Kefir as a therapeutic agent in clinical research: a scoping review

Abstract

Increasing research has been conducted on the role of probiotics in disease treatment. Kefir, a safe, low-cost probiotic fermented milk drink, has been investigated in many in vitro and animal studies, although parameters for human therapeutic dose or treatment time have not yet been determined. Here we perform a scoping review of clinical studies that have used kefir as a therapeutic agent, compiling the results for perspectives to support and direct further research. This review was based on Joanna Briggs Institute guidelines, including studies on the effects of kefir-fermented milk in humans. Using the term KEFIR, the main international databases were searched for studies published in English, Spanish or Portuguese until 9 March 2022. A total of 5835 articles were identified in the four databases, with forty-four eligible for analysis. The research areas were classified as metabolic syndrome and type 2 diabetes, gastrointestinal health/disorders, maternal/child health and paediatrics, dentistry, oncology, women’s and geriatric health, and dermatology. The many study limitations hampered generalisation of the results. The small sample sizes, methodological variation and differences in kefir types, dosage and treatment duration prevented clear conclusions about its benefits for specific diseases. We suggest using a standard therapeutic dose of traditionally prepared kefir in millilitres according to body weight, making routine consumption more feasible. The studies showed that kefir is safe for people without serious illnesses.

Introduction

The idea that gut health and microbiota balance directly affect the homeostasis of human organ systems is gaining increasing acceptance. Accordingly, foods are being researched as therapeutic agents in search of benefits that help maintain the body in a healthy state. The human gut is home to trillions of microbes, and their impact on human health has been extensively studied⁽¹⁾. In addition to being an important digestive organ, the intestine is the largest immune organ in the human body⁽²⁾. Numerous studies on the influence of intestinal microbiota have shown the importance of diet for wellbeing. Interactions between intestinal microorganisms and a series of health problems have been described^(3–6).

Gut microbiota refers to the microbes that reside in the human gastrointestinal tract⁽⁷⁾. It contributes to the integrity of the intestinal epithelial barrier by maintaining cell junctions and promoting damage repair, as well as by helping protect the host against pathogenic microorganisms, neutralising the effects of toxins and/or drugs and providing essential metabolites/vitamins^(8,9) . It also improves nutrient bioavailability and modulates the intestinal epithelium’s absorption capacity⁽¹⁰⁾. Research on gut microbiota is ongoing in a number of fields, including dermatology⁽¹¹⁾, endocrinology^(12,13) , dentistry^(14,15) and oncology^(16,17) .

While the inherited genome is essentially stable over the host’s lifetime, the microbiome is immensely diverse^(18,19) , dynamic⁽²⁰⁾, and responsive to external stimuli, which increases its potential as a target for therapeutic intervention. The composition of the microbiota is influenced by host genotype, environment and diet⁽¹⁰⁾.

Intestinal dysbiosis refers to changes in the quantitative and qualitative composition of our commensal microbes, which can alter host microbial interaction and contribute to an inflammatory disease state, which is associated with the development of many non-communicable diseases⁽²¹⁾. One current strategy for treating dysbiosis is the use of probiotics in an effort to recover microbial diversity and disturbed gut microbiota.

Probiotics refers to microorganisms that, when administered in adequate amounts, confer health benefits to the host^(22–25). A probiotic must be able to survive under gastrointestinal conditions (acidic pH, enzymes, bile salts, etc.), have the ability to adhere to the intestinal mucosa, antagonise pathogens and stimulate the immune system^(22,25) . Among their different mechanisms of action, the following stand out: colonisation and normalisation of disturbed intestinal microbial communities in children and adults, competitive exclusion of pathogens and production of bacteriocins, modulation of faecal enzyme activities associated with bile salt metabolism, inactivation of carcinogens and other xenobiotics, production of short-chain and branched-chain fatty acids (which have broad effects in the gut and peripheral tissue by interacting with short-chain fatty acid receptors, which modulate tissue sensitivity to insulin), cell adhesion and mucin production, immune system modulation and interaction with the brain–gut axis by regulating endocrine and neurological function^(22,26) . Prebiotics, foods that contain non-digestible fibre, stimulate the growth and activity of beneficial microorganisms⁽²⁷⁾, and include human milk oligosaccharides, inulin, fructooligosaccharides and galactooligosaccharides⁽²⁸⁾.

Kefir is fermented milk produced by the action of bacteria and yeast that symbiotically associate in kefir grains⁽²⁹⁾. This slightly effervescent and foamy beverage originates from the action of the natural microbiota present in these grains⁽³⁰⁾, which consists of an inert matrix of polysaccharides and proteins⁽³¹⁾. This matrix is densely populated by species of lactic acid bacteria, acetic acid bacteria and yeast⁽³²⁾.

