Causal associations of waist circumference and waist-to-hip ratio with type II diabetes mellitus: new evidence from Mendelian randomization.

Type II diabetes mellitus (T2DM) is a metabolic disease with high incidence, which has seriously affected human life and health. The associations among waist circumference (WC), waist-to-hip ratio (WHR), and T2DM were discovered in observational studies. However, the causality of these associations still remains unknown. The present study aims to apply two-sample Mendelian randomization (TSMR) using genetic variants as instrumental variables (IVs) to evaluate the causality among WC, WHR, and T2DM. The participants were from three independent studies in genome-wide association studies (GWAS) datasets, which included 127,997 Europeans for WC, 73,137 Europeans for WHR and 659,316 Europeans for T2DM. Furthermore, 16 were associated WC SNPs and eight were associated WHR SNPs as instrument variables were selected for TSMR using P < 5 × 10-8 standard. The pooled odd ratios (ORs) for the assessment of higher WC and WHR on the risk of T2DM for these SNPs were calculated using inverse variance weighted (IVW) method, and validated through extensively complementary analyses. The OR for T2DM per SD (cm) higher WC was 2.623 (95% CI 2.286-3.010, P = 5.000E-43), and the OR for T2DM per SD (cm) higher WHR was 1.751 (95% CI 1.122-2.733, P = 0.014). Consistent results for other methods were obtained. Furthermore, the range of OR fluctuation between WC and T2DM was from 2.623 to 2.986, while that between WHR and T2DM was from 0.990 to 2.931. Overall, these present results provide genetic support that suggests that the use of TSMR, and higher WC and WHR increased the T2DM risk among the European population.