Increased blood cholesterol is associated with heart disease and stroke. The total cholesterol number, however, is not the only important lab value by which to evaluate the risk of disease. Certain subtypes of cholesterol may mean a higher risk of heart disease, while other subtypes of cholesterol can signal a lower risk of heart disease. Two of the subtypes of cholesterol are HDL and LDL.
Definition
HDL and LDL are lipoproteins: a combination of protein, cholesterol, and fats. HDL stands for high-density lipoprotein. LDL stands for low-density lipoprotein. These two substances are responsible for transporting cholesterol and fat back and forth between the liver and the tissues. The liver produces and degrades lipoprotein; the tissues use the cholesterol and lipids within the lipoproteins within their cells.
HDL
High-density lipoprotein consists of approximately 50 percent protein, and 50 percent cholesterol and other fats. HDL is secreted from both the liver and the intestine. Its primary role is scavenging excess cholesterol from tissues and transporting it back to the liver for reuse or disposal. This reduces the total amount of cholesterol in the body; it is thought that HDL may also scavenge cholesterol from the walls of blood vessels. Thus, HDL is known as "the good cholesterol."
LDL
Low-density lipoprotein represents about 50 percent of the total lipoproteins in the blood. LDL is composed of approximately 25percent protein; the rest of the particle consists of cholesterols and fats. LDL transports cholesterol into tissues; the cholesterol is used to make the walls of cells, as well as other substances such as hormones. The fats are used by the cells for energy. However, LDL can cause excess cholesterol to be deposited within the body, including the walls of blood vessels, which can lead to disease. Because of this, LDL is known as "the bad cholesterol."
Numbers
A high amount of HDL and a low amount of LDL will prevent excess cholesterol from being accumulated in the body, especially within the vessel walls, where coronary artery disease can occur.
A high LDL value is a better predictor of heart disease risk than total cholesterol values. An LDL level of less than 100 mg/dl is desirable; values above 160 mg/dl are considered high.
In contrast, a low level of HDL is considered a major risk factor for disease. Values less than 40 mg/dl for men and 50 mg/dl for women are considered a risk factor for heart disease. Higher values of HDL protect against heart disease.
Recommendations
The Mayo Clinic recommends that HDL be kept above the target value, and LDL be kept below the target value. Raising HDL is possible by quitting smoking, exercising, losing weight, eating saturated fat and drinking in moderation. Lowering LDL is possible by limiting the amount of saturated fats in the diet, choosing more healthy fats, eating less cholesterol-containing foods, increasing fiber diet and losing weight. Sometimes lifestyle changes are not enough, and medication may need to be prescribed to lower LDL cholesterol.
References
- "Clinical Diagnosis and Management by Laboratory Methods"; John Bernard Henry, MD; 2001
- American Heart Association: Good vs. Bad Cholesterol
- American Heart Association: What Your Cholesterol Numbers Mean
- MayoClinic.com: Top 5 lifestyle changes to reduce cholesterol