The large number of microorganisms in kefir and their microbial interactions as well as the bioactive compounds that result from microbial metabolism and the benefits associated with this beverage make kefir a natural probiotic. The microbial composition of kefir can vary, being influenced by region of origin, duration of use, substrate and management techniques⁽³³⁾. However, kefir grains generally contain relatively stable and specific microbiota, always including a predominance of certain Lactobacillus species.^(29,34) .

Numerous in vitro and animal studies have demonstrated the beneficial action of milk fermented by kefir grains and its nutraceutical potential, which includes anti-inflammatory^(35–37), antioxidant^(38,39) , anticancer^(39,40) , antimicrobial^(41–43), antidiabetic⁽⁴⁴⁾, antihypertensive^(45–48) and anti-hypercholesterolaemic properties^(49,50) . These effects can be attributed to probiotic microorganisms and the wide diversity of bioactive compounds produced during the fermentation process^(39,51) . However, clinical research on these benefits is still in its infancy, complicated by the heterogeneity of dosages and forms of administration, making it difficult, for example, to compile results through meta-analysis. Thus, the present scoping review aimed to assess clinical studies that have tested kefir as a therapeutic agent for diseases or health conditions, identifying therapeutic patterns and adverse effects. From this analysis, the data were organised to point out strategies and present perspectives for future research.

Methodology

This is a scoping review of human studies on the effects of milk fermented by kefir grains as a therapeutic agent for diseases of organic systems or to improve patient health conditions. This review was structured according to Joanna Briggs Institute guidelines⁽⁵²⁾. Scoping reviews are useful when studies in a topic are heterogeneous, as they map and summarise existing evidence and identify possible knowledge gaps to direct future research⁽⁵²⁾. A search was performed in the main international databases: PubMed, Web of Science, Embase and Scopus, using the term KEFIR. The inclusion criteria were prospective clinical studies, with kefir as the object of the study, published in English, Spanish or Portuguese until 9 March 2022. Duplicated studies and those using water kefir were excluded, since it was not the focus of this review. The inclusion and exclusion criteria were identified by reading the abstracts and, occasionally, if there were any doubts, by reading the full article. Study eligibility was determined by two independent researchers, and a third arbitrated in cases of disagreement. The search results were managed using EndNote.

The titles and abstracts of all articles were evaluated for potential relevance according to the inclusion and exclusion criteria. Data from the selected clinical studies were extracted and summarised regarding the objective, materials and methods (design, dose and intervention time), sample/population, comparison parameters between groups, adverse effects and clinical outcome. The selection process for relevant studies is shown in Fig. 1.

Fig. 1. Flowchart of article inclusion.

Results

The term kefir resulted in a wide range of studies in the selected databases. The vast majority, however, concerned laboratory research on strains of microorganisms in kefir or were in vitro or animal studies testing its disease-fighting properties or its ability to maintain a healthy state. A total of 5835 articles were identified in the four databases, and forty-four were eligible for analysis after applying the inclusion and exclusion criteria and removing duplicates. The selection process for relevant studies is shown in Fig. 1.

Clinical studies published in English (but not Portuguese or Spanish) on kefir as a therapeutic agent can be found beginning in 2002, and this number has shown a growing trend, reaching eight in 2021, as shown in Fig. 2. Turkey, Iran, and the United States have produced the most clinical studies, followed by Brazil and Taiwan, as illustrated in Fig. 3. Most studies are single-centre clinical trials, with some crossover trials, one case series and one case report, all prospective studies. The mean sample size was 45·9 (standard deviation 28·7) participants.

Fig. 2. Year of publication of English-language studies on kefir as a therapeutic agent. *Until 9 March 2022.

Fig. 3. Number of articles according to country of origin (absolute number, percentage).

The mean intervention time could not be calculated, since it was presented in days, weeks, months, medication cycles, continuous use or single dose, or was not daily, as in O’Brien et al. ⁽⁵³⁾, in which it was administered twice a week for 15 weeks. The study population also varied, although none of the subjects had serious diseases or severe comorbidities, which we considered exclusion criteria.

Table 1 ^(54–96) presents the population, sample size, therapeutic dosage, kefir preparation type, intervention time and main findings of each study. Two pairs of publications (Ostadrahimi et al. ⁽⁶⁸⁾ and Alihosseini et al. ⁽⁷²⁾; Fathi et al. ⁽⁷⁰⁾ and Fathi et al. ⁽⁷¹⁾) used the same population to investigate different outcomes.

