Two major lipids (types of fat) circulate in the blood: cholesterol and triglycerides. Because lipids are not water soluble—and because blood contains water—they must first bind with proteins so that they can circulate in the bloodstream. This fusion of lipids and proteins results in compounds called lipoproteins.

Cholesterol lipoproteins belong to a group of molecules called steroids. When people think of cholesterol, they usually think of all the bad things they've heard about, including its ability to clog arteries and increase the risk of heart attacks and strokes. However, cholesterol is important to the human body, and without it we would die. For starters, cells use cholesterol to form and maintain their membranes, which help cells resist temperature changes and also protect and insulate important nerve fibers. Cholesterol is also necessary to produce sex hormones, such as progesterone and testosterone. And it plays a role in producing bile salts, which are needed to digest food. Cholesterol is also converted into vitamin D in the skin when exposed to sunlight.

Even if we did not eat any foods containing cholesterol, our bodies would produce enough cholesterol to do all these necessary functions. In fact, the body produces 85 percent of the cholesterol in the bloodstream. The remaining 15 percent generally comes from the foods we eat. The only foods that contain cholesterol are those that involve animal products, such as meat, dairy, eggs and animal fat. Eating foods high in cholesterol can increase the total amount of cholesterol in the bloodstream, a condition called hypercholesterolemia.

There are both good and bad cholesterol lipoproteins. Density is what separates good lipoproteins from bad lipoproteins. The density of a lipoprotein depends on the amount of protein in the molecule. High density lipoproteins (HDL) contain cholesterol and high amounts of protein, which help mop up excess cholesterol in the bloodstream and transport it to the liver for disposal. HDL is sometimes referred to as "good" cholesterol. Low density lipoproteins (LDL) contain cholesterol and low amounts of protein. LDL is sometimes referred to as "bad" cholesterol.

Under normal circumstances, liver cells process LDL cholesterol. However, the liver can only process a limited amount of it. If the liver can't process any more cholesterol, it can back up in the bloodstream. When this happens, LDL cholesterol binds to artery walls, which can cause plaques to form. Over time, this plaque buildup can harden and narrow the arteries (atherosclerosis), which can slow down or block the flow of blood. The blood carries oxygen to the heart, and if enough blood and oxygen cannot reach your heart, you may suffer chest pain. If a blockage completely cuts off the blood supply to a portion of the heart, the result is a heart attack—or a stroke if the blood supply to the brain is blocked.

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