Q: Can you explain how high cholesterol causes a heart attack or stroke?
A: Cholesterol is a type of fat that travels in the bloodstream. Our bodies need some cholesterol to survive. But high levels of cholesterol in the blood -- particularly low-density lipoprotein (LDL) cholesterol, or "bad" cholesterol -- increase your risk of cardiovascular disease.
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Let's look at the cascade of events that begins with high cholesterol and ends with a heart attack or stroke. (I've put an illustration of this process on my website, AskDoctorK.com.)
• LDL cholesterol enters the artery (blood vessel) wall. When there's too much LDL cholesterol in your bloodstream, some of it sticks to your artery walls and penetrates into them. These form what are called "fatty streaks."
Even young adults in good health develop fatty streaks. They don't do any harm until and unless they grow into fat plaques of atherosclerosis.
Inside such plaques are a lot of LDL cholesterol. The higher your LDL level, the more LDL particles find their way into your artery walls and into the plaques.
• An inflammatory response begins. When LDL cholesterol penetrates an artery's wall, your immune system "sees" it happening. The cholesterol is not supposed to be inside the wall; it's "foreign" in that location. Since the immune system is supposed to eliminate things that are foreign, it goes on the attack.
This leads to inflammation inside the plaque. Inflammation causes certain proteins and a group of white blood cells called monocytes to enter the plaque.
Over time, the monocytes grow and make granules inside themselves. They're now called macrophages.
• The inflammatory cycle continues. Unfortunately, inflammation doesn't eliminate the cholesterol. If anything, it makes the plaque bigger.
Macrophages stay in the artery wall, feeding on LDL and becoming engorged with cholesterol. At this point they are called "foam cells."
If these fat-stuffed foam cells could walk, they would waddle. When the foam cells die, they spill their cholesterol and the inflammatory substances they are making right back into the plaque.
• Fibrous caps form and rupture. The artery attempts to seal off each plaque of atherosclerosis by covering it with a fibrous material. Eventually, what is called a "fibrous cap" forms over the area. This cap keeps the cholesterol and inflammatory substances inside the plaque.
However, the inflammation inside the plaque can eat away at the fibrous cap, weakening it. If and when the cap weakens a lot, it suddenly ruptures. Cholesterol and the inflammatory substances inside the plaque spill out into the central channel of the artery.
This can cause a blood clot to form. When that happens, the plaque and the clot, together, can suddenly block the flow of blood through the artery. If the blockage is severe enough to rob a part of the heart muscle of the blood it needs, that part of the heart dies. That's a heart attack. A similar process in the brain causes a stroke.
That's how constantly high levels of "bad" cholesterol in the blood, year after year, can slowly lead to plaques that can cause a heart attack or a stroke.
• Dr. Komaroff is a physician and professor at Harvard Medical School. To send questions, go to AskDoctorK.com.