Effect of kefir beverage consumption on glycemic control: A systematic review and meta-analysis of randomized controlled clinical trials

Kefir is a fermented beverage obtained as a result of lactic-alcoholic fermentation of milk by using kefir grains or a starter culture. Kefir beverage has numerous health benefits owing to its antimicrobial, anti-inflammatory and anti-diabetic properties, while it could also regulate the gut microbiota. This beverage is recommended as a nutritional approach to glycemic control. This systematic review and meta-analysis of randomized controlled trials (RCTs) aimed to evaluate the effects of kefir beverage on the glycemic control. For this purpose, a literature search was conducted via databases such as PubMed, Scopus, ISI Web of Science, and Google Scholar for the articles published until July 2020 with no limitation. The indicators of glycemic control were fasting blood sugar (FBS), hemoglobin A1c (HbA1c), and insulin. In total, six RCTs on a total sample size of 323 subjects were selected for the meta-analysis. Inverse-variance weighted mean difference (WMD) with a 95% confidence interval (CI) was calculated for the mean changes in HbA1c, FBG and insulin compared to baseline. Considering the clinical heterogeneity caused by the varied duration and dosage of kefir beverage treatment, a random effects model was used to estimate the pooled WMD. Findings indicated a more significant reduction in the FBS (WMD = -10.28; 95% CI: - 16.53-4.02; P = 0.001) and insulin (WMD = -2.87; 95% CI: - 3.96-1.78; P < 0.00001) in the kefir treatment group. Furthermore, kefir consumption could reduce HbA1c (HbA1c = -0.64 -1.36, 0.08), while this effect was not considered significant (-0.64; 95% CI: -1.36-0.08; P = 0.08). The results of our metaanalysis indicated that kefir beverage could reduce the FBS and insulin levels without a significant effect on HbA1c. In general, our findings showed that kefir beverages may have beneficial effects on glycemic control. However, more clinical data and research into the mechanism of probiotics are required to clarify the beneficial effects of probiotics on glycemic markers.