Aims/hypothesis: Higher concentrations of insulin correlate with several coronary heart disease (CHD) risk factors and have been shown to predict incident CHD in several studies, leading to hypotheses concerning the proatherogenic properties of insulin. However, in cross-sectional studies, relationships of concentrations of the insulin precursor molecules, proinsulin and des 31, 32 proinsulin, relate as strongly, or more strongly, to levels of risk factors and to (prevalent) CHD.
Methods: We investigated the relationship between concentrations of insulin, measured with a specific assay, and of proinsulin-like molecules, and risk factors in 1181 non-diabetic men 50-64 years old during Phase II of the Caerphilly Study. We also related concentrations of these molecules to incident CHD during the 10-14 years follow-up.
Results: The relationship between concentrations of insulin, of proinsulin and of des 31, 32 proinsulin and BMI ( r = 0.36-0.45), triglyceride (r = 0.25-0.31), high density lipoprotein- (HDL-) cholesterol (r = -0.17 to -0.21), systolic ( r = 0.05-0.11) and diastolic blood pressure ( r = 0.11-0.15) were similarly close, those with risk factors being somewhat and similarly reduced after adjustment for BMI. The correlation between insulin and of proinsulin-like molecules and those plasminogen activator inhibitor-1 (PAI-1) antigen was also similar (r = 0.28-0.29). There was a negative correlation between concentrations of proinsulin-like molecules - but not insulin - and birth weight. Insulin concentrations correlated positively with height ( r = 0.12). In logistic regression models, concentrations of proinsulin-like molecules, but not insulin, predicted incident of CHD over a follow-up of 10-14 years (insulin - standardised odds ratio (SOR) 1.30 (95 %-CI) 0.91, 1.85), p = 0.15; des 31, 32 proinsulin - SOR 1.38 (95 %-CI 1.02, 1.85), p = 0.034; sum of proinsulin-like molecules - SOR 1.54 (95 %-CI 1.07, 2.20), p = 0.019 after adjusting for age and BMI. The predictive ability of these molecules was reduced by around one third after adjustment for standard risk factors and concentrations of tryglyceride and HDL-cholesterol, and by about half after further adjustment for PAI-1 concentrations.
Conclusion/interpretation: We conclude that concentrations of proinsulin-like molecules provide a better way to predict the incidence of CHD than those of insulin. However, the lack of biological evidence for a causative relationship suggests an association through a common antecedent, and this antecedent is not likely to be intrauterine growth retardation.