Basic Description of Lipids and Lipoproteins

Cholesterol is a fat-like substance (lipid) that is present in cell membranes and is a precursor of bile acids and steroid hormones. Cholesterol travels in the blood in distinct particles containing both lipid and proteins. These particles are called lipoproteins. The cholesterol level in the blood is determined partly by inheritance and partly by acquired factors such as diet, calorie balance, and level of physical activity.

Lipoproteins are a class of heterogeneous particles of varying sizes and densities containing lipid and protein. These lipoproteins include cholesterol esters and free cholesterol, triglycerides, phospholipids and A, C, and E apolipoproteins. Total cholesterol comprises all the cholesterol found in various lipoproteins. Factors that affect blood cholesterol levels include age, sex, body weight, diet, alcohol and tobacco use, exercise, genetic factors, family history, medications, menopausal status, the use of hormone replacement therapy, and chronic disorders such as hypothyroidism, obstructive liver disease, pancreatic disease (including diabetes), and kidney disease. In many individuals, an elevated blood cholesterol level constitutes an increased risk of developing coronary artery disease. Blood levels of total cholesterol and various fractions of cholesterol, especially low density lipoprotein cholesterol (LDL-C) and high density lipoprotein cholesterol (HDL- C), are useful in assessing and monitoring treatment for patients with cardiovascular and related diseases. Blood levels of the above cholesterol components including triglyceride have been separated into desirable, borderline and high risk categories by the National Heart, Lung and Blood Institute in their report in 1993. These categories form a useful basis for evaluation and treatment of patients with hyperlipidemia (See NCEP Guidelines). Therapy to reduce these risk parameters includes diet, exercise and medication, and fat weight loss, which is particularly powerful when combined with diet or exercise.

Three major classes of lipoproteins are found in the blood of a fasting individual: low density lipoproteins (LDL), high density lipoproteins (HDL), and very low density lipoproteins (VLDL). The LDL typically contain 50-70 percent of the total serum cholesterol and both are directly correlated with risk for CHD. The HDL normally contain 20-30 percent of the total cholesterol, and HDL levels are inversely correlated with CHD risk. The VLDL contain 10-15 percent of the total serum cholesterol along with most of the triglyceride in fasting serum; VLDL are precursors of LDL, and some forms of VLDL, particularly VLDL remnants, appear to be atherogenic.

Lesser amounts of cholesterol are also carried in two minor lipoprotein classes, intermediate density lipoproteins (IDL, d 1.006-1.019 kg/L) and Lp(a) (d 1.045-1.080 kg/L).

Chylomicrons (d <1.006 kg/L) appear in the blood transiently after a fat-containing meal and are normally removed completely by 12 hours. They are rich in triglycerides and responsible for the postprandial increase in plasma triglycerides but usually have no significant effect on the total cholesterol concentration. The major lipoprotein fractions can be readily separated from each other, in the ultracentrifuge and are most commonly quantified in terms of their cholesterol .