Knowing your blood lipid (fat) levels—the various forms of cholesterol and triglycerides—is important. But what these numbers mean can be confusing, and how the different types of cholesterol—not to mention other blood fats—relate to coronary artery (heart and blood vessel) disease also can be a puzzle.

Coronary artery disease is the number one killer of Americans. Studies point to certain abnormalities in cholesterol and triglyceride levels as a major contributor to this problem. There is plenty of good news to suggest that public awareness of heart disease and its risk factors is making a difference.

Deaths from cardiovascular disease continue to decrease. Much credit for this encouraging trend goes to improved treatments and modification of the risk factors for heart disease, including lowering cholesterol levels.

Despite these substantial improvements, the American Heart Association reports that cardiovascular disease still kills almost 1 million Americans each year. This is more than all cancer deaths combined. More than 6 million Americans experience symptoms due to coronary artery disease.

As many as 1.5 million Americans will have a heart attack every year, and about half a million of them will die. Blockage of the coronary arteries supplying the heart muscle (which can lead to heart attack) causes more deaths, disability, and economic loss than any other type of heart disease.

The coronary arteries are the heart’s own circulatory system. They supply the heart with blood, oxygen, and nutrients. The heart uses this blood supply for energy to perform its continuous task of pumping. Coronary artery disease can take many different forms, but each has essentially the same effect: the heart muscle does not get enough blood and oxygen through the coronary arteries.

Consequently, its own demands for oxygen and nutrients are not met. This condition can be either temporary or permanent. Most coronary artery disease is caused by atherosclerosis (also known as “hardening of the arteries”). The term “atherosclerosis” comes from the Greek ather (meaning “porridge”) and sklerosis (meaning “hardening”).

Healthy arteries are flexible, strong, and elastic. The inner layer of arteries is smooth, enabling blood to flow freely. Atherosclerosis can be a silent, painless process in which cholesterol-containing fatty deposits accumulate in the walls of the arteries. These accumulations occur as lumps called plaques.

As plaque deposits enlarge, the interior of the artery narrows, and the flow of blood is then reduced (see the illustration below). If reduced flow occurs in the coronary (heart) arteries, it can lead to a type of chest pain called angina pectoris. As a plaque enlarges, the inner lining of the artery becomes rough.

A tear or rupture in the plaque may cause a blood clot to form. Such a clot can block the flow of blood or break free and plug another artery. If the flow of blood to a part of the heart is stopped, a heart attack results. If the blood flow to a part of the brain stops, a stroke occurs.

Many factors influence the clogging of arteries, but cholesterol is a primary one. Cholesterol is a waxy, fat-like substance (a lipid). Although it is often discussed in negative terms, it is an essential component of the body’s cell membranes. It also serves to insulate nerves and is a building block in the formation of certain hormones.

The liver uses it to make bile acids, which help digest food. Confusion about cholesterol is due in great part to the all-purpose use of the term. Cholesterol has two sources: the foods we eat (about 20%) and the cholesterol that is made by the body (about 80%).

Dietary cholesterol is found only in animal products, such as meat and dairy products, or foods made with animal products. Examples include all meats, fish, and poultry, eggs, and milk products.

In addition, both the amount and the type of fat eaten influence the blood cholesterol level. Both saturated (primarily from animals) and trans-saturated (oils that have been processed to make them more solid) fats increase the amount of cholesterol made by the liver.

LDL and HDL Cholesterol and Triglycerides

Cholesterol and triglycerides are fats and are insoluble in the blood. However, when they combine with protein they become lipoproteins and are able to dissolve in and be carried by blood throughout the body. Low-density lipoprotein (LDL) cholesterol is the main cholesterol carrier in the blood.

There is a direct relationship between the level of LDL cholesterol (or total cholesterol) and the rate of coronary artery disease. When there is too much LDL cholesterol circulating in the blood, it can slowly build up in the walls of the arteries that feed the heart and brain. For this reason, LDL is often referred to as the “bad” cholesterol.

If there are too many LDL particles in the blood, or the liver (the normal site of metabolism) does not remove LDL quickly enough from the blood, it builds, particularly in blood vessels. It is the role of high-density lipoprotein (HDL) to counteract this effect. About a third to a fourth of blood cholesterol is carried by HDL.

HDL carries cholesterol away from the arteries and back to the liver, where it is removed from the blood. It is therefore often referred to as the “good” cholesterol. A high level of HDL seems to protect against atherosclerosis and heart attack. The opposite is also true: a low HDL level indicates an increased risk of atherosclerosis.

Thus, the goal is to have a high HDL cholesterol level and a low LDL cholesterol level. Triglycerides in the blood are derived from fats eaten in foods or produced when the body converts excess calories, alcohol, or sugar into fat. Most triglycerides are transported through the bloodstream as very low-density lipoprotein (VLDL).

Some cholesterol is also present in VLDL. A certain amount of triglycerides in the blood is normal. Hormones regulate the release of triglycerides from fat tissue to meet the body’s needs for energy between meals. However, at high levels, triglycerides may contribute to the development of atherosclerosis.

Increased triglyceride levels also may be a consequence of other diseases, such as untreated diabetes mellitus. Calories ingested at a meal and not used immediately by tissues are converted to triglycerides and transported to fat cells to be stored.

The only way to determine whether cholesterol and other blood lipids are in a desirable range is to have them measured by a blood test. The National Cholesterol Education Program guidelines recommend that total cholesterol, HDL cholesterol, and triglycerides be measured at least once every 5 years in all adults age 20 or older.

However, your physician also may recommend that the screening include LDL cholesterol and triglycerides. Triglycerides must be measured after an overnight fast because eating can have a marked effect on blood triglyceride levels. Therefore, fast for at least 12 hours before blood is drawn.

Do not drink alcohol for 24 hours before the test. If you have a risk factor(s) for heart disease, consult your physician regarding the optimal frequency of testing. (See sidebar: Risk Factors for Coronary Artery Disease Other Than LDL Cholesterol, below.)

In a sense, it is incorrect to think of a cholesterol (or triglyceride) level as being strictly abnormal or normal. Although ranges of cholesterol levels have been identified which are considered “too high,” there is no “magic number” that separates risky levels from safe levels. Actually, the ranges for adults are based on a consensus of experts.

They have identified lipid levels in the blood above which the risk for development of coronary complications is high enough to warrant medications or lifestyle changes. People with cholesterol or triglyceride levels in the higher-risk zones are said to be hypercholesterolemic or hypertriglyceridemic (hyper means “high,” and emic means “in the blood”).

But, as with all risk factors, being in the “high” range does not guarantee that coronary artery disease will develop, nor does being in the “low” range guarantee avoiding it. Blood test numbers are only guidelines.

If the numbers stray from the desirable range, a physician can provide advice on what to do. Remember that each number takes on greater meaning in light of the other lipid results and in the presence of other cardiovascular disease risk factors.