What are LDL and HDL cholesterol?

LDL cholesterol is called "bad" cholesterol, because elevated levels of LDL cholesterol are associated with an increased risk of coronary heart disease. LDL lipoprotein deposits cholesterol on the artery walls, causing the formation of a hard, thick substance called cholesterol plaque. Over time, cholesterol plaque causes thickening of the artery walls and narrowing of the arteries, a process called atherosclerosis.

HDL cholesterol is called the "good cholesterol" because HDL cholesterol particles prevent atherosclerosis by extracting cholesterol from the artery walls and disposing of them through the liver. Thus, high levels of LDL cholesterol and low levels of HDL cholesterol (high LDL/HDL ratios) are risk factors for atherosclerosis, while low levels of LDL cholesterol and high level of HDL cholesterol (low LDL/HDL ratios) are desirable.

Total cholesterol is the sum of LDL (low density) cholesterol, HDL (high density) cholesterol, VLDL (very low density) cholesterol, and IDL (intermediate density) cholesterol.

What determines the level of LDL cholesterol in the blood?

The liver not only manufactures and secretes LDL cholesterol into the blood; it also removes LDL cholesterol from the blood. A high number of active LDL receptors on the liver surfaces is associated with the rapid removal of LDL cholesterol from the blood and low blood LDL cholesterol levels. A deficiency of LDL receptors is associated with high LDL cholesterol blood levels.

Both heredity and diet have a significant influence on a person's LDL, HDL and total cholesterol levels. For example, Familial hypercholesterolemia (FH) is a common inherited disorder whose victims have a diminished number or nonexistent LDL receptors on the surface of liver cells. People with this disorder also tend to develop Atherosclerosis and Heart Attacks during early adulthood.

Familial hypercholesterolemia Definition
A dominantly inherited genetic condition that results in markedly elevated LDL (low-density lipoprotein) cholesterol levels beginning at birth, and resulting in heart attacks at an early age.

Diets that are high in saturated fats and cholesterol raise the levels of LDL cholesterol in the blood. Fats are classified as saturated or unsaturated (according to their chemical structure). Saturated fats are derived primarily from meat and dairy products and can raise blood cholesterol levels. Some vegetable oils made from coconut, palm, and cocoa are also high in saturated fats.

Affected people have consistently high levels of low-density lipoprotein, which leads to premature atherosclerosis of the coronary arteries. Typically in affected men, heart attacks occur in their 40s to 50s, and 85% of men with this disorder have experienced a heart attack by age 60. The incidence of heart attacks in women with this disorder is also increased, but delayed 10 years later than in men.