Cholesterol and Its Forms in Blood

Article submitted by Robert M. White, Sr., Ph.D., D.A.B.C.C. (CC&TC), C.H.R.M.

Cholesterol itself is a small molecule whose excess presence in human blood and in certain forms has been implicated in, among other disease states, atherosclerosis. Atherosclerosis is a common form of arteriosclerosis which is a thickening and loss of elasticity of the arterial walls in which deposits of yellowish plaques called atheromas, which contain cholesterol and other naturally occurring materials, are formed within the innermost and medium portions of large- and medium-sized arteries. The formation of atheromas blocks arteries and impedes normal blood flow. The rupture of an atheroma can lead to acute myocardial infarction, which is commonly known as a heart attack, and stroke.

Although only a small fraction of a milliliter of blood is required for the actual determination of a patient's total cholesterol, the measurement of cholesterol in blood serum (the straw-yellow fluid left after blood clots) is expressed in milligrams (abbreviated mg, which is a thousandth of a gram) per deciliter (abbreviated dL, which is 100 milliters). If the total amount of cholesterol in a patient's blood is greater than 200 mg/dL, the level can be taken as a warning sign that the total cholesterol is too high; especially when viewed with respect to other risk factors that include, but are not limited to age, sex, family history, and whether the patient is a smoker or not. However, a patient's total serum cholesterol is not the complete story from a medical perspective.

Every healthy individual has some cholesterol in his or her blood. Although cholesterol can be present in a free or unbound state, some of the cholesterol called esterified cholesterol exists chemically linked to fatty acids.

Lipoproteins arise from diet (exogenous pathway) and from internal synthesis by the liver (endogenous pathway). Lipoproteins that arise from the exogenous pathway are chylomicrons, which are then converted into HDL (high-density lipoprotein) and a chylomicron remnant, which is taken up by the liver. Lipoprotein that is synthesized in the endogenous pathway is VLDL (very low-density lipoprotein), which is then converted into IDL (intermediate density lipoprotein), which finally is turned into LDL (low density lipoprotein). LDL is taken up by a specific receptor on liver cells. The uptake of LDL by liver cells and the internal machinery of the liver cell greatly influence the removal of cholesterol from the bloodstream and the synthesis of cholesterol and putting it back into the blood.

In general, higher levels of HDL cholesterol are desirable. Levels of HDL cholesterol <40 mg/dL are considered undesirable. Lower levels of LDL cholesterol are considered desirable. Depending on other risk factors, desirable levels of LDL cholesterol have been cited as <130 mg/dL, <100 mg/dL, and, most recently, <70 mg/dL as reasonable goals.

In summary, an individual's total cholesterol has been shown to have a close association with the risk of developing coronary artery disease which may lead to future cardiac events such as heart attack. However, the levels of the lipoprotein classes such as HDL and LDL also greatly influence risk and the subclasses of LDL and, possibly, HDL also play a part in the determination of risk of future coronary events.