Introduction
Fats
provide a concentrated source of energy in the diet. The building blocks
of fats are called fatty acids. These can be either saturated, monounsaturated
or poly-unsaturated. Foods rich in saturated fats are usually of animal
origin. Vegetable fats are generally unsaturated.
Saturated fat raises the level
of cholesterol in the blood. Cholesterol is present in animal foods
but not plant foods. It is essential for metabolism but is not needed
in the diet as our bodies can produce all that is needed. Raised blood
cholesterol is associated with an increased risk of heart disease.
Fats and oils are essentially
the same. Fats tend to be solid at room temperature whilst oils are
liquid. The term lipids include both fats and oils.
Structure & Functions
Fats
consist of fatty acids and glycerol. Nearly all the fats in our bodies
and in foods are triglycerides, being made up of three fatty acid molecules
to one glycerol molecule. There are about 16 different fatty acids commonly
present in foods. The nature of fat depends on its constituent fatty
acids.
Fats can be classed as either
saturated, monounsaturated or polyunsaturated. This depends on the
type of chemical bonds present in the fatty acid. If a fatty acid has
all the hydrogen atoms it can hold it is termed saturated. However,
if some of the hydrogen atoms are absent and the usual single bond
between carbon atoms has been replaced by a double bond, then it is
unsaturated. If there is just one double bond then it is monounsaturated.
If there is more than one then it is polyunsaturated. Most fats contain
a proportion of each of these three basic types of fatty acid but are
generally described according to which type predominates.
Saturated fats tend to be animal
fats and are solid at room temperature. Butter, lard, suet and meat
fat are saturated fats. Unsaturated fats are liquid at room temperature.
They are usually of plant origin, though fish oils may also be high
in polyunsaturated fatty acids. Plant oils may be hardened by the addition
of hydrogen atoms, converting double bonds to single bonds. This process
is known as hydrogenation. Hydrogenated vegetable oils are often present
in margarine and other processed foods.
Fats have a number of important
functions in the body. As well as being a concentrated source of energy,
fats act as carriers for fat-soluble vitamins A, D, E and K. Fats are
also essential for the structure of cell membranes and are precursors
of many hormones.
Essential Fatty
Acids
Two fatty acids are termed essential fatty acids. These
are linoleic acid and a-linolenic acid. These must be present in
the diet as the body is unable to make them itself. They are widely
present in plant oils such as sunflower, rapeseed and soyabean
oils.
Linoleic acid is converted into
the body to arachidonic acid from which prostoglandins and other vital
compounds are made. Because of this conversion, arachidonic acid is
not an essential fatty acid as was once believed. a-Linolenic acid
is converted to eicosapentanoic acid (EPA) which is important in proper
nerve function. EPA is present in fish oils and is claimed to be beneficial
in reducing the symptoms of arthritis and the risk of heart disease.
For this reason, fish oils are sometimes used therapeutically. Plant
oils containing large amounts of a-linolenic acid can be used as an
alternative by vegetarians. Linseeds and linseed oil are particularly
rich sources of a-linolenic acid.
Cholesterol
Cholesterol
belongs to the sterol group of fats. It is present in all animal tissues
but is absent from plants. Cholesterol is essential as a component of
cell membranes and a precursor of bile acids and certain hormones. The
body can make its own cholesterol and so a dietary source is not required.
Cholesterol is transported in
to various proteins. These complex molecules are called lipoproteins.
There are four main types of lipoprotein involved in cholesterol transport.
The most commonly refer red to are low density lipoprotein (LDL) and
high density lipoprotein (HDL).
Cholesterol may form plaques
on artery walls if levels in the blood are too high. This can lead
to atherosclerosis. Because of this high blood cholesterol is linked
with heart disease. It is the LDL cholesterol which has been linked
to heart disease. HDL cholesterol may help protect against the risk
of heart disease.
The amount of dietary cholesterol
is not clearly linked to levels of cholesterol in the blood. Blood
cholesterol is more closely related to the amount of saturated fat
in the diet, saturated fat raising blood cholesterol. Unsaturated fats
are not thought to raise blood cholesterol and may indeed lower levels.
Trans Fatty Acids Unsaturated
fatty acids can exist in two different geometric forms. These are called
the cis and trans forms. Unsaturated fatty acids exist naturally in the
cis form. During food manufacturing processes these cis fatty acids may
be changed to trans fatty acids. Hydrogenation of margarine causes this
to occur. It has been suggested that trans fatty acids can increase the
risk of heart disease.
Free Radicals
Free
radicals are highly reactive molecules which have been linked to both
heart disease and cancer. A number of factors, including alcohol, stress
and environmental pollutants can increase the generation of free radicals
in the body. Polyunsaturated fats can also generate free radicals, especially
when exposed to heat or sunlight. Because of this it is suggested that
vegetable oils should be stored out of direct sunlight. Mono-unsaturated
olive oil is less vulnerable to free radical generation and so is a better
choice for frying.
Anti-oxidants such as vitamins
A, C and E offer protection against free radicals. Fresh fruit and
vegetables are rich in these anti-oxidants.
Dietary Sources
Saturated
fats are nearly always from animal foods. Meat, eggs and dairy products
all contain saturated fats. Lard and suet are saturated fats. Coconut
oil and palm oil are vegetable sources of saturated fats. Olive oil is
a monounsaturated fat. Polyunsaturated fats are usually from plant sources.
The ratio of polyunsaturated
to saturated fats in the diet is often called the P:S ratio.
Cholesterol is present in all
animal foods but not plant foods. Egg yolks and high-fat dairy products
are high in cholesterol.
Required Intakes
Currently it is believed that around 42% of energy in the typical British
diet is from fat. Dietary advice is to reduce this. The COMA (Committee
on Medical Aspects of Food Policy) report advocated that no more than 35%
of daily energy requirement should come from fat whilst the NACNE (National
Advisory Committee on Nutritional Education) paper recommends a reduction
to no more than 30%. Special emphasis is placed on reducing the amount
of saturated fat in the diet.
Vegetarian diets tend to be
lower in fat than omnivore diets. However, vegetarians consuming dairy
products and processed foods high in fat may still be consuming too
much. Advice to vegetarians is to keep fat intake to a minimum, avoid
high fat dairy products and processed foods containing dairy fats and
hydrogenated vegetable fats, and to use olive oil for cooking purposes.
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