Once upon a time, we didn't know anything about fat except
that it made foods tastier. We cooked our food in lard or
shortening. We spread butter on our breakfast toast and plopped
sour cream on our baked potatoes. Farmers bred their animals to
produce milk with high butterfat content and meat "marbled" with
fat because that was what most people wanted to eat.
But ever since word got out that diets high in fat are related
to heart disease, things have become more complicated. Experts tell
us there are several different kinds of fat, some of them worse for
us than others. In addition to saturated, monounsaturated and
polyunsaturated fats, there are triglycerides, trans fatty acids,
and omega 3 and omega 6 fatty acids.
Most people have learned something about cholesterol, and many
of us have been to the doctor for a blood test to learn our
cholesterol "number." Now, however, it turns out that there's more
than one kind of cholesterol, too.
Almost every day there are newspaper reports of new studies
or recommendations about what to eat or what not to eat: Lard is
bad, olive oil is good, margarine is better for you than butter--
then again, maybe it's not.
Amid the welter of confusing terms and conflicting details,
consumers are often baffled about how to improve their diets.
FDA recently issued new regulations that will enable consumers
to see clearly on a food product's label how much and what kind of
fat the product contains. (See "A Little Lite Reading" in the
June 1993 FDA Consumer.) Understanding the terms used to discuss
fat is crucial if you want to make sure your diet is within
recommended guidelines (see accompanying article).
Fats and Fatty Acids
Fats are a group of chemical compounds that contain fatty
acids. Energy is stored in the body mostly in the form of fat. Fat
is needed in the diet to supply essential fatty acids, substances
essential for growth but not produced by the body itself.
There are three main types of fatty acids: saturated,
monounsaturated and polyunsaturated. All fatty acids are molecules
composed mostly of carbon and hydrogen atoms. A saturated fatty
acid has the maximum possible number of hydrogen atoms attached to
every carbon atom. It is therefore said to be "saturated" with
hydrogen atoms.
Some fatty acids are missing one pair of hydrogen atoms in the
middle of the molecule. This gap is called an "unsaturation" and
the fatty acid is said to be "monounsaturated" because it has one
gap. Fatty acids that are missing more than one pair of hydrogen
atoms are called "polyunsaturated."
Saturated fats (which contain saturated fatty acids) are
mostly found in foods of animal origin. Monounsaturated and
polyunsaturated fats (which contain monounsaturated and
polyunsaturated fatty acids) are mostly found in foods of plant
origin and some seafoods. Polyunsaturated fatty acids are of two
kinds, omega-3 or omega-6. Scientists tell them apart by where in
the molecule the "unsaturations," or missing hydrogen atoms, occur.
Recently a new term has been added to the fat lexicon: trans
fatty acids. These are byproducts of partial hydrogenation, a
process in which some of the missing hydrogen atoms are put back
into polyunsaturated fats. "Partially hydrogenated vegetable oils,"
such as vegetable shortening and margarine, are solid at room
temperature.
Cholesterol
Cholesterol is sort of a "cousin" of fat. Both fat and
cholesterol belong to a larger family of chemical compounds called
lipids. All the cholesterol the body needs is made by the liver. It
is used to build cell membranes and brain and nerve tissues.
Cholesterol also helps the body produce steroid hormones needed for
body regulation, including processing food, and bile acids needed
for digestion.
People don't need to consume dietary cholesterol because the
body can make enough cholesterol for its needs. But the typical
U.S. diet contains substantial amounts of cholesterol, found in
foods such as egg yolks, liver, meat, some shellfish, and whole-
milk dairy products. Only foods of animal origin contain
cholesterol.
Cholesterol is transported in the bloodstream in large
molecules of fat and protein called lipoproteins. Cholesterol
carried in low-density lipoproteins is called LDL-cholesterol; most
cholesterol is of this type. Cholesterol carried in high-density
lipoproteins is called HDL-cholesterol. (See "Fat Words.")
A person's cholesterol "number" refers to the total amount of
cholesterol in the blood. Cholesterol is measured in milligrams per
deciliter (mg/dl) of blood. (A deciliter is a tenth of a liter.)
Doctors recommend that total blood cholesterol be kept below 200
mg/dl. The average level in adults in this country is 205 to 215
mg/dl. Studies in the United States and other countries have
consistently shown that total cholesterol levels above 200 to 220
mg/dl are linked with an increased risk of coronary heart disease.
(See "Lowering Cholesterol" in the March 1994 FDA Consumer.)
LDL-cholesterol and HDL-cholesterol act differently in the
body. A high level of LDL-cholesterol in the blood increases the
risk of fatty deposits forming in the arteries, which in turn
increases the risk of a heart attack. Thus, LDL-cholesterol has
been dubbed "bad" cholesterol.
On the other hand, an elevated level of HDL-cholesterol seems
to have a protective effect against heart disease. For this reason,
HDL-cholesterol is often called "good" cholesterol.
