ApoB, LDL Cholesterol And Cardiovascular Disease Risk

March 29, 2008

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A statement by the American College of Cardiology and the American Diabetes Association, which will be published in the April issue of Diabetes Care, suggests that ApoB levels should be measured in addition to LDL cholesterol (the “bad” cholesterol) to assess cardiovascular risk in some patients with borderline or slightly elevated LDL cholesterol. ApoB is a protein that found on the surface of with LDL cholesterol particles.

It has long been known that there is a subpopulation of people who even though they have borderlinel or slightly elevated LDL cholesterol are at a high risk for heart attack, stroke etc. Elevated LDL cholesterol definitely puts people at a higher risk for cardiovascular disease, but what about those with borderline or moderate elevations who develop heart and vascular disease?

Generally LDL cholesterol is not actually measured when standard blood lipid panels are done. It is calculated by a formula using measurements of total cholesterol, HDL cholesterol (the “good” cholesterol) and triglyceride. So the calculation just estimates how much cholesterol is in the LDL fraction in blood.

Cholesterol is not free in the blood; it is contained in particles. There are LDL particles, HDL particles etc. LDL particles in the blood are not all the same size or density. They have a size range, with the smallest being the most dense, and the biggest being the least dense. An analogy is comparing lead shot with a Styrofoam packing peanut. The lead shot is smaller but heavy; the Styrofoam is bigger and less dense.

The standard calculation of the amount of LDL cholesterol does not give an indication of the numbers of LDL particles. If there are lots of small dense particles, then the LDL cholesterol measurement might not be very high even though there are lots of LDL particles.

The smallest densest particles are thought to be most atherogenic (most atherosclerosis causing) and most likely to cause cardiovascular disease, while the biggest and “fluffiest” are the least atherogenic.

So, considering LDL cholesterol measurement. If there are lots of predominantly small dense particles in the blood, the calculated LDL cholesterol may not be really high since the small particles do not have very much cholesterol. However, the risk of cardiovascular disease may be significant, since there are actually lots of small dense LDL particles.

Since each LDL particle has one ApoB (apolipoprotein B100) on its surface, then the amount of ApoB can be used to estimate the number of LDL particles. So particularly if LDL cholesterol is borderline or slightly elevated then the amount of ApoB can give an estimate of the number of particles and from this the percentage of atherogenic small dense particles can be calculated.

The measurement of ApoB will therefore be particularly beneficial for predicting which patients with borderline or slightly elevated cholesterol are at risk of cardiovascular disease. Those who have relatively high ApoB would then be candidates for more aggressive cholesterol lowering treatments and could be monitored for early detection of the development of cardiovascular disease.

photo credit: kygp