Table 1. Overview of included studies

n, sample number; i.p., industrial preparation; h.p., home preparation; u.t., unspecified type; AD, Alzheimer’s disease; ALT, alanine aminotransferase; BDC-LDL, baseline diene conjugates of low-density lipoprotein; Beta2-GPI-oxLDL, oxLDL-β2 glycoprotein I; BMD, bone mineral density; BMI, body mass index; CRP, C-reactive protein; ECOG, Eastern Cooperative Oncology Group; ESR, erythrocyte sedimentation rate; FBG, fasting blood glucose; FMT, faecal microbiota transplantation; GI, glycaemic index; HbA1c, glycated haemoglobin; Hgb, haemoglobin; HOMA-IR, insulin resistance Homeostasis Model Assessment; IBD, inflammatory bowel disease; LI, lactose intolerance; MDA, malondialdehyde; MetS, metabolic syndrome; oxLDL, oxidised low-density lipoprotein; OxS, oxidative stress; PBG, postprandial blood glucose levels; QoL, quality of life; STAW, staggered and tapered antibiotic withdrawal; T2DM, type 2 diabetes mellitus; TOS, total oxidant status; WC, waist circumference. Triple therapy* consists of lansoprazole (30 mg), amoxicillin (1000 mg) and clarithromycin (500 mg).

The largest number of clinical studies with kefir was for the prevention or treatment of metabolic and gastrointestinal diseases, and their results were usually positive. Other fields of knowledge in which kefir was tested included maternal/child health and paediatrics, dentistry, oncology, women’s and geriatric health, and dermatology (Table 2).

Table 2. Areas of clinical research on kefir

ATB, antibiotic; IBD, inflammatory bowel disease; MetS, metabolic syndrome; QT, chemotherapy; T2DM, type 2 diabetes mellitus.

Discussion

The discussion of the results will be presented according to the field in which kefir was studied in humans as a therapeutic agent.

Metabolic syndrome and type 2 diabetes

Metabolic syndrome is considered a pro-thrombotic and pro-inflammatory state characterised by a set of clinical findings that include central and abdominal obesity, systemic hypertension, insulin resistance and atherogenic dyslipidaemia, including elevated inflammatory cytokine activity^(97,98) . Therapeutic options involve dietary control, regular exercise and pharmacological treatment for dyslipidaemia, hypertension and hyperglycaemia⁽⁹⁸⁾.

Gut microbes influence host metabolic balance by modulating energy absorption, intestinal motility, appetite, glucose and lipid metabolism, and hepatic fat storage⁽⁹⁹⁾. Dysbiosis favours the translocation of bacterial fragments and can lead to systemic inflammation and insulin resistance^(99,100) . Administration of pre- and probiotics can reduce low-grade inflammation and improve intestinal barrier integrity, aiding metabolic balance. Zonulin, a family of proteins associated with the permeability of the intestinal barrier⁽¹⁰¹⁾, is a physiological modulator of the tight intercellular junctions of the intestinal epithelium, and the loss of barrier function secondary to its up-regulation can lead to an uncontrolled influx of food and microbial antigens. Pražnikar et al. ⁽⁸⁵⁾ studied the effects of supplementing asymptomatic overweight adults with milk or kefir for 3 weeks, finding that kefir supplementation led to lower serum zonulin levels than milk.

Bellikci-Koyu et al. ⁽⁸²⁾ and Da Silva Ghizi et al. ⁽⁹⁵⁾ tested the effects of regular kefir consumption in people with metabolic syndrome. The intervention period in both studies was 12 weeks and they used similar dosages, although the kefir preparation methods differed. The bioactive peptide profile of milk kefir is completely different from that of raw milk, including 236 unique peptides produced during the fermentation process, many with antihypertensive (angiotensin-converting enzyme inhibitors), antimicrobial, immunomodulatory, antioxidant and antithrombotic properties⁽⁴⁶⁾. Bellikci-Koyu et al. ⁽⁸²⁾ found no significant differences between milk and kefir groups, while Da Silva Ghizi et al. ⁽⁹⁵⁾ considered the origin of their fermented beverage key to their more expressive findings than Bellikci-Koyu et al. ⁽⁸²⁾. Da Silva Ghizi et al. used milk inoculated with kefir grains, which contains more than thirty live bacteria species and twelve yeast and fungi species in a complex symbiotic system, a culture that synthesises several bioactive compounds during the fermentation process, such as organic acids, bioactive peptides, bacteriocins and exopolysaccharides^(39,46,51) . Industrial kefir production, which usually involves commercial starter cultures, allows standardisation of this process, although it results in loss of cell viability. Commercial kefir contains a smaller number and variety of microorganisms, especially yeasts^(46,102) . Moreover, in fermented food production (including kefir), the backslopping technique is used to increase production by up to fifty times, maintaining the physico-chemical characteristics and nutritional values, but decreasing the microbiological diversity and producing some consistency deficiencies⁽³¹⁾. In a randomised, double-blind, placebo-controlled clinical study, Da Silva Ghizi et al. ⁽⁹⁵⁾ found significant improvement in several clinical parameters in the kefir group compared with controls (who received homemade curd prepared with the same type of milk as the kefir group), including blood pressure, lipid profile blood, fasting glucose and oxidised LDL, which led to lower cardiovascular risk according to Framingham scores. Abd-Alwahab and Al-Dulaimi⁽⁷⁸⁾ administered grain-fermented kefir drink to volunteers, finding significant improvement in the lipid profile, although their control group received water instead of milk, which prevents direct comparison of the results.

