Fasting plasma C-peptide levels in health and impaired glucose tolerance: relations to blood glucose and relative body weight.

Pancreatic B-cell function was studied as part of a health control examination by measuring fasting plasma C-peptide concentration in 433 44-55 year-old males with normal glucose tolerance. Fasting C-peptide levels were correlated with relative body weight (r = 0.48) and fasting blood glucose concentrations (r = 0.43), yielding a multiple correlation coefficient of 0.56, and the multiple regression equation: FCP (nmol/l) = -0.89 + 0.61 X RBW + 0.16 X FBG (mmol/l). (FCP = fasting plasma C-peptide, RBW = relative body weight, FBG = fasting blood glucose). In 26 subjects with impaired glucose tolerance, fasting plasma C-peptide levels were even more strongly correlated with relative body weight (r = 0.63) and fasting blood glucose concentrations (r = 0.47). Subjects older than 52 years had a significantly higher fasting C-peptide level than younger subjects (p less than 0.01). In the 26 subjects with impaired glucose tolerance, fasting plasma C-peptide levels were not significantly different from those in the 433 men with normal glucose tolerance. However, when compared to a group with normal glucose tolerance matched for relative body weight, the subjects with impaired glucose tolerance had an elevated fasting blood glucose level (p less than 0.01) without difference in C-peptide level, suggesting a reduced insulin sensitivity. It is concluded that, in order to evaluate B-cell secretory function by determining fasting plasma C-peptide concentration, the relative body weight and simultaneous blood glucose concentration should be taken into consideration.