[Why is insulin tied to the prevalence of cardiovascular diseases without being a risk factor for their incidence?]

Insulin is frequently considered to be a risk factor for atherosclerosis (or for coronary and vascular disease). Furthermore, hyperinsulinaemia is claimed to be the primary cause underlying the other features which make up the insulin resistance syndrome. However, if proof of these assertions is based only on prospective studies, its value is limited. Only two studies, both carried out, surprisingly, in policemen, have shown convincingly that insulin was a coronary risk factor. In one of the studies, the Paris Prospective Study, the insulin-coronary disease correlation was shown to subside with increasing duration of follow-up. The other prospective studies have failed to evidence a correlation between insulinaemia and cardiovascular events, even with univariate analysis. One study even showed a negative correlation between insulinaemia and coronary complications. In view of the fact that insulinaemia has been shown repeatedly to be associated with classic cardiovascular risk factors--systolic hypertension, decrease in HDL cholesterol, increase in triglycerides, and abdominal obesity--it is highly surprising that univariate analysis has not been able to show the same correlation between insulin and cardiovascular complications. In fact, the combination of elevated insulinaemia and classic risk factors may result in protection against the deleterious effects of these factors. Another possibility would be that insulinaemia is associated with unknown protective factors. Both hypotheses would account for the existence of a correlation between insulin and current cardiovascular disease, as well as the absence of correlation between insulin and later onset of cardiovascular disease.