Proinsulin-to-C-peptide ratio as a marker of beta-cell function in African American and European American adults

Objective: The primary purpose of the present study was to test the hypothesis that the proinsulin-to-C-peptide ratio (PI:CP), 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 OGTT 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 PI:CP with each phase of beta-cell response (Φb, Φd, Φs, Φtot), 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:CP levels, before and after adjustment for insulin sensitivity. 

Results: African American participants had higher fasting and 2-h PI:CP. Fasting PI:CP was positively associated with Φb, and fasting PI:CP and 2-h PI:CP were inversely associated with DId and insulin sensitivity only in African American participants. 

Conclusions: PI:CP could be useful in identifying African American individuals at highest risk for beta-cell dysfunction and ultimately type 2 diabetes.