DEAR DOCTOR K: I’ve heard it many times: Regular exercise helps prevent cardiovascular disease. But how does it do that?
DEAR READER: First, let’s define terms, to be sure we’re all on the same page. “Cardiovascular disease” (CVD) is a catch-all term. It includes heart attack, stroke, high blood pressure (hypertension), atherosclerosis and heart failure. Regular (not just occasional) exercise improves cardiovascular health in a number of ways:
— Exercise lowers blood pressure. High blood pressure increases the risk of CVD as well as many other health problems. Regular exercise lowers blood pressure.
— Exercise prevents plaque buildup. Atherosclerosis is the buildup of fatty deposits (plaques) inside the walls of arteries. Arteries are the blood vessels that carry blood from the heart to all of the organs of the body. Plaques are filled primarily with LDL cholesterol — “bad cholesterol.” They can narrow blood vessels and block oxygen-rich blood from reaching the body’s organs, including the heart and the brain.
Exercise increases levels of HDL cholesterol — “good cholesterol.” HDL helps prevent fat accumulation in artery walls. Exercise also lowers levels of LDL cholesterol and triglycerides, another type of fat.
— Exercise protects arteries. Artery walls are lined with a very thin layer of cells known as the endothelium. In healthy arteries, the endothelium produces chemicals that cause the artery walls to relax. That makes the arteries grow wider, and that helps blood to flow through them. As you age, the endothelium loses some ability to secrete these chemicals. As a result, an artery in which blood flow is partially blocked by a plaque is less able to widen and help increase the flow of blood. Exercise helps counter this process.
— Exercise makes clots less likely. Plaques have caps made of fibers. These caps normally hold in the fatty material that is inside the plaque. However, some plaques are “vulnerable”: The cap can rupture, causing the fat to spill into the inside of the artery. This causes a blood clot to form that blocks the flow of blood.
A big part of every blood clot are little cell fragments in the blood called platelets. They stick together and form a clot. Regular exercise causes platelets to be less “sticky,” making it less likely that blood will clot.
— Exercise promotes new small arteries in the heart. Aerobic exercise increases your body’s demand for oxygen-rich blood. In response, your body may cause small arteries in the heart to grow larger, or even to create new ones. If an artery were to become blocked in the future, blood could continue flowing to the heart through these alternate blood vessels.
How does regular exercise do all this? Recent research, much of it here at Harvard Medical School, has shown that regular exercise causes muscle and fat cells to release chemicals that travel through the blood to other organs. These chemical signals produce the healthy changes.
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