Vegetarian diets and glycemic control in diabetes: a systematic review and meta-analysis

Introduction: Previous studies have suggested an association between vegetarian diets and improvements in glycemic control in diabetes, although this relationship is not well established. No meta-analysis of these studies has been performed. Methods: To conduct a systematic review and meta-analysis of controlled clinical trials examining the association between vegetarian diets and glycemic control in type 2 diabetes. Data source: The electronic databases Medline, Web of Science, Excerpta Medica Database (EMBASE), and Cochrane Central Register of Controlled Trials were searched for articles published in any language through December 9, 2013. Study selection: The following criteria were used for study inclusion: (I) age of participants > 20 years; (II) vegetarian diet as intervention; (III) mean difference in hemoglobin A1c (HbA1c) and/or fasting blood glucose levels used as outcomes; and (IV) controlled trials, duration >= 4 weeks. Exclusion criteria were: (I) not an original investigation; (II) duplicate samples; (III) diabetes other than type 2; (IV) multiple interventions; and (V) uncontrolled studies. Data extraction and synthesis: The data collected included study design, baseline population characteristics, dietary data, and outcomes. Data were pooled using a random-effects model. Main outcomes and measures: Differences in HbA1c and fasting blood glucose levels associated with vegetarian diets were assessed. Results: Of 477 studies identified, six met the inclusion criteria (n= 255, mean age 42.5 years). Consumption of vegetarian diets was associated with a significant reduction in HbA1c [-0.39 percentage point; 95% confidence interval (CI), -0.62 to -0.15; P= 0.001; I-2= 3.0; P for heterogeneity = 0.389], and a non-significant reduction in fasting blood glucose concentration (-0.36 mmol/L; 95% CI, -1.04 to 0.32; P= 0.301; I-2= 0; P for heterogeneity = 0.710), compared with consumption of comparator diets. Conclusions: Consumption of vegetarian diets is associated with improved glycemic control in type 2 diabetes. PROSPERO registration number is CRD42013004370.