Fat is also known
as lipid and is mainly present in food in a form called 'triglycerides'.
Butter and margarine, for example, are almost entirely made up of
triglycerides. Triglycerides consist of glycerol and three ('tri')
fatty acids. The fatty acids can be mainly 'saturated' as in butter
or mainly 'polyunsaturated' as in some margarines (see
Figure 47). There are also monounsaturated fatty acids,
which occur in quantity in the triglyceride of olive oil and peanut
Food may contain
other fats, such as cholesterol and phospholipids, in addition to
triglycerides. Lecithin is a phospholipid, made up of glycerol, choline
and fatty acids, which again may be mainly saturated or polyunsaturated
48 on lecithin).
Fat is energy
dense, having an Atwater factor of 9 kilocalories, or 37 kilojoules,
per gram. Also, it does not mix with water, so that the food in which
it is found tends to be more energy dense because of the relative
lack of water. If it is a fat of plant origin, it may be associated
with dietary fibre, giving bulk and reducing energy density, as, for
example, with the cereal oats that are relatively high in fat for
a cereal, but also relatively high in dietary fibre. If the fat is
from animal sources, the energy density will be rather high, not only
because of the fat and the low amount of water, but also because there
is no dietary fibre. It is important to realise that the fat content
of meat, and the polyunsaturated to saturated ratio of the fat, varies
considerably depending on the source of the meat, the particular cut
and trim, and the method of preparation.
If we are very
physically active and in need of energy then fat can be a useful source
of the energy. Conversely, if we are physically inactive, too much
fat in our diet can lead to overweight.
It is possible
to eat very little fat and maintain good health. There are traditional
dietary patterns in which the contribution of fat to energy intake
is as low as 10 per cent. However, in the 'affluent' diet, it may
contribute up to 50 per cent of energy intake (see
Figure 5). Nevertheless, it must be remembered that the
quality of the fat eaten as well as its quantity is important (see
Saturated and Polyunsaturated Fats (Chart 9)).
Fat also confers
texture and flavour on food, enhancing its palatability. Many flavours
are fat soluble; unfortunately it is these desirable properties that
range for fat intake:
per cent of energy intake, which for a 35-55 year old man would
be 30-80 grams of fat per day, and for a 35-55 year old woman
would be 20-60 grams of fat per day.