Amino acids that must be provided in the diet to meet an animal's metabolic needs are called essential amino acids (Table 20.1). Those that need not be provided because they can be biosynthesized in adequate amounts are called nonessential amino acids.

Mammals require about half of the amino acids in their diet for growth and maintenance of normal nitrogen balance.

Essential Amino Acids in Mammals	Non-Essential Amino Acids in Mammals
Arginine, Histidine, Isoleucine, Leucine, Lysine, Methionine, Phenylalanine, Threonine, Tryptophan, Valine	Alanine, Asparagine, Aspartic Acid, Cysteine, Glutamic Acid, Glutamine, Glycine, Proline, Serine, Tyrosine