What Is Cholesterol?
Cholesterol is a soft, waxy fat particle (lipid) that circulates in the blood. It is produced in the liver and is the most common steroid in the body. Cholesterol is a building block for cell membranes and it is essential in the formation of bile (which aids in the digestion of fats), vitamin D, and other steroids and hormones (e.g., progesterone, estrogen, testosterone).
The liver produces most of the cholesterol the body needs; however, many popular foods contain cholesterol and substances used by the liver to produce cholesterol. A high intake of these foods can increase the level of cholesterol in the blood.
High cholesterol (hypercholesterolemia) can cause the formation and accumulation of plaque deposits in the arteries. Plaque is composed of cholesterol, other fatty substances, fibrous tissue, and calcium. When it builds up in the arteries, it results in atherosclerosis, or coronary heart disease (CHD). Atherosclerosis can lead to plaque ruptures and blockages in the arteries, which increase the risk for heart attack, stroke, circulation problems and death.
The development of plaques and blockages in the arteries involves several steps.
- When the innermost lining of the arteries (endothelium) is damaged, cholesterol particles deposit into the damaged wall and form plaques (see figure below).
- More cholesterol and other substances incorporate into the plaque and the plaque grows, narrowing the artery (Step 2).
- Plaque deposits can grow large enough to interfere with blood flow through the artery (called a blockage) (Step 3-4). When the arteries supplying the heart with blood (coronary arteries) are blocked, chest pain (angina) may occur; when arteries in the legs are blocked, leg pain or cramping may occur; when arteries supplying the brain with blood are blocked, stroke may occur.
- If a plaque ruptures or tears, a blood clot may develop on top of it (Step 5). If a blood clot completely blocks blood flow through a coronary artery, heart attack (myocardial infarction) occurs; if an artery supplying blood to the brain is completely blocked, stroke occurs.
Cholesterol is measured in milligrams per deciliter of blood (mg/dL). Several different types of blood cholesterol can be measured, and high levels of some types are worse or better than high levels of other types. Types of cholesterol include the following:
- Total blood cholesterol
- HDL (high-density lipoprotein) cholesterol ("good" cholesterol)
- LDL (low-density lipoprotein) cholesterol ("bad" cholesterol)
- Triglycerides ("backbone" of many types of fat)
Total blood cholesterol is the most common cholesterol measurement. It measures the concentration of fat (lipid) in the bloodstream, including cholesterol and triglyceride molecules contained in LDL, HDL, and other lipid particles.
Total blood cholesterol levels can be used to help determine if LDL and triglyceride levels are likely to be normal or elevated. If total cholesterol levels are elevated, a lipid profile is used to determine which lipid level is too high.
HDL ("good") cholesterol may help protect against atherosclerosis by preventing cholesterol from depositing on arterial walls as it circulates in the bloodstream. Low HDL levels may be caused by a genetic predisposition, lack of exercise, smoking and/or obesity.
Some physicians believe it is important to assess the ratio between total blood cholesterol and HDL cholesterol. The ratio is calculated by dividing the HDL number into the total cholesterol number. For example, total cholesterol of 200 mg/dL and an HDL of 40 mg/dL would yield a ratio of 5:1 (200/40). A ratio below 5:1 is desirable and the optimum ratio is about 3.5:1.
Incidence and Prevalence of High Cholesterol
According to the Centers for Disease Control and Prevention (CDC), high cholesterol affects about 20% of adults over the age of 20 in the United States. The highest prevalence occurs in women between the ages of 65 and 74.
The World Health Organization (WHO) reports that high cholesterol contributes to 56% of cases of coronary heart disease worldwide and causes more than 4 million deaths each year. In most parts of the world, the number of female deaths attributed to high cholesterol is slightly higher than the number of male deaths.