One hypothesis about kefir’s mechanism of action on serum cholesterol levels is the deconjugation of bile acids by Lactobacillus spp., while yeasts increase bile acid discharge, which in turn increases cholesterol expenditure during production^(78,103) . In addition, colonic microbiota can metabolise dietary and endogenous cholesterol, reducing it mainly to coprostanol (5β-colestan-3β-ol) and coprostanone, both through low intestinal absorption and elimination through faeces^(104,105) . Higher intake of calcium, which is abundant (about 0·12/100 g) in kefir, can also positively affect the serum lipid profile^(106–108). St-Onge et al. ⁽⁵⁴⁾, Fathi et al. ⁽⁷¹⁾, Pražnikar et al. ⁽⁸⁵⁾, Ostadrahimi et al. ⁽⁶⁸⁾, Gezginc and Maranci⁽⁷⁹⁾ and Bashiti and Zabut⁽⁸⁴⁾ investigated kefir’s effects on the lipid profile, but found different results, probably a consequence of the studies’ different methodologies.

The conflicting outcomes of these studies can be explained, at least in part, by the heterogeneity of their populations, especially the floor (baseline) effect (i.e. the lower the baseline level of an outcome, the less likely a treatment will result in a reduction in that outcome level), which was evident in studies such as that by Gezginc and Maranci⁽⁷⁹⁾, who tested the influence of kefir on the lipid profile of healthy young people.

Hosainzadegn and Hosainzadegan⁽⁹²⁾ reported a case of marked improvement in glycated hemoglobin (HbA1c) levels (from 7·9 to 7·1) and a 4 kg weight loss in a woman with type 2 diabetes after 3 months of using kefir prepared at home. Clinical studies by Judiono et al. ⁽⁶⁷⁾, Ostradahimi et al. ⁽⁶⁸⁾, Alihosseini et al. ⁽⁷²⁾, Bellikci-Koyu et al. ⁽⁸²⁾, Bashiti and Zabut⁽⁸⁴⁾ and Praznikar et al. ⁽⁸⁵⁾ were included in a systematic review with meta-analysis⁽¹⁰⁹⁾ of randomised clinical trials that assessed the effects of kefir drink on glycaemic control. The authors found significant improvement in fasting glucose and insulin levels in participants who consumed kefir, but no significant difference in HbA1c levels. The results of this meta-analysis, however, can be questioned due to the heterogeneity of study populations, dosages, and intervention times.

Weight control and exercise are also involved in the prevention and treatment of metabolic syndrome. Studies by O’Brien et al. ⁽⁵³⁾, Gölünük et al. ⁽⁷³⁾ and Lee et al. ⁽⁸⁸⁾ investigated the effects of kefir on post-exercise oxidative stress parameters. The samples of these studies were healthy young people, although the intervention time, kefir type, quantity and tested parameters differed greatly. Kefir improved total oxidant status levels (an indicator of oxidative stress)⁽⁷³⁾, did not change C-reactive protein levels (which increased in the control group)⁽⁵³⁾, improved exercise endurance, reduced lactic acid production after exercise, accelerated recovery⁽⁸⁸⁾ and did not significantly affect the other parameters.

Regarding weight loss, Fathi et al. ⁽⁷⁰⁾ and Hosainzadegn and Hosainzadegan⁽⁹²⁾ found positive results with kefir, unlike other studies that measured anthropometric parameters such as weight, body mass index and waist circumference^(53,88,95) . Caferoglu and Aytekin Sahin⁽⁹⁴⁾ investigated whether adding kefir to a low or high glycaemic index meal would affect participant appetite and food intake, finding that kefir can help limit appetite and energy intake for high glycaemic index meals but not low glycaemic index meals⁽⁹⁴⁾.

Gastrointestinal health/disorders

The gastrointestinal tract and its microbiome provide unique metabolic functions to the host and are critical to maintaining health. The abundance of different species of microorganisms, their function and their interaction with organic systems have been the focus of numerous medical studies⁽¹¹⁰⁾. Metagenomics and analysis of twin data have shown that environmental factors, such as diet and domestic cohabitation, far outweigh heritable genetic contributions to gut microbiota composition and function⁽¹¹¹⁾.

