Have you ever been about to take a big bite of your triple chocolate fudge cake when someone leaned over and said "you better watch your cholesterol"? That's happening to all of us more frequently because medical research has shown that high levels of cholesterol are associated with heart disease, the nation's number one killer. Statistics say that more than half of all Americans have cholesterol levels that exceed the recommended total and one in five Americans has cholesterol levels that are considered high.

What we don't often hear is the important fact that some cholesterol is vital to human life. In this edition of How Stuff Works we will take a look at cholesterol, both why it is needed for normal human--and animal--functions and why at high levels and in many individuals, it can be deadly. We'll also describe what contributes to and the treatments for high cholesterol levels so that you can take steps to limit your health risks.

What is Cholesterol?

Cholesterol is a waxy, fat-like compound that belongs to a class of molecules called steroids. It's found in many foods, in your bloodstream and in all your body's cells. If you had a handful of cholesterol, it might feel like a soft, melted candle. Cholesterol is essential for:

  • Formation and maintenance of cell membranes (helps the cell to resist changes in temperature and protects and insulates nerve fibers)
  • Formation of sex hormones (progesterone, testosterone, estradiol, cortisol)
  • Production of bile salts, which help to digest food
  • Conversion into vitamin D in the skin when exposed to sunlight.

The formation of cholesterol involves a series of complicated biochemical reactions that begin with the widespread 2-carbon molecule Acetyl CoA: Acetyl CoA (C2) --> mevalonate (C6) --> isopentenyl pyrophosphate (C5) --> squalene (C30) --> cholesterol (C27). Cholesterol is made primarily in your liver (about 1,000 milligrams a day), but also by cells lining the small intestine and by individual cells in the body.

Most of the body's cholesterol is manufactured in the liver.

What's the Difference Between Blood and Dietary Cholesterol?

It may surprise you to know that our bodies make all the cholesterol we need. When your doctor takes a blood test to measure your cholesterol level, the doctor is actually measuring the amount of circulating cholesterol in your blood, or your blood cholesterol level. About 85 percent of your blood cholesterol level is endogenous, which means it is produced by your body. The other 15 percent or so is comes from an external source -- your diet. Your dietary cholesterol originates from meat, poultry, fish, seafood and dairy products. It's possible for some people to eat foods high in cholesterol and still have low blood cholesterol levels. Likewise, it's possible to eat foods low in cholesterol and have a high blood cholesterol level.

So why is there so much talk about cholesterol in our diet? It's because the level of cholesterol already present in your blood can be increased by high consumption of cholesterol and saturated fat in your diet. This increase in dietary cholesterol has been associated with atherosclerosis, the build-up of plaques that can narrow or block blood vessels -- think about what happens to your kitchen drain pipes when you pour chicken fat down the sink. (See How Heart Attacks Work for a good description of atherosclerosis.) If the coronary arteries of the heart become blocked, a heart attack can occur. (See How Your Heart Works for details.) The blocked artery can also develop rough edges. This can cause plaques to break off and travel, obstructing blood vessels elsewhere in the body. A blocked blood vessel in the brain can trigger a stroke.

This coronary artery is becoming dangerously blocked as the cholesterol builds up.

The average American man eats about 360 milligrams of cholesterol a day; the average woman eats between 220 and 260 milligrams daily. So how are we doing? The American Heart Association recommends that we limit our average daily cholesterol intake to less than 300 milligrams. Obviously, people with high levels of cholesterol in the blood should take in even less.

When Is Cholesterol "Good" and When Is It"Bad"?

Comments about "good" and "bad" cholesterol refer to the type of carrier molecule that transports the cholesterol. These carrier molecules are made of protein and are called apoproteins. They are necessary because cholesterol and other fats (lipids) can't dissolve in water, which also means they can't dissolve in blood. When these apoproteins are joined with cholesterol, they form a compound called lipoproteins. The density of these lipoproteins is determined by the amount of protein in the molecule. "Bad" cholesterol is the low-density lipoprotein (LDL), the major cholesterol carrier in the blood. High levels of these LDLs are associated with atherosclerosis. "Good" cholesterol is the high-density lipoprotein (HDL); a greater level of HDL--think of this as the drain cleaner you put in the sink--is thought to provide some protection against artery blockage.

