Proinsulin-to-C-Peptide Ratio as a Marker of β-Cell Function in African American and European American Adults

OBJECTIVE The primary purpose of the current study was to test the hypothesis that the proinsulin-to-C-peptide (PI-to-CP) ratio, as an index of proinsulin secretion, would be higher and associated with indices of beta-cell function in African American adults relative to European American adults without type 2 diabetes. RESEARCH DESIGN AND METHODS Participants were 114 African American and European American adult men and women. A 2-h oral glucose tolerance test was conducted to measure glucose, insulin, C-peptide, and proinsulin and derive indices of beta-cell response to glucose. The Matsuda index was calculated as a measure of insulin sensitivity. The disposition index (DI), the product of insulin sensitivity and beta-cell response, was calculated for each phase of beta-cell responsivity. Pearson correlations were used to investigate the relationship of the PI-to-CP ratio with each phase of beta-cell response (basal, phi b; dynamic, phi d; static, phi s; total, phi tot), disposition indices (DId, DIs, DItot), and insulin sensitivity. Multiple linear regression analysis was used to evaluate independent contributions of race, BMI, and glucose tolerance status on PI-to-CP levels before and after adjustment for insulin sensitivity. RESULTS African American participants had higher fasting and 2-h PI-to-CP ratios. The fasting PI-to-CP ratio was positively associated with phi b, and the fasting PI-to-CP ratio and 2-h PI-to-CP ratio were inversely associated with DId and insulin sensitivity only in African American participants. CONCLUSIONS The PI-to-CP ratio could be useful in identifying African American individuals at highest risk for beta-cell dysfunction and ultimately type 2 diabetes.