**3. Nonessential amino acids**

Nonessential amino acids should not be misconstrued as unimportant amino acids. Of the 20 proteinogenic amino acids, those considered nonessential are still necessary for living organisms. Healthy organisms are able to biosynthesize them and are not required from food and feed consumption. The 20 standard nonessential amino acids for humans found in soybean are: arginine, alanine, asparagine, aspartic acid, cysteine, glutamine, glutamic acid, glycine, proline, serine, and tyrosine [24–26]. As previously mentioned, the necessity of amino acids such as arginine and glycine can differ amongst species. Some nonessential amino acids are also affected by the presence and amounts of essential amino acids.

Cysteine not provided from food consumption is directly biosynthesized from methionine via trans-sulfuration [16, 27–30]. Consequently, if cysteine is not provided in the diet, then enough methionine must be provided to compensate for both amino acid needs. For that reason, feed research for poultry occasionally measures methionine and cysteine jointly [10, 31, 32]. Tyrosine is also directly formed from phenylalanine via hydroxylation [16, 29, 33]. Other amino acids like arginine, glycine, and proline can be required from the diet when animals are young, old, sick, or otherwise deficient in body protein regulation. As human and livestock diets become more sustainably plant-based, it will become more important to evaluate nonessential amino acids, specifically the ones immediately affected by essential amino acids.
