Cholesterol Levels


Most fats in food are made up primarily of triglycerides. In order to use the energy stored in fat, the body breaks down triglycerides into fatty acids, which individual cells burn for energy. Like cholesterol, some fat is normally found in the blood; it travels through the bloodstream to get from its food sources and body stores to the cells that use it. Fat also needs lipoproteins to carry it through the bloodstream.

To illustrate how important these lipoproteins are for fat transport, drop a tablespoon of oil into a glass of water and watch what happens. The fat and water repel each other. This reaction makes transport of fat through blood difficult. When fat is encased in a lipoprotein that prevents it from mixing with blood, however, it can move effortlessly through the bloodstream.

Although all lipoproteins carry some triglycerides, chylomicrons (a lipoprotein) and the very-low-density lipoproteins (VLDLs) are the primary movers of triglycerides. Each transports triglycerides from a particular source.

When fat from foods is digested in the body, the fatty acids are released and then packaged into triglycerides in the intestines. The chylomicrons pick up these triglycerides, along with dietary cholesterol, and transport them through the blood to the muscle cells and fat cells. An enzyme residing on these cells breaks down the chylomicrons so that the fatty acids can enter the cells.

Cholesterol is left behind in the remnant, which makes its way to the liver. The enzyme works quickly: Within five minutes, it can clear from the blood half the triglycerides absorbed from a meal. Within a few hours after a meal the enzyme will have removed all the chylomicrons from the blood.

When your body makes its own fat in order to store extra calories from food, a different lipoprotein takes care of transportation. The VLDLs carry the fat that is made in the liver, along with cholesterol, to the cells where the fat is stored. Once the VLDLs have dropped off their triglycerides, they contain mostly cholesterol and evolve into LDL molecules.

Scientific research appears to show that the blood-triglyceride level, unlike blood-cholesterol level, does not independently predict risk of heart disease in the general population, although it did have predictive value for older women in the Framingham Heart Study. Doctors do not find large amounts of triglycerides in the plaques that clog arteries.

On the other hand, people who have survived heart attacks often do have high blood-triglyceride levels. A high triglyceride level may be one indicator of something called the metabolic syndrome, which is also characterized by abdominal obesity, low levels of HDL cholesterol, high blood pressure, and insulin resistance. People with metabolic syndrome are at an increased risk of coronary heart disease.

Cholesterol is a delicate balancing act. With the newfound knowledge from this article, you'll be better equipped to control your own cholesterol.

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Dr. Neil Stone is a professor of clinical medicine in cardiology at the Feinberg School of Medicine of Northwestern University and a practicing internist-cardiologist-lipidologist at Northwestern Memorial Hospital. He also serves as the Medical Director of the Vascular Center for the Bluhm Cardiovascular Institute. Dr. Stone was a member of the first and third National Cholesterol Education Program Adult Treatment Panels and a past chairman of the American Heart Association Nutrition Committee and Clinical Affairs Committee.