Complement C3 is a risk factor for the development of diabetes: a population-based cohort study.

Cross-sectional studies have reported strong correlations between plasma levels of complement C3, insulin, and glucose. This prospective study explored whether elevated levels of C3, C4, and other inflammation-sensitive plasma proteins (ISPs; fibrinogen, orosomucoid, alpha1-antitrypsin, haptoglobin, and ceruloplasmin) are associated with the development of diabetes. Plasma proteins were measured in 2,815 nondiabetic healthy men, age 38-50 years, who were reexamined after a mean follow-up of 6.1 years. Diabetes development (n = 123) was studied in relation to baseline levels of plasma proteins. After adjusting for age, screening year, and glucose at baseline, the odds ratio (95% CI) for developing diabetes was 1.00, 2.4 (1.1-5.3), 2.9 (1.4-6.0), and 5.6 (2.8-10.9), respectively, for men with C3 in the 1st, 2nd, 3rd, and 4th quartiles (trend: P < 0.00001). Fibrinogen, haptoglobin, C4, and the number of elevated ISPs were also related to future diabetes in this model. Only C3 was significantly associated with diabetes development after further adjustments for potential confounders, including BMI, insulin, and other inflammatory markers. We concluded that the risk of developing diabetes is related to levels of complement C3.