Essential amino acids are
amino acids that the human body cannot produce in sufficient amounts and therefore must be obtained from the diet.
Adult humans have 9 essential amino acids:
- Histidine
- Isoleucine
- Leucine
- Lysine
- Methionine
- Phenylalanine
- Threonine
- Tryptophan
- Valine
Conditionally essential / potentially essential depending on age or circumstances:
- Arginine
- Cysteine
- Glutamine
- Glycine
- Proline
- Tyrosine
The term "conditionally essential" means that the body normally can synthesize the amino acid, but under certain conditions—such as growth, prematurity, illness, injury, metabolic stress, or inadequate precursor availability—dietary intake may become necessary to meet requirements.
The term "essential" does not mean the amino acid is more important than a
nonessential amino acid. It means the body cannot synthesize enough of it to meet its needs, so dietary intake is necessary.