Effects of blueberry and cranberry on type 2 diabetes parameters in individuals with or without diabetes: A systematic review and meta-analysis of randomized clinical trials

Aims: Blueberry and cranberry are rich in polyphenols that are associated with diabetes reduction. This study aimed: 1) to systematically review the literature on the effects of blueberry and cranberry consumption and type 2 diabetes parameters in individuals with or without type 2 diabetes and 2) to quantify these effects by carrying out a meta-analysis. Data synthesis: A systematic review and meta-analysis were performed using articles present in seven databases (PubMed, LILACS, Scielo, Scopus, Web of Science, Cochrane, and Embase), including publications until May 2021. We included randomized clinical trials that compared blueberry or cranberry effects on type 2 diabetes parameters, such as fasting blood glucose, insulin resistance, and glycated hemoglobin. Quality of the studies was performed using the Cochrane scale, while the Egger test assessed the publication bias and meta-regression the estimated effect sizes with potential moderator variables. From the 2034 studies identified, 39 were read in full and 22 were included in meta-analysis. In individuals with diabetes, the consumption of blueberry or cranberry significantly reduced fasting blood glucose [MD: −17.72 mg/dl; 95% CI: −29.62, −5.82; p = 0.03; I2 = 57%] and glycated hemoglobin [MD: −0.32%; 95% CI: −0.57, −0.07; p = 0.15; I2 = 39%], whereas for insulin resistance the effects were null. Results were not significant for the general population, except in the sensitivity analysis for fasting blood glucose. Conclusions: The consumption of blueberry and cranberry significantly reduced fasting blood glucose and glycated hemoglobin levels in individuals with diabetes, with high credibility of the evidence.