A high level of LDL in the blood may mean that cell membranes in the liver have reduced the number of LDL receptors due to increased amounts cholesterol inside the cell. After a cell has used the cholesterol for its chemical needs and doesn't need any more, it reduces its number of LDL receptors. This enables LDL levels to accumulate in the blood. When this happens, the LDLs begin to deposit cholesterol on artery walls, forming thick plaques. In contrast, the HDLs--the "good" guys--act to remove this excess cholesterol and transport it to the liver for disposal.

A third group of carrier molecules, the very low-density lipoproteins (VLDL) are converted to LDL after delivering triglycerides to the muscles and adipose (fat) tissue.

The levels of HDL, LDL and total cholesterol are all indicators for atherosclerosis and heart attack risk. People who have a cholesterol level of 275 or greater (200 or less is desirable) are at significant risk for a heart attack, despite a favorable HDL level. In addition, people who have normal cholesterol levels but low HDL levels are also at increased risk for a heart attack.

What Factors Affect Cholesterol Levels?

There are a number of factors that influence a person's cholesterol levels. They include diet, age, weight, gender, genetics, diseases and lifestyle.

Diet

There are two dietary factors associated with increases in blood cholesterol levels:

  • Eating foods that are high in saturated fats, even if the fats themselves do not contain cholesterol. (These include foods containing high levels of hydrogenated vegetable oils, especially palm and coconut oils, avocados and other high-fat foods of vegetable origin.)
  • Eating foods containing high levels of cholesterol. (This group includes eggs and red meat--the most maligned of the cholesterol culprits--as well as lard and shrimp. These foods can significantly raise blood cholesterol levels, especially when combined with foods that are high in saturated fat.)
It's important to note that only foods of animal origin contain cholesterol. Lack of awareness of this fact has led to some confusing labels at the grocery store. For example, some items that are high in saturated fats from plant sources bear labels claiming that they are 100 percent cholesterol free. The statement may be true, but it's generally misleading because it implies that the product is definitely beneficial to your health.

Age

The blood levels of cholesterol tend to increase as we age--a factor doctors consider when when deciding treatment options for patients with certain cholesterol levels.

Weight

People who are overweight are more likely to have high blood cholesterol levels. They also tend to have lower HDL levels. The location of the excess weight also seems to play a role in cholesterol levels. A greater risk of increased cholesterol levels occurs when that extra weight is centered in the abdominal region, as opposed to the legs or buttocks.

Gender

Men tend to have higher LDL levels and lower HDL levels than do women, especially before age 50. After age 50, when women are in their post-menopausal years, decreasing amounts of estrogen are thought to cause the LDL level to rise.

Genetics

Some people are genetically predisposed to having high levels of cholesterol. A variety of minor genetic defects can lead to excessive production of LDLs or a decreased capacity for their removal. This tendency towards high cholesterol levels is often passed on from parents to their children. If your parents have high cholesterol, you need to be tested to see if your cholesterol levels are also elevated.

Diseases

Diseases such as diabetes can lower HDL levels, increase triglycerides and accelerate the development of atherosclerosis. High blood pressure, or hypertension, can also hasten the development of atherosclerosis, and some medications used to treat it can increase LDL and triglycerides and decrease HDL levels.

Lifestyle

Factors that negatively affect cholesterol levels also include high levels of stress, which can raise total cholesterol levels, and cigarette smoking, which can lower a person's HDL level as much as 15 percent. On the other hand, strenuous exercise can increase HDL levels and decrease LDL levels. Exercise also can help reduce body weight, which, in turn, can also help reduce cholesterol. Recent research has shown that moderate alcohol use (one drink per day for women, two drinks a day for men) can raise HDL cholesterol and therefore reduce the risk of heart attack. Despite such research, it is difficult to recommend the habitual use of alcohol, because there are also negative health consequences associated with alcohol use and a high potential for abuse.

Always remember that risk factors for high cholesterol and cardiovascular disease don't exist in a vacuum--they tend to amplify each other. Reducing the risk of a cardiovascular disease involves eliminating all of the risk factors that we can control and seeking medical advise for those we can't.

How Often Should I Get My Cholesterol Tested and What Do the Numbers Mean?

You should get your cholesterol tested every three to five years, more often if you have high cholesterol levels. Please refer to the table below for guidelines for total cholesterol, LDL and HDL levels.

