What are the 20 amino acid building blocks of proteins?

In the human body, there are 20 amino acids that function as building blocks of proteins.

Nine of these amino acids are considered essential—they must be consumed in the diet—while five are considered nonessential in that they can be made by the human body. The remaining six protein-building amino acids are conditional, being essential only at certain life stages or in certain disease states.

The essential amino acids are histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine.

The nonessential amino acids are alanine, asparagine, aspartic acid, glutamic acid, and serine.

Conditional amino acids include arginine, cysteine, glutamine, glycine, proline, and tyrosine.

Some authorities recognize a 21st amino acid, selenocysteine, which is derived from serine during protein biosynthesis.