Macronutrient Intakes and Development of Type 2 Diabetes: A Systematic Review and Meta-Analysis of Cohort Studies

Objective: To synthesize the best available evidence on the association between macronutrient intake and type 2 diabetes risk. Data Sources: MEDLINE, EMBASE, CINAHL, Cochrane Library, ProQuest, Mednar, and the JBI Library of Systematic Reviews were searched up to July 2012 to identify published and unpublished studies. The review was restricted to human participants only but was not restricted by date or by language. Study Eligibility: Studies were included in the review if they were a cohort study examining the relationship between dietary macronutrient intake and type 2 diabetes risk, included healthy participants with no history of type 2 diabetes at the baseline assessment, and reported risk estimates (odds ratios, hazards ratios, or relative risks [RRs]) and corresponding 95% confidence intervals for type 2 diabetes risk by comparison of the highest with the lowest level of macronutrient consumption. Methods: Data extraction and risk of bias assessments were performed in duplicate by 2 reviewers. Random-effects meta-analyses were performed to pool RR estimates from individual studies to assess the relationship between dietary macronutrient (carbohydrate, fat, protein, and macronutrient subtypes) intake and type 2 diabetes risk. Statistical heterogeneity was assessed using the I-2 statistic. Sensitivity analyses were performed to assess robustness of results, and publication bias was evaluated by the visual inspection of funnel plots and was formally assessed using Egger's test. Results: Twenty-two relevant cohort studies were eligible for inclusion in this review. High intake of total dietary carbohydrate was associated with an increased type 2 diabetes risk (relative risk [RR] = 1.11, 95% confidence interval [CI]: 1.01 to 1.22, p = 0.035); however, this effect was not observed in an analysis stratified by gender. High vegetable fat intake was associated with a reduced type 2 diabetes risk in females (RR = 0.76, 95% CI: 0.68 to 0.85, p < 0.001). Other macronutrients were not significantly associated with type 2 diabetes risk. Conclusions and Implications: The results of this systematic review and meta-analysis indicate that total carbohydrate is associated with an increased risk of type 2 diabetes; however, this effect was not observed in an analysis stratified by gender. High vegetable fat intake may decrease type 2 diabetes risk in females. There is a need for further well-designed prospective cohort studies to examine the potential association between macronutrient intake and type 2 diabetes risk.