It has been found that the protein content and the proportion of various lipid components, the size and density of the lipoprotein molecules are different in different lipoproteins, which have been categorized into the following four classes.

(1) High-density lipoproteins, abbreviated as HDL, are also called alpha lipoproteins. This lipoprotein has the smallest particle size of 5 to 20 millimeters and the largest specific gravity. It is composed of high levels of protein and phospholipids and very low levels of triglycerides, which are protective factors for the heart and blood vessels.

(2) Low-density lipoprotein is abbreviated as LDL and is also called called aim protein. The particles are 20 to 25 millimeters, high in cholesterol and low in triglycerides.

(3) Very low-density lipoprotein is abbreviated as VLDL, also called pre-B lipoprotein. It is low in protein and high in triglycerides and is a contributing factor to atherosclerosis.

(4) Coeliacs abbreviated as CM, 90% of which are triglycerides, are metabolized quickly in the blood. It does not play an important role in the formation of atherosclerosis.

In addition, there are free fatty acids in the plasma, but the content is very small.

Now, it has been further discovered that lipids are important in inducing related diseases, and apolipoproteins are very important. Nearly a dozen types of apolipoproteins have been identified, of which apolipoprotein A is the main component of high-density lipoproteins, which remove cholesterol deposited in the vessel wall and carry it to the liver for processing or bile excretion, thus lowering plasma cholesterol; and apolipoprotein B is found in low-density lipoproteins. It is this LDL that is thought to cause the greatest risk of atherosclerosis, whereas HDL acts as a vascular protector. So when predicting the risk of atherosclerosis, it is important to look not only at changes in blood lipids, but more importantly at the levels of LDL and HDL and the ratio of the two. Under normal circumstances, the ratio of LDL to HDL should be less than 4, and when the ratio is greater than 4, the harm caused by LDL becomes more significant.