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A Cholesterol Primer

Keep an eye on the lipoproteins and you'll keep your arteries clear

If you're confused about the difference between "good" and "bad" cholesterol, you have lots of company. Here, Dr. Dearing Johns, a cardiologist at the University of Virginia Medical School, explains how the biochemistry of cholesterol works and what the tests mean.

What is cholesterol?

Cholesterol is a pearly-white, waxy substance found in animal fats and oils. It is found throughout the body but is produced primarily by the liver. Cholesterol circulates in the blood, commonly in association with other saturated and unsaturated fats, and can be either beneficial or harmful. For instance, it is necessary for making cell membranes and many important hormones. However, cholesterol may also form gallstones and harmful deposits in blood vessels.

Why is it dangerous?

Too much cholesterol in the bloodstream leads to atherosclerosis, which is the build-up of cholesterol and fats in the walls of blood vessels, causing permanent blood vessel damage and blockage of blood flow. Blocked blood flow to the heart will cause a heart attack and death of heart muscle. Lack of blood flow to the brain will cause a stroke and permanent brain damage. Similarly, obstruction of the blood supply to any vital organ will lead to impairment of its normal function. For instance, poor circulation to the kidney may result in the need for dialysis or kidney transplantation, and reduction in blood flow to the eye can lead to blindness.

Just as important as the cholesterol content of the body is whether or not cholesterol is oxidized. Oxidation is an energy-consuming process in which hydrogen ions are removed from a substance. During this process, high-energy molecules, known as free radicals, may damage blood vessels and other tissue. The oxidation of cholesterol in the body increases the likelihood of tissue injury and damage to blood vessel walls.

Is there a difference between dietary cholesterol and blood cholesterol?

Most of the cholesterol in the body is formed by the liver, which is capable of producing (and degrading) all the cholesterol the body needs. Usually, about two-thirds of the body cholesterol is produced by the liver and only one-third is derived from the diet. Reducing dietary cholesterol will lower blood cholesterol.

However, blood cholesterol is reduced more efficiently if the intake of saturated fat is decreased. That's because saturated fat slows elimination of cholesterol. Unsaturated fats signal the liver to speed up its elimination of cholesterol. By judiciously altering the proportion of unsaturated fat to saturated fat in your diet, you can protect the body from atherosclerosis (blood vessel damage from cholesterol and fats) very effectively.

What makes fats saturated or unsaturated?

Fats in the body are usually in the form of long chains of carbon atoms connected by double or single bonds. A single-bond connection is considered a "saturated" bond (there is no room for additional binding). A double bond connection is an "unsaturated" bond that is potentially able to bond other molecules. Unsaturated fats can be polyunsaturated (multiple double-bond connections) or monounsaturated (mostly single bonds with one double bond).

Saturated fats, such as butter and lard, tend to be solid at room temperature. Saturated fats come from animal products and are seen on the outside or form the "marbling" of meats. Coconut and palm oil are also heavy in saturated fats.

Unsaturated fats are usually liquid at room temperature and are derived from plant sources such as safflower oil, sunflower seed oil, and corn oil. Olive oil and cannola oil are monounsaturated oils.

How can some cholesterol be harmful and some good for you?

Cholesterol does not dissolve in water-based fluids such as blood. In order to circulate in the blood stream, cholesterol and fats are surrounded by protein, thus forming larger cholesterol-rich particles called lipoproteins. Most of the circulating cholesterol is packaged in particles called low- density lipoproteins, or LDL-cholesterol particles. LDL-cholesterol is often called "bad" cholesterol because it leads to clogged arteries and impaired circulation. High-density lipoprotein, or HDL-cholesterol, is often called "good" or "cardioprotective" cholesterol because it is the cholesterol particle that is destined for elimination by the liver.

The more the HDL-cholesterol, the less the chance for cholesterol deposits in the blood vessel walls and the lower the risk of impaired circulation due to blood vessel blockage. Measurement of total cholesterol includes LDL-cholesterol and HDL-cholesterol. Thus it is important, when measuring blood cholesterol, to break down the total cholesterol value into LDL-cholesterol values and HDL-cholesterol values.

How much cholesterol is unhealthy?

That is a difficult question because it depends on a person's innate (genetic) ability to handle cholesterol and fats as well as on other factors that place a person at high risk for heart disease, such as the presence of cigarette smoking, high blood pressure, or family members with heart disease at a young age. If there are no such adverse risk factors, a total cholesterol of less than 200 mg/dl is considered desirable. A total cholesterol between 200 and 239 mg/dl is considered borderline high and greater than 240 mg/dl is definitely too high.

Also important is how high the HDL-cholesterol is. An HDL-cholesterol of less than 35 mg/dl is associated with increased risk of heart disease, especially in women. An HDL-cholesterol above 60 mg/dl confers a protective effect. The lower the LDL-cholesterol, the better. With no risk factors, an LDL-cholesterol less than 160 mg/dl is adequate, but below 130 mg/dl is desirable. If a person already has coronary artery disease (e.g., if they have had a heart attack), then the LDL-cholesterol goal should be less than 100 mg/dl.

The best way to limit high cholesterol is to reduce the total fat intake and to aim to have most dietary fat in the form of monounsaturated or polyunsaturated fats. The American Heart Association recommends no more than 30 percent of total daily calories be derived from fats (the average American diet is 40 percent to 50 percent fat). Of these fats, only 10 percent should be saturated fats.

How effective are cholesterol-lowering drugs?

Cholesterol-lowering drugs will reduce cholesterol in most people. LDL-cholesterol can be cut between 15 percent and 40 percent, depending on which drug is selected. Many of these drugs also will raise the HDL-cholesterol by 5 percent to 25 percent. Your doctor will use your cholesterol profile to guide selection of the most appropriate cholesterol-lowering drug.

Newer cholesterol-lowering drugs, called HMG-CoA reductase inhibitors, actually prevent cholesterol production in the liver. These drugs lower LDL-cholesterol by 20 percent to 40 percent and raise HDL-cholesterol up to 15 percent in some people. These drugs (Mevacor, Zocor, Pravachol, and Lescol) are very effective, but close monitoring is necessary to prevent serious muscle and liver damage.

Several studies have shown that these agents can reduce death from heart disease. More recent studies also show that they slow progression of atherosclerosis and blockages of coronary arteries, and in some cases, actually seem to reverse blockages previously seen in blood vessels.

Overall, it is estimated that for every 1 percent reduction in LDL-cholesterol, the risk of coronary heart disease is reduced 2 percent.

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Updated on April 8, 1996