TABLE OF CONTENTS Proteins can be used as an energy source for the body, if required. However, their primary function is in the formation of body structures, such as muscle. Proteins are made up of amino acids, of which there are around 20 different types in all. The body requires these in various quantities and cannot easily store them. Those surplus to immediate needs are excreted, and a shortage of any one particular amino acid will mean that building work on a particular protein is halted. Therefore it is important to eat a mixture of proteins that will provide enough of each of the 20 individual amino acids.
Amino acids are all based on the common structure shown in Box 11, but they vary considerably in their chemical properties because of the differences in the R groups.
The body can make many of the amino acids, but nine of them (histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine) either cannot be made at all or not in sufficient quantities for the body s needs. These are termed essential amino acids, and must be provided from the diet. Others may also be required from the diet in certain circumstances, e.g. tyrosine, which is normally made from phenylalanine (Whitney et al, 1998).
In general, individual animal products have a better balance of amino acids for our needs than plant products. This is not surprising, bearing in mind the use that the animal would have made of the proteins and individual amino...
