Amino acids are the essential building blocks of all proteins.  There are twenty amino acids, twelve of which are produced by the body.  The other eight amino acids are known as essential amino acids; they cannot be produced by the body, and must be supplied through dietary intake.  The essential amino acids are isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine.  All twenty amino acids, however, are necessary for protein synthesis.  They are also the building blocks for hormones, enzymes and neurotransmitters in the body.

Amino acid supplements are often used by athletes to add lean muscle mass, and by vegetarians who find it more difficult to get complete proteins (proteins including all essential amino acids) from dietary sources. 

In addition, individual amino acids and related compounds are often used as supplements for specific conditions.  Lysine, for example, inhibits the proliferation of viruses, including genital herpes and shingles, tryptophan is essential for niacin synthesis and for alleviation of insomnia, and the amino acid compound SAM-e is useful in treating depression.