The study found that patients diagnosed with high levels of low-density lipoprotein cholesterol often failed to make the necessary lifestyle changes, such as cutting down on saturated fat. (CBC)

Though levels of "bad" cholesterol have fallen dramatically among Americans, a high percentage of people with high cholesterol are still failing to get treatment.

A U.S. study found that in people 20 and older, levels of low-density lipoprotein cholesterol (LDL-C), considered to be more harmful than high-density lipoprotein cholesterol (HDL-C), fell by approximately one-third between 1999 and 2006. Prevalence of LDL-C in that age group went from 31.5 per cent in 1999-2000 to 21.2 per cent in 2005-2006.

The study, conducted by researchers at the U.S. Centers for Disease Control and Prevention, is published in the Nov. 18 issue of the Journal of the American Medical Association.

The authors of the study attribute the drop to the use of cholesterol-lowering medications. Yet despite the progress, more could be done to lower LDL-C levels in high-risk people, they say.

"It's very encouraging when LDL levels decrease," lead author Elena Kuklina told CBC News. "But on the other hand, when you look at people, especially those at high risk, it's discouraging."

"Nearly one in five people in the U.S. have high LDL levels."

High levels of LDL-C can lead to a build-up of plaque in the arteries, which can narrow them, leading to a condition called atherosclerosis. This condition can increase a person's risk of stroke and heart attack, according to the Heart and Stroke Foundation of Canada.

The study involved 7,044 participants who were divided into three groups based on their relative risk of developing atherosclerosis: high risk, intermediate risk and low risk. Factors such as evidence of heart disease, high blood pressure, obesity, level of physical activity and levels of LDL-C determined in which category a participant was placed.

High levels of LDL-C were classified as equal to or greater than 100 mg/dL for those people deemed high risk; as equal to or greater than 130 mg/dL for those of intermediate risk and equal to or greater than 160 mg/dL for people considered low risk.

People categorized as high risk had the greatest prevalence of high LDL-C levels. In 1999-2000, 69.4 per cent of high-risk participants had high LDL-C levels versus 58.9 per cent in 2005-2006. To be deemed high risk, participants had to have a self-reported history of coronary heart disease, angina, heart attack, stroke or diabetes, and to have a fasting blood sugar level of 126 mg/dL.

In this high-risk category, one-fifth of the study's participants qualified for cholesterol-lowering drugs but did not receive them in 2005-2006. The authors suggest that many people with high levels of LDL-C are not screened often enough, or their doctors fail to adequately assess their cholesterol levels and prescribe lipid-lowering medications.

"In a study of 500 randomly selected U.S. physicians, almost half failed to classify their patients' risk levels correctly," write the authors.

Finally, patients diagnosed with high levels of LDL-C often failed to make the necessary lifestyle changes to lower their levels. These included consuming more fibre, soy and nuts, cutting back on saturated fat and becoming more physically active.