Kefir: A Potential Gut Microbiota Modulator: A Systematic Review of Human Interventional Studies

Kefir is a fermented dairy product containing live and active microbial culture, including lactic acid bacteria and yeast. Preclinical studies and narrative reviews have reported potential modulatory effects of kefir on gut microbiota composition. Despite this, there isn't yet a thorough analysis of human intervention studies that fills this gap of research. Therefore, the aim of this review is to examine the role of kefir consumption on gut microbiota composition in humans. Comprehensive research was conducted using three major databases including Web of Science, PubMed, and Scopus. The risk of bias was assessed using Cochrane risk-of-bias tool and Risk of bias of non-randomized trials. The search resulted in 2743 studies, of which eight studies met our eligibility criteria. Overall, kefir resulted in minor changes in phyla and class levels. On the other hand, the Bifidobacterium genus increased in 3 out of 4 studies. Kefir-specific bacterial species and strains were found in participants fecal samples suggesting colonization properties. Kefir consumption was associated with modest and heterogenous changes in gut microbiota composition. Proposed mechanisms include transient persistent of kefir-associated taxa and modulation of the intestinal environment, although direct functional evidence in humans remains limited.