LDL, HDL, and Triglycerides

Fats, also called lipids, are energy-rich substances that serve as a major source of fuel for the body's metabolic processes. Fats are obtained from food or formed in the body, mostly in the liver, and can be stored in fat cells for future use. Fat cells also insulate the body from cold and help protect it from injury. Fats are essential components of cell membranes, of the myelin sheaths that surround nerve cells, and of bile.

The two major fats in the blood are cholesterol and triglyceride. The fats attach themselves to certain proteins so they can travel throughout the bloodstream; the combined fats and proteins are called lipoproteins. The major lipoproteins are chylomicrons, very low density lipoproteins (VLDL), low-density lipoproteins (LDL), and high-density lipoproteins (HDL).

Each type of lipoprotein serves a different purpose and is broken down and excreted in a slightly different way. For example, chylomicrons originate in the intestine and carry certain types of digested fat from the intestines into the bloodstream. A series of enzymes then remove the fat from the chylomicrons for use as energy or for storage in fat cells. Ultimately, the remaining chylomicron, stripped of much of its fat (triglyceride), is removed from the bloodstream by the liver.

The body regulates lipoprotein levels in several ways. One way is by reducing the synthesis of lipoproteins and their entry into the bloodstream. Another is by increasing or decreasing the rate at which lipoproteins are removed from the blood.

Abnormal levels of fats circulating in the bloodstream, especially cholesterol, can lead to long-term problems. The risk of having atherosclerosis and coronary artery or carotid artery disease (and therefore the risk of having a heart attack or stroke) increases as a person's total cholesterol level increases. Low cholesterol levels are therefore generally better than high ones, although extremely low cholesterol levels may not be healthy either. An ideal total cholesterol level is probably 140 to 200 milligrams of cholesterol per deciliter of blood (mg/dL) or less. The risk of a heart attack more than doubles when the total cholesterol level approaches 300 mg/dL.

Not all cholesterol increases the risk of heart disease. The cholesterol carried by LDL (the so-called bad cholesterol) increases the risk; the cholesterol carried by HDL (the so-called good cholesterol) lowers the risk and is beneficial. Ideally, LDL cholesterol levels should be below 130 mg/dL, and HDL cholesterol levels should be above 40 mg/dL. The HDL level should account for more than 25 percent of the total cholesterol. The total cholesterol level is less important as a risk factor for heart disease or strokes than the total cholesterol to HDL cholesterol ratio or the LDL to HDL ratio.

Whether high levels of triglycerides increase the risk of heart disease or strokes is uncertain. Blood levels of triglycerides above 250 mg/dL are considered abnormal, but high levels don't appear to uniformly increase the risk of atherosclerosis or coronary artery disease. However, extraordinarily high levels of triglycerides (above 800 mg/dL) may lead to pancreatitis.