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 plaque 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.

According to results of a study conducted by researchers from the National Institutes of Health (NIH) and released in May 2014, high levels of total blood cholesterol ("free cholesterol") may increase the risk for infertility. Published online in The Journal of Endocrinology and Metabolism (a publication of the Endocrine Society), the study showed that couples in which both partners had high cholesterol, as well as those in which the woman had high cholesterol and the man had normal cholesterol levels, took longer to achieve pregnancy. More research is needed to learn more about this possible link.

The development of plaques and blockages in the arteries involves several steps.

1. When the innermost lining of the arteries (endothelium) is damaged, cholesterol particles deposit into the damaged wall and form plaques (see figure below).
2. More cholesterol and other substances incorporate into the plaque and the plaque grows, narrowing the artery (Step 2).
3. Plaque deposits can grow large enough to interfere with blood flow through the artery (called ablockage) (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.
4. 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.

Incidence and Prevalence of High Cholesterol

According to the Centers for Disease Control and Prevention (CDC) in December 2015, almost 37 percent of adults over the age of 21 in the United States meet the recommended criteria for taking cholesterol-lowering medication. Yet within this group, the CDC reports that nearly half are not are on the medicine(s). The highest prevalence of high cholesterol occurs in women between the ages of 65 and 74.

The World Health Organization (WHO) reports that high cholesterol contributes to about 56 percent 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.