Researchers from the University of California at San Diego have found that when proinsulin levels rise, so does the risk of heart disease for both men and women. The good news is a blood test is all that's needed to make the determination.
One of the newest "buzz" words in medical circles is proinsulin, a hormone that helps the body make insulin, which clears sugar from the bloodstream. In the current online edition of Circulation, scientists present the newest of several studies showing the importance of proinsulin as a predictor of heart disease."This is a relatively new area of study and we don't know a lot about proinsulin, but what we do know seems to indicate that it is in some way connected to cardiovascular disease," says study co-author Dr. Elizabeth Barrett-Connor, a professor of family and preventive medicine at the University of California at San Diego.
Indeed, the study found men and women with increased levels of proinsulin had double the risk of heart disease than those who had normal blood levels of this hormone.
While previous studies believed it was high levels of insulin that increased the risk of heart disease, those findings only proved true in men.
Studies began focusing on the effects of proinsulin in the 1990s, when the first links between the hormone and heart disease were established.
Unfortunately, study statistics were never broken down according to gender, which is what makes Barrett-Connor's research so important: It is the first to show that elevated levels of proinsulin can double the risk of heart disease in both sexes.
For New York endocrinologist Dr. Valerie Peck, the findings are intriguing, but far from conclusive.
"It's an interesting study, but there just isn't enough data to know if they have really hit on a significant cause of heart disease, and I'm especially concerned with the methods they used to determine who had heart disease," says Peck, adding she's unsure if medical records and cardiograms were enough to draw accurate conclusions.
The current research involved 1,456 middle-aged and elderly white people, originally part of the Rancho Bernardo (California) Heart and Chronic Disease study, begun more than 25 years ago by Barrett-Connor. In this group, there were 554 men and 902 women, all of whom tested negative for diabetes.
The researchers evaluated a body of data on each of the participants, including past medical records, particularly cardiograms, as well as blood samples. They looked for three key factors: levels of insulin, proinsulin and C-peptide, a natural chemical byproduct of insulin production.
What they found: About one-quarter of the group had heart disease, with equal distribution among men and women. More important, each of these people were also found to have significantly higher levels of proinsulin than the heart-healthy patients in the study. This finding held true in equal numbers for both men and women.
Barrett-Connor reports an increase in proinsulin levels more than doubled the risk of heart disease in men, with only slightly less impact in women. The finding held up even after "we took into consideration other risk factors for heart disease, including age, weight, blood pressure and cholesterol."
The study also found some evidence indicating C-peptide levels could point to increased risk of heart disease, perhaps more reliably than insulin.
While researchers aren't sure how proinsulin encourages heart disease, Barrett-Connor believes it may be due to its effect on clotting.
"Some people believe it's associated with blood-clotting factors, some think it is itself a clotting factor, but we don't know. This is a fairly new area of research," says Barrett-Connor, who adds she believes proinsulin may impair the body's ability to break up blood clots.
For Peck, the findings show the need for more research, but she is less convinced that proinsulin is an initiating factor for heart disease.
"It could turn out that proinsulin levels rise in response to heart disease, rather than initiating it. So, while rising levels may help us to pinpoint those who have heart disease, I'm not sure it can tell us who is going to get heart disease soon enough to allow for a significant difference in either medical care or lifestyle intervention," Peck says.