is an essential part of the diet. It is made up of various combinations
of small organic chemicals called amino acids. When we eat food containing
protein it is broken down during digestion into its constituent amino
acids. These amino acids are absorbed by our bodies and are used to
produce new proteins and other necessary substances. Our bodies can
make some of the amino acids needed to manufacture proteins, but others
must be obtained from the diet; these are the eight so-called 'essential'
amino acids. In addition, one other amino acid is needed by infants
during early growth and development.
part of the structure of the body, so that a continual supply of amino
acids is needed. Our bodies are able to put these basic amino acid
units together, using different arrangements of amino acids, to produce
specific proteins, which can only be produced if all the necessary
amino acids are available.
value of a protein food can be judged by its ability to provide both
the quantity and number of essential amino acids needed by the body.
Different food sources contain different groups of proteins, which
are made up of different arrangements and amounts of amino acids.
In general, proteins from animal sources are of greater nutritional
value because they usually contain all the essential amino acids.
Proteins from plant sources, such as cereals and vegetables, may be
deficient in one or other of the essential amino acids. For example,
the proteins obtained from wheat lack adequate quantities of one essential
amino acid, and those from beans are deficient in another.
Because the deficiency
is different in each food, when they are eaten together they complement
each other and the mixture is of higher nutritional value than the
separate foods, and is as good as animal protein. It is important,
particularly for strict vegetarians who do not consume dairy or egg
Figure 21), that a variety of different types of protein
foods are eaten.
Cooking can alter
the amino-acid composition of protein and this usually results in
desirable flavour and browning development. Very little nutritional
value is lost.
DAILY DIETARY INTAKE OF PROTEIN IN AUSTRALIA
recommended dietary intake (RDI) in Australia is one gram per kilogram
of body weight per day. The protein intake for a 70-kilogram man is
70 grams and for a 58-kilogram woman, 58 grams per day. Growing children
and pregnant and lactating women have a greater requirement for protein
because of the additional needs of these conditions (see
Figure 6). People who have had severe infections or surgery may
require additional protein. Because of the margin of safety in the
RDI for protein it is usually not necessary to increase protein intake
for additional muscular activity such as required for heavy work or
A deficiency of
protein in the diet can lead to muscle wasting, oedema, anaemia and,
in children, a slowing or stopping of growth. These conditions are
usually seen as a result of chronic protein malnutrition. Having an
adequate energy intake (see Figure
11 and Figure
12) will almost always ensure an adequate protein intake.
of protein consumption appear to be neither beneficial nor harmful.
However, it is possible that additional calcium may be required to
counterbalance an excessive protein intake. Also there is a higher
load of protein breakdown products, which must be excreted by the