Inflammatory bowel disease is characterised by chronic and relapsing inflammation of the gastrointestinal tract and includes 2 chronic idiopathic inflammatory diseases: Crohn’s disease and ulcerative colitis^(83,112,113) . These heterogeneous and complex immune disorders of the gastrointestinal tract share many common clinical features but differ in inflamed areas and are treated differently^{(110,113,114)} . Considered a serious and debilitating condition that affects general health and quality of life, inflammatory bowel disease has been associated with intestinal dysbiosis, which decreases microbial biodiversity and slows or stops important functions of intestinal barrier integrity and immune system regulation, which results in inflammation and increased immune response⁽¹¹⁴⁾. In addition, mucolytic and pathogenic bacteria are also increased, which leads to degradation of the mucosal barrier and greater pathogen penetration into intestinal tissues^(114,115) .

Probiotics and faecal microbial transplantation, both aimed at reintroducing beneficial microbes into dysbiotic guts, are currently being used to treat inflammatory bowel disease^(114,116) . Since kefir contains a diverse range of microorganisms, many of which have already been studied as probiotics⁽¹¹⁷⁾, it has promise as an alternative treatment for intestinal dysbiosis. Wang⁽⁸⁷⁾ found that 21 d of freeze-dried (industrialised) kefir had positive effects on gastrointestinal symptoms, such as abdominal pain and bloating, and increased abundance of bifidobacteria. Forejt et al. ⁽⁵⁷⁾ found significant improvement in gastrointestinal discomfort and lower Enterococcus faecalis after 2 weeks of kefir therapy in a sample of women. Figler et al. ⁽⁵⁶⁾ investigated how consuming two types of kefir influenced the levels of primary probiotic Streptococcus, Lactobacillus and Bifidobacterium among the total number of microbes, finding that these populations increased in both groups after 4 weeks, with a more expressive increase in the intervention group (Biofir^® kefir). In contrast, Sepp⁽⁸⁰⁾ found increased diversity of Lactobacillus spp. after 8 weeks only in the group who received kefir (industrialised) with added L. fermentum ME-3. The studies’ samples, however, were healthy people with no diagnosed gastrointestinal disease, and different intervention times were used. In a sample of patients with Crohn’s disease, Yilmaz et al. ⁽⁸³⁾ found that regular kefir use can improve both symptoms and short-term quality of life and has a positive effect on biochemical parameters (such as haemoglobin, C-reactive protein and erythrocyte sedimentation rate).

Kefir has also been studied as an adjuvant treatment in two other gastrointestinal pathologies, Helicobacter pylori infection and recurrent Clostridioides difficile infection. Among patients who received kefir, Bekar et al. ⁽⁶¹⁾ found a lower prevalence of side effects, such as headache, nausea, diarrhoea and abdominal pain, in a group that received a triple therapy consisting of lansoprazole (30 mg), amoxicillin (1000 mg) and clarithromycin (500 mg). This may have important implications for increasing treatment adherence. In a case series of patients at risk of recurrent Clostridioides difficile infection, Bakken⁽⁶⁴⁾ used continuous kefir ingestion in association with a regimen of staggered and tapered antibiotic withdrawal, finding the same efficacy as faecal microbial transplantation.

Microorganisms present in kefir, such as Lactococcus spp., Lactobacillus spp. and some strains of Kluyveromyces spp. hydrolyse lactose to glucose and galactose⁽¹¹⁸⁾. During the fermentation process, milk proteins are also extensively hydrolysed, releasing functional peptides and improving digestibility⁽¹¹⁹⁾. Two studies tested kefir for digestibility in healthy adults with poor digestion⁽⁵⁵⁾ or lactose intolerance⁽⁶³⁾, both using a single intervention dose. They concluded that milk fermented with kefir, as well as yogurt, causes less severe discomfort than normal milk.

Turan et al. ⁽⁶⁶⁾ and Maki et al. ⁽⁷⁷⁾ studied the effects of kefir on chronic intestinal constipation, a highly prevalent condition for which diet is a possible treatment⁽⁹⁷⁾. The first study enrolled patients who met Rome II criteria for chronic constipation, without metabolic or structural disorders that could be responsible for the disease; the second enrolled patients with some physical or mental disability who were admitted to a Japanese hospital. In addition to the different study populations, the type and amount of kefir used were completely different. Turan et al. ⁽⁶⁶⁾ used a higher dose of kefir (250 ml twice per day) for 28 d, while Maki et al. ⁽⁷⁷⁾ used lyophilised kefir at a much lower dose (about 1/8 of Turan et al.) for 84 d. Turan et al. found that kefir supplementation was associated with a statistically significant decrease in laxative use, increased defecation frequency, higher bowel satisfaction scores and shortened colonic transit times. Maki et al. ⁽⁷⁷⁾ found that kefir worked better in patients with non-severe chronic constipation and suggested identifying individuals who could benefit from its use. Clinical trials with larger sample sizes and a control group are highly recommended to determine kefir’s benefits in preventing and/or treating these gastrointestinal disorders.

