Insulin action and insulinemia are closely related to the fasting complement C3, but not acylation stimulating...

An elevated C3 concentration has been reported in people with obesity, type 2 diabetes, hypertension, and dyslipidemia, and has been proposed to play a role in the development of atherosclerosis. We hypothesized that an elevated C3 concentration might be linked to insulin resistance and/or hyperinsulinemia, abnormalities commonly observed in association with the above conditions.

Fasting concentrations of C3 and acylation stimulating protein (ASP, C3adesarg), a cleavage product of C3 recently found to stimulate glucose uptake in vitro, were measured in 33 healthy nondiabetic Pima Indians (14 women and 19 men; age 27 +/- 1 and body fat 33 +/- 1%, means +/- SEM). Subjects were characterized for body composition dual-energy X-ray absorptiometry, insulin action (insulin-stimulated glucose disposal [M], hyperinsulinemic glucose clamp), and glucose tolerance (75-g oral glucose tolerance test).

Fasting C3 and ASP concentrations were positively correlated (r = 0.43, P < 0.05). Fasting C3 concentration was closely related to percent body fat (r = 0.77), M (r = -0.75), and fasting insulin concentration (r = 0.72) (all P < 0.0001). Fasting C3 concentrations remained significantly related to M and fasting insulin after adjusting for percent body fat (partial r = -0.53 and 0.33, both P < 0.05). In subjects with impaired glucose tolerance, fasting C3 concentrations were higher than in those with normal glucose tolerance--a difference that remained after adjustment for percent body fat and M. We found that fasting ASP concentrations were significantly related to percent body fat (r = 0.37, P < 0.05), but not to M or fasting insulin.

In Pima Indians, fasting C3 concentration is closely related to adiposity, insulin action, and fasting insulin levels and may thus be a mediator for the postulated link between obesity, insulin resistance, hyperinsulinemia, and possibly atherosclerosis.