Cholesterol has been a health issue for decades, but there are still many people who don't fully understand it, especially since cholesterol can be both bad for the body and essential for health.

Cholesterol is a fat, produced by the liver and also found in many foods. It builds and stabilizes the outer membrane of cells; it is used to manufacture hormones such as testosterone and estrogen; it helps produce bile for digestion; and it is part of the myelin sheath, which routes nerve impulses to the brain. It also helps metabolize fat-soluble vitamins, such as A, D, E and K.

It is transported through the blood via particles called lipoproteins. Low-density lipoproteins, or LDL, deliver cholesterol to the body and are known as "bad cholesterol," while high-density lipoproteins, or HDL, are called "good cholesterol" because they remove this fat from the bloodstream. What's bad about LDL cholesterol is that if the body contains more than it needs, it stores it along the walls of the arteries. These cholesterol deposits can restrict the flow of blood, and in extreme cases can block the artery entirely, causing heart attack or stroke.

Your doctor will test your levels of LDL and HDL, along with your level of triglycerides, the main form of fat in the body. If you eat more calories than your body needs right away, they are converted into triglycerides and stored in fat cells, to be released and burned off later if the body needs energy but doesn't have an immediate food source. Many of the lipoproteins that transport triglycerides through the bloodstream also carry cholesterol, so high levels of this type of fat can also be a risk factor for artery problems.

In some cases, high cholesterol levels are just the "luck of the draw." You're more at risk for high cholesterol if you have a family history of heart disease or stroke, or close relatives with high cholesterol. Men are more at risk than women, while women are more at risk if they have early menopause.

However, the most likely culprit is lifestyle, starting with diet. Excess weight is a factor in high cholesterol levels, as is lack of exercise: excess calories are converted to triglycerides, which carry cholesterol in the bloodstream. Researchers are now finding that saturated fat, and foods that contain cholesterol, such as eggs, are not the factors in high cholesterol levels as they were once thought to be. However, trans fats, which are liquid oils processed with hydrogen for improved shelf life, have been found to both raise the levels of LDL, or "bad" cholesterol, and lower the levels of HDL, or "good" cholesterol, and are considered a factor in poor health.

Soluble fiber - the type that dissolves in water and forms a soft gel in the intestines - reduces LDL, but most Americans don't eat enough fiber.

Toxic chemicals in cigarette smoke affect the way the body processes cholesterol, increasing the amount of LDL, while excessive alcohol use raises LDL and lowers the amount of HDL in the body.

Some diseases will cause LDL levels to rise, such as diabetes, kidney and liver diseases, underactive thyroid, and high blood pressure.

Cholesterol levels are diagnosed by a blood test, which measures the levels of LDL ("bad") and HDL ("good") cholesterol, as well as triglyceride levels. In some people, the risk factors are genetic, and doctors will usually prescribe drugs, called statins, that block an enzyme in the liver responsible for producing cholesterol.

For most people, though, a change in lifestyle is necessary. This includes losing weight, quitting smoking, reducing stress, getting more exercise, moderating alcohol intake, and getting sufficient restful sleep.

A diet to reduce cholesterol has been controversial over the years. Researchers now say that foods containing cholesterol, such as eggs and some types of seafood, do not substantially increase cholesterol in the body. As well, saturated fat, found in fatty meat, dairy products, and coconut and palm oils, is no longer considered as much of a culprit for cholesterol, although these fats are high in calories and should be eaten in moderation. However, studies show that trans fats raise the levels of bad cholesterol, and lower those of good cholesterol, and should be avoided.

Soluble fiber, the type that dissolves in water, can lower the levels of bad cholesterol. It's found in such foods as oatmeal, oat bran, barley, apples, prunes and beans. Omega-3 fatty acids, found in oily fish such as salmon, mackerel, halibut and sardines, or in dietary supplements, also play a role in healthy cholesterol levels. Other important foods include whole grains, fresh vegetables, soy, brown rice, olive oil, and small quantities of nuts.