Flavonoid subclasses and type 2 diabetes mellitus risk: a meta-analysis of prospective cohort studies

Epidemiological studies have suggested controversial associations between flavonoid subclasses and type 2 diabetes mellitus (T2DM) risk. The aim of the present meta-analysis was to quantitatively estimate these associations with prospective cohort study. A systematic literature search in PubMed and Scopus databases was performed up to May 2018. Multivariate-adjust relative risks (RRs) with corresponding 95% confidence intervals (CIs) for the highest versus the lowest category were pooled by using a random-effects model. Using restricted cubic spline regression model, non-linear dose-response analysis was estimated. Nine independent prospective cohort studies with 172,058 participants and 16910 events were included. Dietary intakes of flavanols, flavonols, flavan-3-ols and isoflavones were inversely associated with T2DM risk, and the summary RRs were 0.86 (95%CI: 0.77, 0.97), 0.91 (95%CI: 0.85, 0.98), 0.90 (95%: 0.82, 0.99) and 0.91 (95%CI: 0.84, 0.98), respectively. Dose-response analysis showed that 135 mg/day increment of flavanols (95%CI: 0.92, 0.96; P for trend <0.001), 50 mg/day increment of flavonols (95%CI: 0.88, 0.99, P for trend = 0.021), 68 mg/day increment of flavan-3-ols (95%CI: 0.92, 0.96, P for trend <0.001), or 1.8 mg/day increment of isoflavones (95%CI: 0.92, 0.97, P for trend <0.001) were associated with 6% reduction in T2DM risk. Non-significant association was observed with respect to flavanones and flavones. The present meta-analysis provides substantial evidence that dietary intakes of flavanols, flavonols, flavan-3-ols and isoflavones were inversely associated with T2DM risk, respectively. Higher dietary intakes of flavanol-, flavonol-, flavan-3-ol- and isoflavone-foods would have beneficial effects for protection against T2DM.