Maternal/child health and paediatrics

Intestinal bacterial colonisation in the first years of life (generally the first three) has a major impact on the immune system, and the immunological influences of microbiota during this specific window can determine resistance or susceptibility to diseases, affecting the host’s health throughout life^(120–122). Breast milk is a critical factor in the development and composition of intestinal microbiota in neonates. One possible origin of the bacteria in this fluid, many of which are potentially probiotic, is the maternal gastrointestinal tract via bacterial translocation through the lymphatic system⁽¹²³⁾. Tunay et al. ⁽⁹⁶⁾ tracked transmission of bacteria unique to kefir grains (Lactobacillus kefiranofaciens, Lentiactobacillus kefiri, Lentiactobacillus parakefiri) in breast milk and the faeces of newborns, finding that these microorganisms were transmitted to milk through maternal consumption of kefir, resulting in infant intestinal colonisation. Kurt et al. assessed the effects of milk kefir in nursing mothers in relation to the carbohydrate profile of their breast milk⁽⁹¹⁾. The authors detected a trend towards more carbohydrates, including galactoligosaccharides (structures with prebiotic properties), in the milk of mothers who consumed kefir. They also reported that neither the mothers nor their infants experienced abdominal discomfort from using kefir. While a good body of evidence about the effects of probiotics on paediatric populations already exists^(124–126), studies about the benefits of kefir in children are still incipient.

Merenstein et al. ⁽⁶⁰⁾ tested kefir’s effects on prevention of antibiotic-associated diarrhoea, a disease with a high morbidity rate and low treatment adherence⁽¹²⁷⁾. The sample consisted of children aged between 1 and 5 years who received antibiotics to treat upper airway infections. Although they found no difference in diarrhoea rates between the intervention and control groups, they did find different absolute numbers of diarrhoea incidents in children aged 3–5 years (14% in the control group versus 6% in the kefir group) and in boys (32% in the control group versus 24% in the kefir group, compared with a 2% difference among girls in these groups). The differences in absolute numbers of diarrhoea incidents in the older groups were quite expressive (57% higher in the placebo group), which suggests that a study with a larger sample in this population could help elucidate the promising role of kefir for preventing antibiotic-associated diarrhoea. The authors further reported that patient safety was excellent, as expected for a food.

De Araújo et al. studied the effects of lyophilised kefir on the clinical parameters of wheezing among infants aged 6–24 months and on cytokine expression via T-helper 1 and T-regulatory cells⁽⁷⁵⁾, finding no significant decrease in the clinical parameters of wheezing, only a trend towards lower recurrence (perhaps the sample was too small to demonstrate significance). However, they suggested that this probiotic mixture triggers immunomodulation due to the production of T-helper 1 and T-regulatory cell cytokines, including IL-10 and IL-12, since there they increased significantly in the intervention group. Intestinal bacteria contribute to proper development of the immune system in the first years of life, and the intestinal flora and its metabolites (such as short-chain fatty acids) actively participate in the proliferation and differentiation of B cells and T cells, inducing a protective antibody response⁽¹²⁸⁾.

Dentistry

The human mouth harbours a complex microbiome whose imbalance can lead to dental caries and periodontal disease⁽¹²⁹⁾. When metabolising carbohydrates, cariogenic microorganisms produce lactic, formic, acetic and propionic acids, which decrease the mouth’s pH to below 5·5, resulting in demineralisation of enamel hydroxyapatite crystals and proteolytic breakdown of the hard tissue structure of the teeth. Streptococcus mutans is the most important bacterial species related to oral health⁽¹³⁰⁾; it is more abundant in disease and is the main bacteria involved in early childhood caries⁽¹³¹⁾. Three included studies analysed the effects of kefir on the salivary count of S. mutans. Nevertheless, the results differed depending on the population, intervention time and dosage. The results showed that kefir was as effective as sodium fluoride for reducing salivary S. mutans counts in young adults⁽⁶⁵⁾. Daily consumption of kefir and the use of probiotic toothpaste decreased salivary microbial colonisation in orthodontic patients⁽⁷⁶⁾; probiotic products together with dental restorations effectively reduced S. mutans in children aged 8–12 years⁽⁸⁹⁾.

