Six other amino acids are considered conditionally essential in the human diet, meaning their synthesis can be limited under special pathophysiological conditions, such as prematurity in the infant or individuals in severe catabolic distress These six are arginine, cysteine, glycine, glutamine, proline, and tyrosine. Six amino acids are non-essential (dispensable) in humans, meaning they can be synthesized in sufficient quantities in the body. These six are alanine, aspartic acid, asparagine, glutamic acid, serine,and selenocysteine (considered the 21st amino acid). Pyrrolysine (considered the 22nd amino acid),which is proteinogenic only in certain microorganisms, is not used by and therefore non-essential for most organisms, including humans.

The limiting amino acid is the essential amino acid which is furthest from meeting nutritional requirements.This concept is important when determining the selection, number, and amount of foods to consume because even when total protein and all other essential amino acids are satisfied if the limiting amino acid is not satisfied then the meal is considered to be nutritionally limited by that amino acid.