Cholesterol
Cholesterol is a lipidic, waxy alcohol found in the cell membranes and transported in the
blood plasma of all animals or a waxy fat-like substance (lipid). About 80 percent of
cholesterol is manufactured in the liver. The remaining cholesterol is consumed in
cholesterol-rich foods such as meat, eggs or dairy products. Cholesterol is vital to good
health. It is an essential component of mammalian cell membranes where it is required
to establish proper membrane permeability and fluidity. Cholesterol is the principal sterol
synthesized by animals, but small quantities are synthesized in other eukaryotes, such
as plants and fungi.
Cholesterol itself is vital for survival. However, it can also contribute to coronary artery
disease. To understand how cholesterol is related to heart disease, it is necessary to
understand how it is transported through the body. Cholesterol is carried in the
bloodstream in specialized protein packages called lipoproteins. These are comprised of
another building block called apolipoproteins.
The body uses cholesterol to:
Assist in the manufacture of hormones
Break down carbohydrates and proteins
Help form a protective coating around nerves
Build cell walls and produce bile (the word cholesterol is Greek for “bile solids”)
Cholesterol is carried through the bloodstream by lipoproteins. Lipoproteins are proteins
that wrap around cholesterol and other fatty materials and transport them through the
bloodstream. There are five major kinds of lipoproteins, each of which has a different
function in the body. They are:
High-density lipoproteins (HDL). “Good” cholesterol, HDLs move easily through
the blood and are actually beneficial. They are stable and do not stick to artery
walls. They help prevent heart disease by carrying cholesterol away from the
arteries and back to the liver, where the process of its removal from the body
begins. Liver damage, from alcohol abuse or other conditions, can undo the
beneficial effects of HDLs.
Low-density lipoproteins (LDL). “Bad” cholesterol, LDLs contain more fat and
less protein than HDLs. LDLs are unstable and tend to fall apart. They are more
likely to adhere to the walls of the artery and penetrate the protective inner lining
of cells (e.g., the endothelium). Once cholesterol has migrated into the inner wall
of the artery, it oxidizes and attracts other fatty substances (e.g., triglycerides),
sticky blood-clotting materials (e.g., fibrin and platelets) and white blood cells.
Together, these substances form the building materials for plaque deposits, which
are the hallmark of atherosclerosis, or "hardened arteries." Atherosclerosis is the
leading cause of coronary artery disease, which is the leading cause of heart
attack and death in the United States. LDL reduction is a primary therapeutic goal
among people with abnormal cholesterol levels (e.g., dyslipidemia). Many people
with high levels of "bad" cholesterol also have high triglyceride levels because
both types of fats have similar risk factors (e.g., obesity and diabetes).
Very low-density lipoproteins (VLDL). These are extremely harmful lipoproteins
that carry triglycerides and cholesterol.
Intermediate-density lipoproteins. Like VLDLs, these also carry triglycerides and
cholesterol.
Chylomicrons. These are very large particles that are rich in triglycerides.
Cholesterol Education Program classifies cholesterol levels as follows (all measurements
are in milligrams per deciliter):
Total cholesterol levels less than 200 are desirable.
Total cholesterol levels between 200 and 239 are borderline-high.
Total cholesterol levels that are 240 or higher are high.
HDL levels of less than 40 for men and 50 for women are considered low, and
levels of 60 or more are considered protective against heart disease.
HDL levels should be 40 or above (the American College of Cardiology
recommends levels of 45 or above for women).
LDL levels should optimally be less than 100 (levels greater than 129 are
considered borderline-high, and levels greater than 159 are considered
high). Among some patients, physicians might aim for an even lower LDL target
of 70.
Regular cholesterol screenings are particularly important for people who have risk factors
such as diabetes, obesity or a family history of cardiovascular disease
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