Hypovitaminosis D Is Associated with Liver Insulin Resistance in Obese Subjects

Hypovitaminosis D is highly prevalent in obese subjects. Serum 25-hydroxy vitamin D3 [25(OH)D] concentration, the best marker of human vitamin D status, has been reported to be associated with glucose status, insulin resistance (IR) and beta cell function. To specifically investigate the relationship between 25(OH)D and liver IR we performed a comprehensive metabolic assessment (2-h OGTT, hyperinsulinemic euglycemic clamp [HEC], body composition by DXA) in 20 obese non-diabetic subjects (42.9±2.7 yrs; BMI 37.7±0.8 kg/m2) with 25(OH)D insufficiency (<30 ng/mL). Liver IR was estimated using the index by Vangipurapu et al. (-0.091 + [log insulin AUC 0-120 min x 0.400] + [log fat mass% x 0.346] - [log HDL cholesterol x 0.408] + [log BMI x 0.435]). There was a significant inverse correlation between 25(OH)D and the liver IR index (r = -0.514, p = 0.02). This correlation maintained its significance after adjusting for age, gender, total cholesterol, triglycerides and whole body insulin sensitivity (M value assessed by HEC) in multiple linear regression analysis. There was no significant correlation between 25(OH)D and beta cell function estimated by the Disposition Index. Our data suggest that, in obese subjects, low 25(OH)D levels are independently associated with liver insulin resistance, but not with beta cell function. Further studies are needed to clarify the relationship between glucose homeostasis and vitamin D levels.