Proteins are built from sub-units called amino acids which are linked together in chains of various lengths and sequences, thus accounting for the vast variety of different proteins.
Some amino acids are ‘essential’ which means that they cannot be made by the body and have to be included in the diet. Others can be made by the body and are thus termed ‘non-essential’.
It should be noted that the terms ‘essential’ and ‘non-essential’ relate only to dietary requirements, and not at all to the relative importance of the individual amino acids.
An ‘L -’ prefix before the name of an amino acid denotes a naturally occurring form which the body can use. The mirror image ‘D-form’ is not normally useful to human body cells but is important in the structure and metabolism of bacteria.
CLASSIFICATION OF AMINO ACIDS
| Essential | Non – Essential | |
| Isoleucine | Alanine | Serine* |
| Leucine | Aspartic Acid | Tyrosine* |
| Lysine | Asparagine | Arginine* |
| Methionine | Cysteine* | Histidine* |
| Phenylalanine | Glutamic Acid | |
| Threonine | Glutamine* | |
| Tryptophan | Glycine* | |
| Valine | Proline* |
* These amino acids are regarded as ‘conditionally essential’ in that they can be made by the adult body, but babies and young children may not be able to make enough for their needs and so also require a dietary supply.