and composition. However, further research is required to define optimal doses and microbial compositions for specific clinical applications.

From a clinical nutrition perspective, the current evidence suggests that milk-based kefir may be incorporated as part of a balanced diet, particularly in individuals with mild metabolic disturbances, gastrointestinal complaints, or lactose intolerance. However, 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.

These findings also have implications for clinical nutrition practice, dietary guidelines, and the development of functional fermented beverages. 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 [6]. Routine measurement and declaration of microbial counts and composition by manufacturers would enable more systematic clinical research, allowing meaningful comparisons across studies [110,116]. Such standardization is essential for high-quality systematic reviews and meta-analyses, which in turn could help identify effective doses and formulations for specific health outcomes [116]. Furthermore, consistent with ISAPP recommendations [15,16], it is important to clearly distinguish fermented foods from probiotic products unless strain-specific microorganisms are defined and shown to confer a health benefit at an effective dose.

Although kefir shows promising health benefits, it does not meet the formal definition of a probiotic. Greater transparency in labeling, including reporting microbial composition and viable counts where feasible, would support both clinical research and informed consumer choice. Overall, the available clinical evidence supports the inclusion of milk-based kefir as a functional fermented food within a diverse dietary pattern. However, further well-designed and adequately powered clinical trials using standardized kefir preparations are required before specific health claims or formal dietary recommendations can be established.

4.9. Limitations and Recommendations

This review has several limitations, mainly connected to the properties of kefir. Blinding is particularly challenging in kefir research due to its characteristic taste, aroma, texture, and effervescence. Most trials were open-label or single-blind, which may introduce expectation bias. Only three studies included in this review reported double-blind designs: da Silva Ghizi et al. [44] reported identical packaging without clear sensory matching, while Bekar et al. [56] did not specify methods. The most rigorously described approach was Merenstein et al. [59], which used heat-inactivated, matched flavoring, appearance, and packaging to achieve perceptual equivalence. These organoleptic constraints should be considered when interpreting clinical evidence. Future trials should aim to improve blinding or use carefully matched sensory controls to minimize expectation bias and strengthen methodological rigor.

A major methodological limitation is that the included studies also differed in kefir production methods, microbial composition, daily dose, and duration, which complicates direct comparisons and interpretation of findings. Standardized reporting of microbial content, daily intake, intervention duration and differentiation between grain-fermented and starter-culture kefir is recommended. Future studies should investigate longer intervention periods, larger sample sizes, and more homogeneous populations to enhance comparability and strengthen conclusions.

Besides these two major limitations, sample sizes of clinical trials were often small, and populations were heterogeneous, ranging from healthy individuals—such as athletes—to patients with inflammatory bowel disease, metabolic syndrome, or dental conditions, limiting generalizability. Assessed outcomes were also heterogeneous, including metabolic and inflammatory parameters, gut microbiota modulation, cognitive and oral health, as well as exercise-related parameters. Several studies were short-term, preventing the evaluation of long-term effects and trials not published in English were not included [117,118].

A further limitation is that microbiota analyses were largely limited to compositional profiling, with little assessment of microbial function. This restricts the interpretation of how kefir may influence metabolic or inflammatory pathways.

5. Conclusions

Kefir consumption appears to offer broad health benefits, including support for the gut microbiota, metabolic and immune function, and positive effects on oral, cognitive, and exercise-related outcomes. Evidence from 28 clinical trials, including recent randomized studies, shows these benefits across diverse populations, although their magnitude can vary depending on microbial composition, daily dose, and duration of kefir consumption. Traditional grain-based kefirs, with higher microbial diversity, may produce stronger effects than starter culture-based versions, though both forms were found to be beneficial.

The heterogeneity across the studies points to the need for more standardized research, including detailed reporting of microbial composition and viable counts, consistent dosing, longer interventions, and, where feasible, double-blind designs with matched sensory controls. While this review may not have included every published study on kefir consumption, the overall pattern of findings supports the conclusion that kefir is a safe, functional fermented milk beverage with important health potential. Continued high-quality clinical studies are needed to clarify optimal consumption patterns and the mechanisms behind kefir’s health-promoting effects.