Slideshow: Cholesterol 101 -- What Your Levels Mean

We may associate cholesterol with fatty foods, but most of the waxy substance is made by our own bodies. The liver produces 75% of the cholesterol that circulates in our blood. The other 25% comes from food. At normal levels, cholesterol actually plays an important role in helping cells do their jobs. But cholesterol levels are precariously high in nearly 94 million Americans.

High cholesterol does not cause any symptoms. But it does cause damage deep within the body. Over time, too much cholesterol may lead to a buildup of plaque inside the arteries. Known as atherosclerosis, this condition narrows the space available for blood flow and can trigger heart disease. The good news is high cholesterol is simple to detect, and there are many ways to bring it down.

People older than 20 should have their cholesterol levels checked at least once every four to six years. This is done with a simple blood test known as a fasting lipid profile. It measures the different forms of cholesterol that are circulating in the blood after you avoid eating for nine to 12 hours. The results show your levels of "bad" cholesterol, "good" cholesterol, and triglycerides.

Guidelines for cholesterol have changed over time. The most important factor is not necessarily measured by a number but your overall risk for heart disease and/or stroke. 

Most of the cholesterol in the blood is carried by proteins called low density lipoproteins or LDL. This is known as bad cholesterol because it combines with other substances to clog the arteries. A diet high in saturated fats and trans fats tends to raise the level of LDL cholesterol. For most people, an LDL score below 100 is healthy, but people with heart disease may need to take medications to lower their LDL.

Up to a third of blood cholesterol is carried by high-density lipoproteins or HDL. This is called good cholesterol because it helps remove bad cholesterol, preventing it from building up inside the arteries. The higher the level of HDL cholesterol, the better. People with too little are more likely to develop heart disease. Eating healthy fats, such as olive oil, may help boost HDL cholesterol.

The body converts excess calories, sugar, and alcohol into triglycerides, a type of fat that is carried in the blood and stored in fat cells throughout the body. People who are overweight, inactive, smokers, or heavy drinkers tend to have high triglycerides, as do those who eat a very high-carb diet. A triglycerides score of 150 or higher puts you at risk for metabolic syndrome, which is linked to heart disease and diabetes.

Total cholesterol measures the combination of LDL, HDL, and VLDL (very low density lipoprotein) in your bloodstream. VLDL is a precursor of LDL, the bad cholesterol. Your total cholesterol number should be looked at together with other risk factors for heart attack.

Cholesterol-rich foods, like eggs, shrimp, and lobster are no longer completely forbidden. Research shows that the cholesterol we eat has only a small effect on blood cholesterol levels for most people. A few people are "responders," whose blood levels spike up after eating eggs. But for most, saturated fat and trans fats are bigger concerns. Daily cholesterol limits are 300 mg for healthy people and 200 mg for those at higher risk. One egg has 186 mg of cholesterol.

Cholesterol comes from two sources -- the body and food -- and either one can contribute to high cholesterol. Some people inherit genes that trigger too much cholesterol production. For others, diet is the main culprit. Saturated fat and cholesterol occur in animal-based foods, including meat, eggs, and dairy products made with milk.  In many cases, high cholesterol stems from a combination of diet and genetics.

Several factors can make you more likely to develop high cholesterol:

• A diet high in saturated fats and cholesterol
• A family history of high cholesterol
• Being overweight or obese
• Getting older

Until menopause, women typically have lower total cholesterol levels than men of the same age. They also have higher levels of HDL cholesterol, the good kind. One reason is estrogen: The female sex hormone raises the level of HDL cholesterol. Estrogen production peaks during the childbearing years and drops off during menopause. After age 55, a woman's risk of developing high cholesterol begins to climb.

There's evidence that cholesterol can begin clogging the arteries during childhood, leading to atherosclerosis and heart disease later in life. The American Heart Association recommends kids and teenagers with high cholesterol take steps to bring it down. Ideally, total cholesterol should be below 170 in people ages 2 to 19.