In 1992, a panel of medical experts convened by the National
Institutes of Health (NIH) recommended that individuals should have
their level of HDL-cholesterol checked along with their total
cholesterol.
According to the National Heart, Lung, and Blood Institute
(NHLBI), a component of NIH, a healthy person who is not at high
risk for heart disease and whose total cholesterol level is in the
normal range (around 200 mg/dl) should have an HDL-cholesterol
level of more than 35 mg/dl. NHLBI also says that an LDL-
cholesterol level of less than 130 mg/dl is "desirable" to minimize
the risk of heart disease.
Some very recent studies have suggested that LDL-cholesterol
is more likely to cause fatty deposits in the arteries if it has
been through a chemical change known as oxidation. However, these
findings are not accepted by all scientists.
The NIH panel also advised that individuals with high total
cholesterol or other risk factors for coronary heart disease should
have their triglyceride levels checked along with their HDL-
cholesterol levels.
Triglycerides and VLDL
Triglyceride is another form in which fat is transported
through the blood to the body tissues. Most of the body's stored
fat is in the form of triglycerides. Another lipoprotein--very low-
density lipoprotein, or VLDL--has the job of carrying triglycerides
in the blood. NHLBI considers a triglyceride level below 250 mg/dl
to be normal.
It is not clear whether high levels of triglycerides alone
increase an individual's risk of heart disease. However, they may
be an important clue that someone is at risk of heart disease for
other reasons. Many people who have elevated triglycerides also
have high LDL-cholesterol or low HDL-cholesterol. People with
diabetes or kidney disease--two conditions that increase the risk
of heart disease--are also prone to high triglycerides.
Dietary Fat and Cholesterol Levels
Many people are confused about the effect of dietary fats on
cholesterol levels. At first glance, it seems reasonable to think
that eating less cholesterol would reduce a person's cholesterol
level. In fact, eating less cholesterol has less effect on blood
cholesterol levels than eating less saturated fat. However, some
studies have found that eating cholesterol increases the risk of
heart disease even if it doesn't increase blood cholesterol levels.
Another misconception is that people can improve their
cholesterol numbers by eating "good" cholesterol. In food, all
cholesterol is the same. In the blood, whether cholesterol is
"good" or "bad" depends on the type of lipoprotein that's carrying
it.
Polyunsaturated and monounsaturated fats do not promote the
formation of artery-clogging fatty deposits the way saturated fats
do. Some studies show that eating foods that contain these fats can
reduce levels of LDL-cholesterol in the blood. Polyunsaturated
fats, such as safflower and corn oil, tend to lower both HDL- and
LDL-cholesterol. Edible oils rich in monounsaturated fats, such as
olive and canola oil, however, tend to lower LDL-cholesterol
without affecting HDL levels.
How Do We Know Fat's a Problem?
In 1908, scientists first observed that rabbits fed a diet of
meat, whole milk, and eggs developed fatty deposits on the walls of
their arteries that constricted the flow of blood. Narrowing of the
arteries by these fatty deposits is called atherosclerosis. It is a
slowly progressing disease that can begin early in life but not
show symptoms for many years. In 1913, scientists identified the
substance responsible for the fatty deposits in the rabbits'
arteries as cholesterol.
In 1916, Cornelius de Langen, a Dutch physician working in
Java, Indonesia, noticed that native Indonesians had much lower
rates of heart disease than Dutch colonists living on the island.
He reported this finding to a medical journal, speculating that the
Indonesians' healthy hearts were linked with their low levels of
blood cholesterol.
De Langen also noticed that both blood cholesterol levels and
rates of heart disease soared among Indonesians who abandoned their
native diet of mostly plant foods and ate a typical Dutch diet
containing a lot of meat and dairy products. This was the first
recorded suggestion that diet, cholesterol levels, and heart
disease were related in humans. But de Langen's observations lay
unnoticed in an obscure medical journal for more than 40 years.
After World War II, medical researchers in Scandinavia noticed
that deaths from heart disease had declined dramatically during the
war, when food was rationed and meat, dairy products, and eggs were
scarce. At about the same time, other researchers found that people
who suffered heart attacks had higher levels of blood cholesterol
than people who did not have heart attacks.
Since then, a large body of scientific evidence has been
gathered linking high blood cholesterol and a diet high in animal
fats with an elevated risk of heart attack. In countries where the
average person's blood cholesterol level is less than 180 mg/dl,
very few people develop atherosclerosis or have heart attacks. In
many countries where a lot of people have blood cholesterol levels
above 220 mg/dl, such as the United States, heart disease is the
leading cause of death.
High rates of heart disease are commonly found in countries
where the diet is heavy with meat and dairy products containing a
lot of saturated fats. However, high-fat diets and high rates of
heart disease don't inevitably go hand-in-hand.
Learning from Other Cultures
People living on the Greek island of Crete have very low rates
of heart disease even though their diet is high in fat. Most of
their dietary fat comes from olive oil, a monounsaturated fat that
tends to lower levels of "bad" LDL-cholesterol and maintain levels
of "good" HDL-cholesterol.
