Vitamin D Insufficiency Predicts Elevated Levels of Complement 3 Independent of Insulin Resistance and BMI

This study was to determine serum 25-hydroxyvitamin D (25(OH)D), the complement 3 (C3), and C-reactive protein (CRP) levels, and their association with the risk of insulin resistance (IR). A case-control study was carried out among 134 participants with body mass index (BMI) ≥30 kg/m² and BMI=18.5-24.99 kg/m². Anthropometric and body composition indicators were measured. Serum levels of C3, CRP, 25(OH)D, insulin, and glucose were also measured. IR was assessed by the homeostasis model assessment (HOMA-IR). C3, CRP, insulin, and HOMA-IR levels were higher in participants with obesity than that of controls (p<0.001). After adjustment for the potential confounders, anthropometric and body composition indicators were correlated positively with C3 (p<0.001), and negatively with 25(OH)D (p<0.05). C3, and 25(OH)D were correlated with HOMA-IR (r=0.350; r=-0.212; p<0.05). In logistic regression analyses, C3 and CRP were significantly related to increased odds of IR among participants with obesity as compared to controls after progressively adjusting for the potential confounders (p<0.001), whereas 25(OH)D was negatively, but insignificantly, related to decreased odds of IR among participants with obesity (p>0.05). C3 was associated positively with 25(OH)D insufficiency/deficiency independent of HOMA-IR and/or BMI (β=0.183, p<0.05). Obesity is associated with elevated levels of proinflammatory biomarkers and IR. 25(OH)D insufficiency/deficiency was associated with C3 regardless of HOMA-IR or BMI, which could in turn, have a role in the augmentation of IR during obesity.