People who are just moderately overweight have an increased risk developing heart disease, even if they are otherwise healthy, according to pooled data from published studies.

As study chief Dr. Rik P. Bogers noted in an email to Reuters Health, the data show that "even if overweight and obese persons succeeded in lowering blood pressure and cholesterol to normal levels, they would still have a higher risk of coronary heart disease than their normal-weight peers."

Thus, the worldwide increase in the number of people who are moderately overweight "may drive the incidence of coronary heart disease upward," Bogers and colleagues warn in a report in the Archives of Internal Medicine.

They reviewed 21 studies that evaluated the impact of being overweight or obese on the heart. When the data were pooled together, a total of 18,000 heart-related events or deaths occurred among 302,296 study participants.

After factoring out the effect of age, sex, activity level and smoking, people who were moderately overweight, with a body mass index (BMI) between 25 and 29.9, had a 32-percent increased risk of heart disease, compared to people who were of normal weight. The body mass index is the ratio between height and weight, commonly used to determine if an individual is within a healthy weight range.

People who were obese, with a BMI of 30 or higher, had an 81 percent increased risk of heart disease compared with their normal-weight counterparts.

"Adverse effects of overweight or obesity on blood pressure and cholesterol levels could not totally explain this link," noted Bogers, who is from the National Institute for Public Health and the Environment, Bilthoven, the Netherlands.

"After we took into account blood pressure and cholesterol levels, those who were moderately overweight still had a 17-percent increased risk of coronary heart disease and those who were obese had a 49 percent increased risk," the researcher noted.

Moreover, for every 5-unit increase in BMI, the risk of heart disease increased by 29 percent before factoring in blood pressure and cholesterol and by 16 percent after these two heart disease risk factors were considered.

The findings imply that even with optimal treatment for high blood pressure and high cholesterol levels, overweight people still have an increased risk of developing heart disease, the investigators warn.

It could be that being overweight promotes a state of constant low-grade inflammation, problems with blood vessel function or an imbalance in blood chemicals that could lead to more clotting - all of which may contribute to the development of heart disease.