Most heart attacks stem from coronary artery disease (CAD)—the narrowing of the arteries that carry oxygenated blood to the heart. CAD is a complex process that is far from completely understood. But the underlying cause of CAD is atherosclerosis—a slow, progressive buildup of deposits called plaques within the walls of arteries, including the coronary arteries.
Scientists now think that the process of plaque formation may begin with LDL cholesterol. When the bloodstream carries more LDL than can be utilized by the body’s cells, the excess becomes embedded within the walls of arteries. (Excess cholesterol can also be picked up by another type of lipoprotein, high-density lipoprotein, or HDL, which transports it back to the liver for reprocessing or excretion. For this reason, HDL is sometimes referred to as “good” cholesterol, while LDL is termed “bad” cholesterol.)
The higher the LDL level in the bloodstream, the more LDL works its way into the artery walls. As the LDL cholesterol accumulates, it becomes oxidized (that is, it combines with oxygen), provoking an inflammatory reaction that results in the formation of plaques, which are made of cholesterol-rich fatty deposits, proteins and other substances. Other factors, such as smoking and high blood pressure, can further damage the artery wall and aggravate inflammation.
Over time, plaques stiffen and narrow the arteries, and some plaques may eventually limit the flow of blood. When this occurs in the coronary arteries, the result is CAD. One of the characteristic symptoms of CAD is the chest pain or discomfort called angina, which occurs because the heart does not get enough blood. Angina most often occurs during physical activity, when the heart requires more oxygen because of its increased workload but can’t get a sufficient supply because of the narrowing of one or more arteries.
Inflammation not only causes a plaque to grow further, but also promotes the formation of a thin, fibrous cap over the plaque’s lipid core, adding to the bulk of the plaque. Sometimes a plaque becomes so large that it cuts off blood flow in an artery, producing a heart attack—the death of cardiac tissue. But scientists now think that most heart attacks occur because the inflammatory process acts to make a plaque increasingly unstable, and at some point the fibrous cap ruptures. This triggers the release of clotting factors (the way a cut in your skin does) and other responses that cause a blood clot to form at the site of the plaque, which can then cause a heart attack.
What causes high LDL?
A variety of factors affect how much LDL cholesterol is circulating in your blood. Your body—and especially your liver—is constantly producing cholesterol and may produce more than your cells can efficiently process. In addition, your intake of dietary cholesterol can increase total cholesterol and LDL levels. Even more significant, though, is your intake of saturated fat: In both observational studies and clinical trials, researchers have found a much stronger link between blood cholesterol levels and saturated fat and trans fats. Saturated fat is found primarily in animal foods but also in coconut, palm and palm kernel oils. Trans fats, which occur in partially hydrogenated oils used in margarines and many processed foods, are chemically akin to saturated fats. But the effect of trans fats on blood lipids is actually worse, since trans fats not only raise levels of LDL and total cholesterol (as do saturated fats) but also lower levels of heart-protective HDL.
There are people who, no matter how low their intake of saturated fat and dietary cholesterol, nevertheless have high levels of LDL cholesterol. Some of them have a genetic predisposition toward elevated LDL levels. Other factors that can affect cholesterol levels are excess weight and smoking (smokers have lower levels of HDL and higher levels of LDL and triglycerides— another type of blood lipid—than nonsmokers). Certain medical conditions, including diabetes, thyroid disorders and some liver and kidney diseases, can raise LDL. For most people, however, diet is linked to elevated LDL—and is the first defense against it. (HDL is more affected by elevated triglycerides, excess body weight, smoking and lack of exercise, all of which contribute to low HDL levels. Genetic factors also can have a big impact on HDL.)
Some people suffer from genetic abnormalities that severely affect their blood cholesterol levels. For example, an inherited disorder called familial hypercholesterolemia is caused by a genetic defect that leads to extremely high levels of LDL cholesterol and total cholesterol (above 300 mg/dL). About one in 500 children worldwide is born with one form of the disorder; a rarer form affects one person in a million.
Published January 01, 2014