Types of Lipoproteins

The human body is 45 to 75 percent water by weight, reports nutritionist Roberta Larson Duyff, M.S., in the "American Dietetic Association Complete Food and Nutrition Guide." Fats, or lipids, do not dissolve in water. Your body works around this problem by chemically coupling lipids with carrier proteins, which coat the fats and make them water-soluble. Fat molecules coupled with carrier proteins are called lipoproteins. Different types of lipoproteins circulate in your body. The relative proportion and total amounts of lipoproteins in your bloodstream can significantly affect your health and risk for heart disease.

HDL

Your liver produces high-density lipoprotein, commonly known as HDL or "good cholesterol." HDL acts a cholesterol scavenger. It circulates in your bloodstream, absorbing excess cholesterol from your body tissues. HDL then delivers the excess cholesterol to your liver, which excretes the fat from your body through the bile. By removing excess cholesterol from your body, HDL helps protect you from developing fat deposits, or plaques, in your arteries. The American Heart Association reports that an HDL level of less than 40 mg per deciliter of blood increases your risk for coronary artery disease, a condition characterized by poor blood flow to the heart caused by plaques in the vessels that carry blood to the heart muscle.

LDL

You probably know of low-density lipoprotein, or LDL, as the "bad cholesterol." LDL has the opposite effect of HDL. LDL contains high concentrations of cholesterol and is the principal transporter of the fat to the body tissues. Optimally, your LDL level should be less than 100 mg/dL, according to the National Cholesterol Education Program. An elevated LDL level increases your risk for coronary artery disease.

Your diet can significantly affect your LDL level. If your LDL is abnormally elevated, the National Institutes of Health recommends adopting the therapeutic lifestyle changes, or TLC, diet. With the TLC diet, your total intake of fat should account for no more than 25 to 35 percent of your daily calories, with less than 7 percent coming from saturated fats. Additionally, your daily intake of cholesterol is limited to less than 200 mg and sodium to 2,400 mg.

VLDL and IDL

Your liver produces another lipoprotein known as very low-density lipoprotein, or VLDL. When you eat a meal or snack containing a large amount of sugar, your liver converts the excess sugar to a fat called triglyceride and couples it with cholesterol and a carrier protein to form VLDL. VLDL delivers the triglyceride to your body tissues, which use it or store it as fat. Triglyceride-depleted VLDL is known as intermediate-density lipoprotein, or IDL. Most IDL undergoes a chemical conversion to become cholesterol-rich LDL, explains the medical reference text "Harrison's Principles of Internal Medicine." Thus, VLDL and IDL are the link between a high-carbohydrate diet and an elevated LDL level, with an associated increased risk for heart disease.