Oncology

In vitro and animal studies have found positive results with probiotic dietary products like kefir in the prevention and treatment of various types of cancer^(40,132,133) . Studies by Topuz et al. ⁽⁵⁸⁾ and Can et al. ⁽⁵⁹⁾ explored the use of kefir in very similar populations, using the same dose in almost identical intervention times, although their objectives differed. Topuz et al. measured serum levels of pro-inflammatory cytokines, the antimicrobial effects of kefir and the mucositis rate in patients with colorectal cancer receiving chemotherapy (5-fluorouracil or oral fluoropyrimidine), but found no statistically significant differences between the intervention and control groups. Can et al. ⁽⁵⁹⁾ explored kefir’s ability to prevent treatment-related gastrointestinal symptoms and its effects on the quality of life of cancer patients. They detected no differences in quality-of-life indices and reported more gastrointestinal complaints, but found better sleep quality in patients who used kefir. The amount of fermented milk used in the intervention may explain the increased complaints, since the ingestion of 250 ml of any liquid can be uncomfortable and increase symptoms such as nausea, vomiting and diarrhoea. Furthermore, the control group did not receive any type of similar liquid.

Smoak et al. ⁽⁹³⁾ studied patients with cancer who had undergone chemotherapy or radiotherapy in the previous 2 years and were enrolled in an exercise programme at the University of Northern Colorado Cancer Rehabilitation Institute. The intervention group drank approximately 240 ml of kefir up to 30 min after the exercise sessions, which took place three times a week for 12 weeks. The control group performed the same exercises but received no placebo. The kefir group improved in lean body mass, depression symptoms, fatigue, gastric discomfort and a biomarker of intestinal dysbiosis, which suggests that including kefir as part of a post-exercise diet can have significant psychological and physical benefits for cancer survivors⁽⁹³⁾.

Women’s and geriatric health

Özcan et al. ⁽⁸¹⁾ studied perimenopause middle-aged women, in whom hormonal changes can lead to sleep and mood disturbances, sexual problems and, in the long term, decreased bone density^(134,135) . The authors used the Women’s Health Insomnia Rating Scale, the Menopause-Specific Quality of Life Questionnaire and the Beck Depression Inventory to determine whether kefir benefitted post-menopausal women suffering from sleep disorders. After 1 month of intervention there was significant improvement in the first two parameters but not in Beck Depression Inventory results, which showed a non-significant trend towards improvement. Despite the study’s small sample size, the authors found kefir to be a non-pharmacological alternative for minimising some of the discomforts of the menopausal transition period.

Tu et al. ⁽⁶⁹⁾ studied patients with osteoporosis, a disorder especially prevalent in postmenopausal women and older men, characterised by decreased bone mass and increased fracture risk⁽¹³⁶⁾. Therapy for patients with osteoporosis includes non-pharmacological measures (exercise, adequate calcium intake, etc.), medications to increase bone density and improve bone strength, and strategies to reduce the risk of falling^(136,137) . In their controlled, parallel, double-blind intervention study, Tu et al. ⁽⁶⁹⁾ divided participants into an intervention group, which received 1600 mg of freeze-dried milk kefir plus CaCO₃ (1500 mg) daily, while the control group received the same amount of CaCO₃ plus placebo (1600 mg of freeze-dried unfermented raw milk) daily for 6 months. By 6 months, the intervention had promoted short-term changes in bone turnover markers and greater increases in hip bone mineral density.

In the field of neurodegenerative diseases, Ton et al. ⁽⁸⁶⁾ conducted an uncontrolled clinical investigation to explore the antioxidant effects of milk fermented by kefir grains in patients with Alzheimer’s disease. Alzheimer’s disease causes progressive functional and cognitive decline in older adults and is the most common cause of dementia worldwide⁽¹³⁸⁾. From a pathological point of view, it is characterised by extracellular deposition of β-amyloid peptides and intracellular accumulation of hyperphosphorylated and aggregated hyperphosphorylated tau (a protein abundant in neurons), forming neurofibrillary tangles⁽¹³⁹⁾. The central mediators in the pathogenesis of Alzheimer’s disease are oxidative stress and neuroinflammation^(140–142). Numerous studies have shown that gut microbiota play an important role in brain function^(143–145). The brain and microbiota communicate through a complex bidirectional connection known as the ‘microbiota–gut–brain axis’, which involves the immune system, neuroendocrine mechanisms, tryptophan metabolism, vagus nerve and enteric nervous system.^(3,143,146) . Lipopolysaccharides (endotoxins) and bacterial amyloids synthesised by the gut microbiota can activate brain immune response and lead to neuroinflammation^(143,144) . Neuroinflammatory cytokines may compromise β-amyloid clearance, leading to its accumulation in the brain^(8,147) . Ton et al. ⁽⁸⁶⁾ evaluated the benefits of kefir supplementation for 90 d at a dose of 2 ml/kg/d on cognitive function and biomarkers of oxidative stress, inflammation and cell damage in older adults with Alzheimer’s disease. They found improvement in every test (memory, visual–spatial function and abstraction skills, executive and language functions, constructive skills and attentive function), a protective effect for mitochondria, and cytoprotective and anti-apoptotic action, whose effects slow neurodegeneration.

