A low level of high-density lipoprotein, or HDL cholesterol, in the blood is a risk factor for cardiovascular disease, heart attack and stroke. Scientific studies indicate that HDL can remove cholesterol from fatty plaques in artery walls. However, recent evidence challenges the theory that high HDL cholesterol levels will uniformly lower heart attack risk. Several subtypes of HDL circulate in the blood. The relationship among HDL particle subtype, quantity, composition and function and cardiovascular risk is a complex and ongoing topic of investigation.
Low HDL and Heart Disease
HDL carries 25 to 33 percent of total blood cholesterol. Other lipoproteins -- including LDL or "bad" cholesterol -- carry the balance of blood cholesterol. In contrast to the other lipoprotein carriers, HDL is often referred to as the "good" cholesterol carrier. The different types of HDL may remove cholesterol from artery walls and slow the disease process of atherosclerosis -- a buildup of cholesterol-rich fatty plaques.
A blood HDL level less than 40 mg/dL for men and 50 mg/dL for women increases heart disease risk, according to the American Heart Association. A study published in August 2012 in the "Journal of the American College of Cardiology" suggests that a large number of HDL particles confer protective benefits.
Raising HDL Cholesterol
An analysis of 68 long-term population-based studies involving more than 300,000 people found that a 15 mg/dL or higher HDL level with an 80 mg/dL lower non-HDL cholesterol level was associated with a 65 percent reduction in heart disease risk, according to a study published in November 2009 in the "Journal of the American Medical Association."
Lifestyle changes -- such as quitting smoking, losing weight, increasing physical activity, eating more unsaturated fat and less saturated fat and using alcohol in moderation -- have been suggested to increase HDL. Nevertheless, having high HDL cholesterol does not always mean a lower risk of heart attack, especially if hereditary factors cause the high HDL.
Reverse Cholesterol Transport
A primary function of HDL is to transport cholesterol from body tissues to the liver for secretion into the small intestine with bile -- a process called reverse cholesterol transport. Proteins in HDL particles interact with other proteins in tissue cells that mediate cholesterol transfer from cells to HDL particles.
Although cholesterol transfer from cells to HDL cannot be directly measured inside people, laboratory research suggests that HDL can remove cholesterol from sites of fatty plaque accumulation in artery walls. A higher capacity of isolated cells to transfer cholesterol to HDL rather than a person’s HDL cholesterol level may be associated with decreased atherosclerosis.
The liver and small intestine produce and secrete very small protein-rich HDL that accumulates cholesterol, triglycerides and phospholipids, growing in size as they circulate in the blood. The various subtypes of HDL are classified according to size. Larger HDL particles contain more cholesterol than smaller particles.
People with coronary artery disease are more likely to have higher levels of the smaller HDL subtypes. All HDL subtypes can remove cholesterol from tissues, including artery walls. However, larger HDL subtypes contain a protein called apoE, which permits HDL particles to take up larger amounts of cholesterol. ApoE also facilitates cholesterol delivery to the liver for export with bile.