High Blood Cholesterol

Cholesterol is a waxy, fat-like substance that is found in all cells of the body. In addition to being a structural component of cell membranes, cholesterol also plays important roles in making hormones, vitamin D, and bile acids that aid in the digestion of foods.

Cholesterol itself is not harmful, but too much cholesterol in the blood, or high blood cholesterol, can be dangerous. Heart attack and stroke, two of the most common cardiovascular diseases, originate from high blood cholesterol, or more specifically, high concentration of cholesterol carried by low-density lipoproteins (LDLs).

Blood is watery, and cholesterol is fatty. Just like oil and water, the two do not mix. To travel in the bloodstream, cholesterol is carried in small packages called lipoproteins. There are two kinds of lipoproteins that carry cholesterol throughout the body.

• Low-density lipoprotein (LDL) - The higher the LDL level in the blood, the greater the chance to get cardiovascular diseases. The cholesterol carried by LDL is often called "bad cholesterol".
• High-density lipoprotein (HDL) - The higher the HDL level in the blood, the lower the chance to get cardiovascular diseases. The cholesterol carried by HDL is often called "good cholesterol". 

Why LDL is Bad

LDL plays an essential role in the pathogenesis of atherosclerosis, which can cause many complications, including heart attack and stroke. Diseases caused by atherosclerosis are the leading cause of illness and death in the United States. The initial step leading to atherosclerosis is the trap of LDL particles in the tunica intima (or simply called intima), which is the innermost layer of the artery wall. The intima contains endothelium, basement membrane and underlying connective tissue. The endothelium is in direct contact with the blood flow and the basement membrane separates epithelium from the underlying connective tissue.

The size of LDL particles varies from 18 to 25 nm in diameter. They can penetrate the endothelium efficiently. The LDL particle contains mainly the apolipoprotein B (ApoB), which can bind to proteoglycans in the complex matrix beneath the endothelium. The trapped LDL particles could be oxidized and eventually form a plague. The rupture of plague can induce the formation of thrombus (blood clot) and block blood flow. This will result in stroke or heart attack, depending on whether the blockage is in the brain or the heart.. 

Why HDL is Good

The size of HDL particles varies from 5 to 17 nm in diameter. They can penetrate the endothelium even more efficiently than LDL particles. However, they are less likely to be trapped in the intima, because the HDL particle contains mainly apolipoprotein A-I (ApoA-I) which does not bind to proteoglycans. Only a small amount of HDL particles may be trapped because they contain apolipoprotein E (ApoE) which can bind with proteoglycans.

Generally, LDL carries cholesterol from the liver to the rest of the body, while HDL carries cholesterol from the blood back to the liver. Not only can HDL remove the cholesterol in the blood, it also possesses anti-oxidant and anti-inflammatory properties to prevent atherosclerosis (reference).

Symptoms and Diagnosis

Because there are usually no signs or symptoms of high blood cholesterol, it is best to have cholesterol level checked regularly. A blood test called a lipoprotein profile will give the following information:

• Total cholesterol.
• LDL cholesterol.
• HDL cholesterol.
• Triglycerides: another form of fat in the blood.

Cholesterol levels are measured in milligrams (mg) of cholesterol per deciliter (dL) of blood.

Triglycerides can also raise the risk for heart disease.  150?99 mg/dL are borderline high; 200 mg/dL or more are high.

Causes

Two factors may affect the cholesterol level in the blood: genetics and foods.

An inherited genetic condition (familial hypercholesterolemia) results in very high LDL cholesterol levels. It begins at birth, and may result in heart attack at an early age.

The following types of fats may raise the cholesterol level:

Saturated fats
Saturated fats raise total blood cholesterol level. They are found in meat, seafood, cheese, milk,  ice cream and egg yolks.

Trans fats
Trans fats are more harmful than saturated fats because they raise bad LDL and lower good HDL. They are produced by heating liquid vegetable oils in the presence of hydrogen (a process called hydrogenation). They are found in French fries, deep-fried chips and most margarines. 

References:

Anti-inflammatory Properties of HDL - Circulation Research, 2004.

National Heart, Lung, and Blood Institute, USA.