Dermatology

The skin and gut appear to share a series of indirect bidirectional metabolic pathways known as the ‘gut–skin axis’⁽⁹⁰⁾. Patients with atopic dermatitis, a highly prevalent inflammatory skin disease worldwide, present with intestinal and cutaneous dysbiosis^{(120,148,149)} . This condition of microbiome imbalance, along with skin barrier dysfunction, immune dysregulation and environmental risk factors, contributes to disease onset and the atopic march^(11,150) . In their controlled crossover intervention study, Alves et al. ⁽⁹⁰⁾ compared the effects of kefir ingestion (grain-fermented at home) on the skin of adults with and without atopic dermatitis. The primary outcomes were transepidermal water loss and stratum corneum hydration in all participants and the severity scoring of atopic dermatitis (SCORAD) index in patients with atopic dermatitis. Significant improvements were found in both groups, including a significant SCORAD index decrease in the atopic dermatitis group.

Kefir production and dosage

Commercially produced kefir models may contain different species of Lactobacillus than those produced with the inoculation of the grains, which can make an important difference, because species exclusive to kefir grains such as L. kefiranofaciens and L. kefiri have already demonstrated beneficial health effects^(151–154). Furthermore, commercial kefir samples usually do not contain acetic acid bacteria, which are abundant in traditionally prepared kefir^(155–157). Another noteworthy difference between commercially and traditionally prepared kefir is the variety of yeasts, radically smaller in commercial models⁽¹⁵⁶⁾. Bourrie et al. ^(158,159) demonstrated the impact of these microbial differences on kefir’s ability to improve metabolic parameters in obese mice.

On the basis of these animal studies and on the analysed intervention studies, our suggestion is to use a therapeutic dose pattern in millilitres of traditionally prepared kefir (kefir grains inoculated in milk), due to its greater microbiological complexity and potential bioactive compounds.

The randomised, double-blind, placebo-controlled trial by Da Silva Ghizi et al. ⁽⁹⁵⁾ was the only one to use individualised dose per kilogram (1·6 ml/kg for males and 1·9 ml/kg for females), calculated on the basis of studies by Reagan-Shaw et al. ⁽¹⁶⁰⁾ and Rosa et al. ⁽¹⁶¹⁾. This dose was close to the lowest standardised doses used in other studies that showed benefits. So, we suggest this individualised dose to be used in future studies avoiding underdose/overdose error. For example, a dose of 1·6 ml/kg for a man weighing 90 kg would be 144 ml of kefir per day, making routine consumption much more viable. When such individualisation is not possible, doses of 100–200 ml/d are suggested.

These doses were determined by the perception that a minimum amount necessary is better tolerated by people and is more economically viable, increasing the probability of the consumption of this food to become a habit and its possible benefits to extend over time. Such doses would also make the methodology of future studies closer to the reality of the population. Very high amounts of kefir can hamper the viability of longer-term use due to palatability and maintenance costs, even when prepared at home. Daily doses of 500 or 600 ml, used in some studies, are not feasible long term for the general population, thus preventing any possible benefits.

Conclusions

Despite a good body of evidence and interesting and promising findings in several areas of research, the included studies involved many limitations and their results cannot be generalised. The small sample sizes, methodological differences and varying kefir types, dosage and therapy times prevent us from drawing clear conclusions about its benefits for specific diseases. However, this review indicates fruitful paths for further research on kefir, facilitating the compilation of data and strengthening the results of future meta-analyses.

We suggest using a daily therapeutic dose pattern in millilitres of traditionally prepared kefir according to body weight (1·6 ml/kg for males and 1·9 ml/kg for females) or, alternatively, doses of 100–200 ml/d.

The studies included in this review found kefir to be a safe food for people without serious illnesses. However, further research is necessary before generalising this to people with severe disabilities or more advanced diseases. A healthy, balanced diet is fundamental for quality of life and the prevention of numerous diseases. Easy access to the initial culture (kefir grains), often available by donation, makes this healthy food a viable nutritional alternative for low-income populations, guaranteeing, at the very least, an optimal source of bioactive compounds and essential nutrients.