Higher fasting insulin but lower fasting C-peptide levels in African Americans in the US population

Background Fasting serum insulin and fasting serum C‐peptide are risk factors for developing type 2 diabetes. Because of the higher incidence of type 2 diabetes in African Americans and Hispanic Americans, it is likely that these groups may differ from non‐Hispanic whites in their levels of insulin and C‐peptide. Methods We analyzed data from a nationally representative sample of adults in the US population for whom sociodemographic, clinical, and laboratory information were obtained. The data were used to describe distributions of fasting insulin and fasting C‐peptide in non‐Hispanic white, non‐Hispanic black, and Mexican American men and women aged ⩾20 years without a medical history of diabetes. Results Among men, Mexican Americans had higher insulin values than non‐Hispanic whites and blacks. Among women, both Mexican Americans and blacks had higher insulin values than whites. For C‐peptide, differences by sex and race‐ethnicity paralleled those seen for fasting insulin with the exception that black men had significantly lower C‐peptide values than whites and Mexican Americans. After adjustment for age, fasting plasma glucose (FPG), body mass index (BMI), and waist‐to‐hip ratio (WHR), the higher levels for insulin in blacks and Mexican Americans remained; both black men and women had significantly lower C‐peptide values than whites and Mexican Americans. The molar ratio of fasting C‐peptide to fasting insulin was similar for men and women in each race‐ethnic group. However, blacks had substantially lower ratios than whites and Mexican Americans. Conclusions We found wide variations in fasting insulin and fasting C‐peptide levels by race and ethnicity in US adults that were not explained by confounding factors, primarily measures of obesity. Most notably, the higher fasting insulin and lower fasting C‐peptide levels in blacks implies that there is a derangement in insulin clearance and an impairment in beta‐cell function in blacks compared with whites and Mexican Americans. Published in 2002 by John Wiley & Sons, Ltd.