A new study found that the improvement in U.S. heart disease rates leveled off from 1999 through 2004, after 30 years of steady progress.

Lead researcher Dr. Francisco Lopez-Jimenez said that while there have been gains made in health resources and in addressing heart disease risk factors like lowering cholesterol and blood pressure and smoking cessation, these improvements could be upended by increases in obesity and diabetes.

The scientists looked at data on heart disease risk factors from three periods: 1976-1980, 1988-1994 and 1999-2004.

In comparing the data they found that the risk of heart disease dropped from 10 percent in the first period to 7.9 percent in the second. However, from the second period to the third, the risk factor only dropped to 7.4, which they call a flattening of the downward trend.

Even with the decline, heart disease is the leading cause of death for men and women in the U.S., according to the Centers for Disease Control and Prevention.

The CDC reports that in 2003, 37 percent of adults reported having two or more of the six risk factors – high blood pressure, high cholesterol, diabetes, smoking, physical inactivity and obesity – associated with heart disease and stroke.