Desirable Borderline Undesirable
Total Cholesterol Below 200 200-240 Above 240
HDL Cholesterol Above 45 35-45 Below 35
LDL Cholesterol Below 130 130-160 Above 160
Total Cholesterol/HDL Below 4.5 4.5-5.5 Above 5.5
LDL/HDL Below 3 3-5 Above 5

Figure 1 - Blood Type Relationships

How Can You Reduce Your Cholesterol?

There are several steps you can take to reduce your cholesterol levels. The first is to eat a low-fat, low-cholesterol diet. That means keeping your total fat consumption--saturated, polyunsaturated and monounsaturated--to fewer than 30 percent of your daily intake of calories. (See How Fat Works for details.) Remember to keep your cholesterol intake to fewer than 300 milligrams per day. Saturated fats contained in butter, whole milk, hydrogenated oils, chocolate (See How Chocolate Works), shortening, etc. should comprise no more than one third of your total fat consumption. To reduce your total fat and cholesterol intake, limit your consumption of meats such as beef, pork, liver and tongue (always trim away excess fat). In addition, avoid cheese, fried foods, nuts and cream, and try to curb your intake of eggs to no more than four per week. Try to eat meatless meals several times a week, use skim milk and include fish in your diet. Eat a wide variety of vegetables, pasta, grains and fruit. Another good tip is to look at the package label of the foods you buy, and restrict your choices to foods containing 3 grams of fat or less per serving.

There is evidence that water-soluble fibers can aid in lowering cholesterol; these foods include the fiber in oat or corn bran, beans and legumes, pectin found in apples and other fruits, and guar that is used as a thickener. Although highly touted by the media and health food stores, the phospholipid Lecithin has not been confirmed as a reducer of blood cholesterol levels.

If you are overweight, trying to lose weight and including aerobic exercise in your routine can help raise those desirable HDL levels. Diet and exercise alone can decrease cholesterol levels by up to 15 percent.

It probably comes as no surprise to you that, if you smoke, you should quit to avoid a wide range of health problems, including lower HDL levels and increased risk of heart attack.

When Is Medication Used to Lower Cholesterol?

Sometimes positive changes in diet, lifestyle and exercise are not enough. In these cases, doctors may consider the use of medication that lowers cholesterol. The decision to have a patient begin medication is often based on (high) levels of LDL cholesterol and other risk factors for cardiovascular disease. For example, medication may be indicated if your LDL level is over 190 or is over 160 and you have several other risk factors for cardiovascular disease.

Drugs that reduce LDL blood levels can prevent or reduce the build-up of artery blocking plaques and can limit the possibility of the release of those plaques as dangerous blood clots. There are several types of drugs that can help reduce blood cholesterol levels! The most commonly prescribed are the statins, HMG-CoA reductase inhibitors, including:

  • Lovastatin (Mevacor)
  • Simvastatin (Zocor)
  • Atorvastatin (Lipitor), a new, highly potent drug
These drugs work within the liver to directly prevent the formation of cholesterol and can lower LDL cholesterol by as much as 40 percent. Research also shows that these drugs can reduce the risk of death from cardiovascular disease. Another major drug category is the resins, which bind bile acides, causing the liver to produce more of them and using up cholesterol in the process. By "tying" it up, these drugs make cholesterol less available in the blood. They include:
  • Cholestyramine (Questran)
  • Colestipol (Colestid)
The B vitamin Niacin, in high doses, can lower triglycerides and LDL levels and increase HDL levels. Niacin has been proven to reduce a person's risk of having a second heart attack. Last are the drugs in the fibrates category, which lower triglycerides and can increase HDL levels. These include:
  • Gemfibrozil (Lopid)
  • Fenofibrate (Tricor)

The decision to take cholesterol- or lipid-lowering drugs is not taken lightly by your doctor. These drugs can be fairly expensive and are often required for many years or even the rest of your life. It is also important to note that some of these drugs can have dangerous side effects, such as damage to the liver.

The Best Advice?
Adopt a healthy lifestyle and visit your doctor regularly. Have your cholesterol levels checked there rather than risk incorrectly interpreting numbers in self test kits currently on the market. Remember, cholesterol is necessary for life but it can also be very harmful and requires monitoring. So, watch your cholesterol and take heart from the fact that, for every 1 percent drop in your cholesterol level, your risk of heart attack is lowered by 2 percent. If you'd like to learn more about your own cholesterol, Check Your Cholesterol and Heart Disease IQ.

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