Monograph: |
PROTEIN
Proteins are made up of carbon, hydrogen, oxygen.
and nitrogen; most proteins also contain sulphur and
some contain phosphorus. They are required for the
regulation of body processes such as growth and tis-
sue maintenance, and excess protein can be convert-
ed into carbohydrate and used to provide energy.
Proteins consist of chains of amino acids of which
there are essential and non-essential types. Essential
amino acids cannot be synthesised in sufficient
amounts in the body and must therefore be present
in food; non-essential amino acids can be synthe-
sised in the body. Essential amino acids are also
known as indispensable amino acids. There are eight
essential amino acids: isoleucine, leucine, lysine.
methionine, phenylalanine. threonine, tryptophan.
and valine. Arginine and histidine are also essential
for infant growth while synthesis of cysteine. tau-
rine. and tyrosine may be inadequate in premature
infants.
These are made up of some 20 different amino acids, of
which nine are indispensable for normal synthesis of the
different proteins in the body and for maintaining nitrogen
balance in adults. These are listed in the information box.
Arginine is also needed in infants. Methionine can sub-
stitute for cysteine and phenylalanine for tyrosine but not
vice versa. The term 'indispensable' is preferred to the
older term 'essential' because dispensable (non-essential)
amino acids (glycine, alanine etc.) as well as indispensable
amino acids are all essential at the cellular level.
Indispensable' is used to mean 'required in the diet'. The
dispensable amino acids can be synthesised in the body by
transamination provided there is a sufficient supply of
amino groups.
The nutritive value of different proteins depends on the relative proportions of indispensable amino acids they contain (sometimes called its biological value). Proteins of animal origin, particularly from eggs, milk and meat, are generally of higher biological value than the proteins of
vegetable origin which are low in one or more of the
indispensable amino acids. However, when two different
vegetable proteins are eaten together (e.g. a cereal and a
legume), their amino acid patterns can complement one
another and produce a mix of indispensable amino acids
with an adequate protein nutritive value. This is the principle
of protein supply in a vegan or totally vegetarian diet.
|