Cholesterol is a necessary component of cell membranes and provides the structural backbone for the production of steroid hormones, and a minimal level is essential for good health. However, high levels of one type of cholesterol in the blood, called low-density lipoprotein or LDL, can lead to heart disease including atherosclerosis. High levels of high-density lipoprotein or HDL, the "good" cholesterol, combat the negative effects of LDL by helping to remove it from the blood, and protect against atherosclerosis.
Atherosclerosis
A specific type of arteriosclerosis, or hardening of the arteries, atherosclerosis occurs when fat build-up inside of blood vessels blocks the smooth flow of blood. In addition to constricting blood flow and increasing blood pressure, the fatty plaques can break loose as dangerous blood clots, potentially causing a heart attack or stroke. The fatty plaques that block the blood vessels primarily consist of LDL cholesterol that deposits on the blood vessel walls when circulating levels are too high.
HDL and LDL
Because cholesterol is fatty, it does not dissolve into the blood. Instead it circulates in the blood bound to carrier molecules called lipoproteins. The vast majority of cholesterol binds to LDL in the blood, while the remaining one-third to one-fourth binds to HDL. LDL primarily functions to deliver cholesterol into cells, binding to LDL receptors with a protein constituent called apolipoprotein-B. HDL does not have apolipoprotein B, so it cannot deliver cholesterol to cells. Instead, HDL primarily removes cholesterol from cells.
HDL Levels
Levels of HDL less than 40 ng/dL for men and less than 50 ng/dL for women are considered unhealthy and increase the risk of heart disease. An HDL level greater than 60 ng/dL is protective against heart disease including atherosclerosis.
HDL and Atherosclerosis
High levels of LDL and low levels of HDL are independent risk factors for heart disease. High levels of HDL protect against heart disease and counteract the negative effects of LDL. While LDL molecules can deposit cholesterol on the walls of damaged arteries and form dangerous plaques, HDL molecules generally carry cholesterol away from blood vessels and deliver it to the liver, where it eventually is processed to leave the body. HDL might be able to remove cholesterol from plaques as well, reducing their size and the severity of their effects.
Boosting HDL
According to the Mayo Clinic, as little as 30 minutes of exercise several times a week can help boost levels of HDL. Quitting smoking can also increase HDL levels. Some medications that lower total cholesterol also increase HDL levels. Examples include niacin and fibrates.
References
- MayoClinic.com: Arteriosclerosis/atherosclerosis
- American Heart Association: Good vs. Bad Cholesterol
- "Journal of Lipid Research"; Structure of apolipoprotein B-100 in low density lipoproteins; Jere Segrest et al.; September 2001
- American Heart Association: What Your Cholesterol Levels Mean
- MayoClinic.com: High cholesterol
- MayoClinic.com: Cholesterol medications
