Cholesterol is a substance found in cell membranes that is essential for proper health, but too much cholesterol can contribute to heart disease. Lipoproteins, or lipids, carry cholesterol in the blood. There are several types of lipoproteins, including low-density lipoproteins, or LDL, and high-density lipoprotein, or HDL. When HDL carries cholesterol, the structure is called high-density lipoprotein cholesterol, or HDL cholesterol.
Structure
Of the several types of lipoproteins, HDL is the smallest and densest, as it contains the most protein, according to Neil Campbell, Ph.D., in "Biology." HDL can pick up cholesterol in the arteries and blood. An enzyme changes the molecular structure of the cholesterol, so HDL can transport it. The HDL also changes shape from straight-like to spherical as it picks up cholesterol. HDL increases in size as it picks up more cholesterol in the bloodstream.
Function
According to Campbell, HDL carries cholesterol it picked up from the arteries and blood to the liver or to organs in the body that produce hormones made from cholesterol, such as the ovary, testes and adrenal gland. The cholesterol brought to the liver is excreted into the bile and is converted into bile acids, which process fats that are eaten.
HDL Cholesterol and Health
HDL cholesterol is sometimes called the "good" cholesterol. Cholesterol and other substances can build up along artery walls, causing a narrowing and hardening of blood vessels, or atherosclerosis. Atherosclerosis can lead to such conditions as high blood pressure, heart attack and stroke. The ability of HDL to pick up cholesterol from the arteries and blood and deliver it to the liver and other organs for excretion or reuse is why HDL cholesterol is sometimes called the "good" cholesterol.
Measuring HDL Levels
A blood test can measure the amount of cholesterol contained in HDL. According to the American Heart Association, a HDL cholesterol concentration of 60mg/dL, is the "high" level and considered optimal for preventing heart disease. A HDL cholesterol level between 40 and 59mg/dL is the "medium" level. The "low" HDL cholesterol level is less than 40mg/dL for women and less than 50mg/dL for men. Increased risk for heart disease occurs at the "low" level.
Raising HDL Cholesterol Levels
Although clinical evidence suggests that raising HDL cholesterol levels can help protect against heart disease, the National Cholesterol Control Program recommends focusing on lowering LDL cholesterol levels to help improve health. LDL cholesterol is considered to be the "bad" cholesterol, as LDL deposits cholesterol along artery walls. To lower LDL cholesterol concentrations, the National Cholesterol Control Program suggests making lifestyle changes, such as increasing exercise, maintaining a healthy weight, eating less dietary fat, refraining from smoking and drinking alcohol in moderation.
References
- American Heart Association: Cholesterol
- "Biology"; Neil Campbell, Ph.D., Jane Reece, Ph.D., and Lawrence Mitchell, Ph.D.; 2001
- "Detection, Evaluation and Treatment of High Blood Cholesterol in Adults"; National Cholesterol Control Program of the National Institutes of Health; May 2001
