DALLAS, Texas, Aug. 4, 1998 -- Many people with diabetes develop heart disease, but a new study says that determining who has high levels of insulin in the blood -- a condition that precedes diabetes -- may better predict who is at risk for having a heart attack.
In today's Circulation: Journal of the American Heart Association, Finnish researchers say measuring a person's extent of hyperinsulinemia -- a condition that occurs when the body's cells are unable to effectively use the hormone insulin to break down sugar in food -- was superior to measuring other risk factors in determining whether individuals would have heart attacks.
"Over 22 years of follow-up, the predictive power of insulin levels was of the same magnitude as that of cholesterol levels," says the study's senior author, Kalevi Pyorala, M.D., of the department of medicine at the University of Kuopio in Finland. "With additional adjustment for other risk factors, insulin levels remained, with the exception of the first five years, a significant independent predictor of heart attack risk."
When compared to other risk factors, insulin levels were the most statistically significant predictor of heart attack risk during the study. As participants in the study grew older, blood pressure became a significant predictor. After the first 10 years of the study, smoking predicted heart attack risk. The researchers say that body mass index, blood levels of the fat triglyceride, and physical inactivity showed no association to heart attack risk.
The study was based on a group of 970 men, 34 to 64 years of age, who were free of cardiovascular disease and diabetes. During the 22-year span of the study, 164 of them had a heart attack.
At the first five-year follow-up, the scientists found that those people with the highest levels of insulin were more than three times as likely to have a heart attack as those with the lowest levels of insulin. In a follow-up examination 10 years after the study began, those with highest insulin levels were at 2.7 times greater risk than those at the lowest levels.
Although hyperinsulinemia was shown to be a strong predictor of heart attack risk, its ability to predict future heart attacks diminished over time. The predictability of risk by use of insulin levels continued to slide during the later years of the study. Fifteen years after the study began, the risk had dropped to 2.1 times greater risk, and at 22 years, those with high insulin levels were at 1.6 times greater risk.
"A relatively strong association of hyperinsulinemia to heart attack risk during the early part of the follow-up, as was the case in our study, may lead to selective morbidity and mortality, which could weaken the predictive value of hyperinsulinemia," says Pyorala. "People with the highest insulin levels may die and become ill sooner than those with normal levels of the hormone.
"Second, the cumulative impact of other risk factors may override the impact of hyperinsulinemia or insulin resistance during long follow-up periods."
The researchers say that this study adds to the growing body of evidence showing that diabetes and insulin levels are important predictors of heart attack risk. However, they point out that the various findings of these studies raise more questions for research.
"Although the results of our study clearly demonstrated a statistically independent association of hyperinsulinemia to heart attack risk, we want to emphasize that this association may still be explained through other factors clustering with hyperinsulinemia and insulin resistance," says Pyorala. "On the basis of present evidence, doctors do not necessarily need to screen patients for hyperinsulinemia."
The American Heart Association says that people with diabetes can avoid or delay heart disease and stroke by controlling weight and blood cholesterol with a low-saturated-fat, low-cholesterol diet and regular exercise coupled with lowering blood pressure and not smoking.
Co-authors are Marja Pyorala, M.D.; Heikki Miettinen, M.D.; and Markku Laakso, M.D.