Legume and soy intake and risk of type 2 diabetes: a systematic review and meta-analysis of prospective cohort studies

Background: Previous findings on the associations of legume and soy intake with the risk of type 2 diabetes are conflicting. Objective: We aimed to summarize the longitudinal associations between legume and soy intake and risk of type 2 diabetes. Methods: We searched for relevant prospective cohort studies in PubMed, EMBASE, and Ovid up to August 2019. Study-specific, multivariable-adjusted RRs and 95% CIs were pooled by random-effects models. Results: We identified 15 unique cohorts including 565,810 individuals and 32,093 incident cases. The summary RRs (95% CIs) of incident type 2 diabetes were 0.95 (0.79, 1.14; NS) for total legumes, 0.83 (0.68, 1.01; NS) for total soy, 0.89 (0.71, 1.11; NS) for soy milk, 0.92 (0.84, 0.99) for tofu, 0.84 (0.75, 0.95) for soy protein, and 0.88 (0.81, 0.96) for soy isoflavones, respectively. High heterogeneity was found for total legumes (I-2 = 84.8%), total soy (I-2 = 90.8%), and soy milk (I-2 = 91.7%). Potential sources of heterogeneity were not evident for total legumes or soy milk, whereas for total soy, geographic location (Asia, United States; P = 0.04) and study quality (high, moderate, or low; P = 0.02) significantly predicted heterogeneity. In dose-response analysis, significant linear inverse associations were observed for tofu, soy protein, and soy isoflavones (all P < 0.05). Overall quality of evidence was rated as moderate for total legumes and low for total soy and soy subtypes. Conclusions: Dietary intakes of tofu, soy protein, and soy isoflavones, but not total legumes or total soy, are inversely associated with incident type 2 diabetes. Our findings support recommendations to increase intakes of certain soy products for the prevention of type 2 diabetes. However, the overall quality of evidence was low and more high-quality evidence from prospective studies is needed. This trial was registered as PROSPERO CRD42019126403 (https://www.crd.york.ac.uk/PROSPERO).