The association between low sex hormone binding globulin and increased risk of type 2 diabetes is mediated by increased visceral and liver fat: results from observational and Mendelian randomization analyses

The aim of this study was to investigate the associations between sex hormone binding globulin (SHBG), visceral fat (VAT), liver fat content, and risk of type 2 diabetes. In the Netherlands Epidemiology of Obesity study 5690 women (53%) and men without pre-existing diabetes were included and followed for incident type 2 diabetes. SHBG concentrations were measured in all, VAT with MRI, and liver fat content with proton-MR spectroscopy in n=1822. We examined associations between SHBG and liver fat with linear regression and bidirectional Mendelian randomization analyses, and between SHBG and type 2 diabetes with Cox regression adjusted for confounding, and additionally for VAT and liver fat to examine mediation. The mean(SD) age was 56(6) years, BMI 30(4) kg/m², median(IQR) SHBG was 47 (34,65) nmol/L in women and 34 (26,43) nmol/L in men, median(IQR) liver fat was 3.4(1.6,8.2)% in women and 6.0 (2.9,13.5)% in men. Compared with the highest SHBG quartile, liver fat was 2.9-fold (95%CI: 2.4,3.4) increased in women and 1.6-fold (95%CI: 1.3,1.8) in men, and the hazard ratio (95%CI) of type 2 diabetes was 4.9 (2.4,9.9) in women and 1.8 (1.1,2.9) in men. Genetically predicted SHBG was associated with liver fat content (women: SD (95%CI) -0.45(-0.55,-0.35), men: ln(95%CI) -0.25 (-0.34,-0.16)). VAT and liver fat together mediated 43% (women) and 60% (men) of the SHBG-type 2 diabetes association. To conclude, in a middle-aged population with overweight, the association between low SHBG and increased risk of type 2 diabetes was for a large part mediated by increased VAT and liver fat.