Amino Acids may occur in two possible optical isomers, called D or L-amino acids. The L-amino acids represent the vast majority of amino acids found in proteins. The D-amino acids are found in some proteins produced by exotic sea-dwelling organisms. The basic function of most amino acids is to supply essential material for duplication of the genetic code, for cell division and the formation of muscles and connective tissues. They are utilised in the body in various ways including antibodies, endorphins, enzymes, hormones, neurotransmitters and nutrient carriers.

Essential Amino Acids must be obtained from the diet, and include arginine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine. Arginine and histidine are generally only considered essential in children, because the metabolic pathways that synthesise these amino acids are not fully developed in children. It is important to obtain sufficient amounts of essential amino acids in the diet to ensure healthy protein synthesis in the body.

Non-Essential Amino Acids are synthesised by the body from essential amino acids, and include alanine, asparagine, aspartate, cysteine, glutamine, glutamate, glycine, proline, serine and tyrosine. Although nonessential amino acids are synthesised in the body, some individuals may require additional intake, for example, people with phenylketonuria (PKU) must keep their intake of phenylalanine extremely low to prevent various metabolic complications, however, as phenylalanine is the precursor for tyrosine synthesis, tyrosine becomes essential in the diet of PKU patients.