High cholesterol is one of the major risk factors for coronary artery disease, heart attacks, and strokes. It also appears to boost the risk of Alzheimer's disease. As we saw earlier, high cholesterol leads to a buildup of plaque that narrows the arteries. This is dangerous because it can restrict blood flow. If the blood supply to a part of the heart or brain is completely cut off, the result is a heart attack or stroke.

Diet changes offer a powerful way to fight high cholesterol. If you've ever wondered why some cereals claim to be heart-healthy, it's the fiber. The soluble fiber found in many foods helps reduce LDL, the bad cholesterol. Good sources of soluble fiber include whole-grain breads and cereals, oatmeal, fruits, dried fruits, vegetables, and legumes such as kidney beans.

No more than 35% of your daily calories should come from fat. But not all fats are equal. Saturated fats -- from animal products and tropical oils -- raise LDL cholesterol. Trans fats carry a double-whammy, boosting bad cholesterol, while lowering the good kind. These two bad fats are found in many baked goods, fried foods (doughnuts, fries, chips), stick margarine, and cookies. Unsaturated fats may lower LDL when combined with other healthy diet changes. They're found in avocados, olive oil, and peanut oil.  

Meat and full-fat milk offer plenty of protein, but they are also major sources of cholesterol. You may be able to reduce LDL cholesterol by switching to soy protein, such as tofu, at some meals. Fish is another great choice. Some varieties, like salmon, are rich in omega-3 fatty acids, which can improve cholesterol levels. The American Heart Association recommends eating fish at least twice a week.

There's growing evidence that low-carb diets may be better than low-fat diets for improving cholesterol levels. In a two-year study funded by the National Institutes of Health, people who followed a low-carb plan had significantly better HDL (good cholesterol) levels than those who followed a low-fat plan.

If you're overweight, talk to your doctor about beginning a weight loss program. Losing weight can help you reduce levels of triglycerides, LDL, and total cholesterol. Shedding even a few pounds can also boost levels of good cholesterol -- it tends to go up one point for every 6 pounds you lose.

Giving up tobacco is tough, but here's one more reason to try. When you stop smoking, your good cholesterol is likely to improve by as much as 10%. You may be more successful if you combine several smoking cessation strategies. Talk to your doctor about which options are best for you.

If you're healthy but not very active, starting an aerobic exercise program could increase your good cholesterol by 5% in the first two months. Regular exercise also lowers bad cholesterol. Choose an activity that boosts your heart rate, such as running, swimming, or walking briskly, and aim for at least 30 minutes on most days of the week. It doesn't have to be 30 continuous minutes; two 15-minute walks works just as well.

If high cholesterol runs in your family, diet and exercise may not be enough to get your numbers where you want them. In that case, medication can give cholesterol levels an extra nudge. Statins are usually the first choice. They block the production of cholesterol in the liver. Other options include cholesterol absorption inhibitors,and  bile acid resins. Your doctor may recommend a combination of these medications.

Certain dietary supplements may help improve cholesterol levels. These include plant sterols, barley and oats, fiber, and green tea. 

Some studies suggest garlic can knock a few percentage points off total cholesterol. But garlic pills can have side effects and may interact with medications. Other herbs that may reduce cholesterol include:

• Fenugreek seeds
• Artichoke leaf extract
• Yarrow
• Holy basil

Many people are able to lower cholesterol levels through a combination of medication and lifestyle changes. But how low is low enough? Recent guidelines do not have a target number. If you have diabetes, are at high risk of developing heart disease, or you already have coronary artery disease, you will probably be put on a drug called a statin to lower your cholesterol.

It takes years for high cholesterol to clog the arteries with plaque. But there is evidence that atherosclerosis can be reversed, at least to some degree. Dean Ornish, MD, has published several studies showing that a low-fat vegetarian diet, stress management, and moderate exercise can chip away at the build-up inside the coronary arteries. Other research supports the idea that big drops in cholesterol can somewhat help open clogged arteries.