HDL and LDL are two types of lipoprotein particles that carry the water-insoluble lipids in the blood. One of the lipids they carry is cholesterol, which may come from diet or be synthesized in the body. HDL stands for high-density lipoprotein, so called because the particles are about 50 percent proteins, which have higher density. By contrast, LDLs are low-density lipoproteins that consist mostly of lipids--about 43 percent of LDL consists of cholesterol.
Function
HDL and LDL are part of the standard lipid panel from a blood test, according to Lab Tests Online. Their values correlate with the risk of cardiovascular disease. Both synthesized in the liver, they are like little buses that transport cholesterol and other lipids to and from the liver. But the "bus routes" are quite different. LDL takes cholesterol from the liver and delivers it to other cells, whereas HDL takes cholesterol from the cells and brings it back to the liver. In the liver, cholesterol can be incorporated into the bile and excreted. Because of this difference, it's generally considered that LDLs are pro-cardiovascular diseases, whereas HDLs are protective.
Oxidized LDL
While LDL is circulating in the blood, it can be modified by oxidation. The oxidized LDLs have higher affinity for the blood vessels and tend to stick to injury sites in the vessel walls, where they are modified further to become fatty streaks and plaques that eventually clog the artery. The longer LDL lingers in the blood, the more likely it will be oxidized. The more oxidized LDL, the more plaque formation. Hence, higher LDL in the blood increases the risk of atherosclerosis.
HDL Score
HDL is considered a negative risk factor for cardiovascular disease, with lower HDL precipitating the disease. People who have HDL below 40 mg/dl are prone to cardiovascular disease. This is of particular importance to men under the age of 50. Men who get heart disease early in their lives tend to have low HDL, as reviewed by UCSF cardiologists.
On the other hand, people with blood HDL higher than 60 mg/dl are protected from heart problems. High HDL is believed to be mostly genetic. People who are blessed with HDLs greater than 90 mg/dl generally do not develop cardiovascular disease, regardless of other risk factors. For others, it's possible to raise HDL through regular exercise and moderate drinking.
LDL Score
High LDL is associated with increased risk for cardiovascular disease. Blood LDL should be kept low, not exceeding 130 mg/dl, and optimally below 100 mg/dl. This can be achieved through a combination of diet, exercise and medical therapies. Diabetic patients, in particular, need to lower the LDL to less than 100 mg/dl, given the fact that they have a two to four times increased risk for cardiovascular disease than the general public.
Lipoprotein Ratios
The ratio of LDL over HDL, or the ratio of total cholesterol over HDL, is sometimes used as an adjunct evaluation of heart disease risk. From the Framingham study, which is a landmark study of the risk factors for heart diseases, it's recognized that the higher the ratio of total cholesterol over HDL, the greater the risk of heart disease. A ratio of 4.4 is the cutoff point, which roughly corresponds to 200 mg/dl of total cholesterol and 45 mg/dl of HDL. A ratio of 3.0 presents only half of the average risk of developing cardiovascular disease.