The Inuit, or Eskimo, people of Alaska and Greenland also are
relatively free of heart disease despite a high-fat, high-
cholesterol diet. The staple food in their diet is fish rich in
omega-3 polyunsaturated fatty acids.
Some research has shown that omega-3 fatty acids, found in
fish such as salmon and mackerel as well as in soybean and canola
oil, lower both LDL-cholesterol and triglyceride levels in the
blood. Some nutrition experts recommend eating fish once or twice a
week to reduce heart disease risk. However, dietary supplements
containing concentrated fish oil are not recommended because there
is insufficient evidence that they are beneficial and little is
known about their long-term effects.
Omega-6 polyunsaturated fatty acids have also been found in
some studies to reduce both LDL- and HDL-cholesterol levels in the
blood. Linoleic acid, an essential nutrient (one that the body
cannot make for itself) and a component of corn, soybean and
safflower oil, is an omega-6 fatty acid.
At one time, many nutrition experts recommended increasing
consumption of monounsaturated and polyunsaturated fats because of
their cholesterol-lowering effects. Now, however, the advice is
simply to reduce dietary intake of all types of fat. (Infants and
young children, however, should not restrict dietary fat.)
The available information on fats may be voluminous and is
sometimes confusing. But sorting through the information becomes
easier once you know the terms and some of the history.
The "bottom line" is actually quite simple, according to John
E. Vanderveen, Ph.D., director of the Office of Plant and Dairy
Foods and Beverages in FDA's Center for Food Safety and Applied
Nutrition. What we should be doing is removing as much of the
saturated fat from our diet as we can. We need to select foods that
are lower in total fat and especially in saturated fat." In a
nutshell, that means eating fewer foods of animal origin, such as
meat and whole-milk dairy products, and more plant foods such as
vegetables and grains. n
Eleanor Mayfield is a writer in Silver Spring, Md.
Fat Words
Here are brief definitions of the key terms important to an
understanding of the role of fat in the diet.
Cholesterol: A chemical compound manufactured in the body. It is
used to build cell membranes and brain and nerve tissues.
Cholesterol also helps the body make steroid hormones and bile
acids.
Dietary cholesterol: Cholesterol found in animal products that are
part of the human diet. Egg yolks, liver, meat, some shellfish, and
whole-milk dairy products are all sources of dietary cholesterol.
Fatty acid: A molecule composed mostly of carbon and hydrogen
atoms. Fatty acids are the building blocks of fats.
Fat: A chemical compound containing one or more fatty acids. Fat is
one of the three main constituents of food (the others are protein
and carbohydrate). It is also the principal form in which energy is
stored in the body.
Hydrogenated fat: A fat that has been chemically altered by the
addition of hydrogen atoms (see trans fatty acid). Vegetable oil
and margarine are hydrogenated fats.
Lipid: A chemical compound characterized by the fact that it is
insoluble in water. Both fat and cholesterol are members of the
lipid family.
Lipoprotein: A chemical compound made of fat and protein.
Lipoproteins that have more fat than protein are called low-density
lipoproteins (LDLs). Lipoproteins that have more protein than fat
are called high-density lipoproteins (HDLs). Lipoproteins are found
in the blood, where their main function is to carry cholesterol.
Monounsaturated fatty acid: A fatty acid that is missing one pair
of hydrogen atoms in the middle of the molecule. The gap is called
an "unsaturation." Monounsaturated fatty acids are found mostly in
plant and sea foods.
Monounsaturated fat: A fat made of monounsaturated fatty acids.
Olive oil and canola oil are monounsaturated fats. Monounsaturated
fats tend to lower levels of LDL-cholesterol in the blood.
Polyunsaturated fatty acid: A fatty acid that is missing more than
one pair of hydrogen atoms. Polyunsaturated fatty acids are mostly
found in plant and sea foods.
Polyunsaturated fat: A fat made of polyunsaturated fatty acids.
Safflower oil and corn oil are polyunsaturated fats.
Polyunsaturated fats tend to lower levels of both HDL-cholesterol
and LDL-cholesterol in the blood.
Saturated fatty acid: A fatty acid that has the maximum possible
number of hydrogen atoms attached to every carbon atom. It is said
to be "saturated" with hydrogen atoms. Saturated fatty acids are
mostly found in animal products such as meat and whole milk.
Saturated fat: A fat made of saturated fatty acids. Butter and lard
are saturated fats. Saturated fats tend to raise levels of LDL-
cholesterol ("bad" cholesterol) in the blood. Elevated levels of
LDL-cholesterol are associated with heart disease.
Trans fatty acid: A polyunsaturated fatty acid in which some of the
missing hydrogen atoms have been put back in a chemical process
called hydrogenation. Trans fatty acids are the building blocks of
hydrogenated fats.
--E.M.
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