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

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.