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,
- 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
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
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
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.
There are two dietary factors associated with increases in blood cholesterol
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.
- 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.)
The blood levels of cholesterol tend to increase as we age--a factor doctors
consider when when deciding treatment options for patients with certain
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.
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
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
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
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.
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:
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:
- Lovastatin (Mevacor)
- Simvastatin (Zocor)
- Atorvastatin (Lipitor), a new, highly potent drug
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:
- Cholestyramine (Questran)
- Colestipol (Colestid)
- 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 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.
About the Author
To learn more about Dr. Gordon and his practice, please click here.