What Are Amino Acids? Essential, Conditionally Essential and Non-Essential Explained

The three groups as MedlinePlus lists them

MedlinePlus (U.S. National Library of Medicine, reviewed 2025) gives the following lists.

1. Essential

   The body cannot make them, so they must come from food. [1]

   Amino acids named: Histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, valine [1]

2. Nonessential

   The body can produce them even if food does not supply them. [1]

   Amino acids named: Alanine, arginine, asparagine, aspartic acid, cysteine, glutamic acid, glutamine, glycine, proline, serine, tyrosine [1]

3. Conditionally essential

   Usually not essential, except in times of illness and stress. [1]

   Amino acids named: Arginine, cysteine, glutamine, tyrosine, glycine, proline, serine [1]

Together the essential and nonessential lists name 20 amino acids. Notice that seven amino acids appear in both the nonessential and the conditionally essential lists. That is not an error: "conditionally essential" describes a subset of nonessential amino acids that may fall short during illness or stress. Other sources may draw the lines a little differently, so this page follows one named source rather than claiming a single universal list. A second MedlinePlus page adds that nonessential amino acids are made from essential amino acids or in the normal breakdown of proteins. [2]

What the labels do and do not mean

• Essential does not mean "better" or "more important". It means food must supply them, and it is why overall protein quality matters. See Protein Quality and Complete Proteins.
• Nonessential does not mean "unneeded". The body needs them; it just can make them.
• Conditionally essential is a clinical idea about illness and stress. MedlinePlus does not say a healthy person needs a supplement of them, and this page does not either.
• You do not need to eat essential and nonessential amino acids at every meal, but getting a balance of them over the whole day is important. [1]

Where each group comes from

Amino acids are found in animal foods such as meat, milk, fish and eggs, and in plant foods such as soy, beans, legumes, nut butters and some grains. [2] Food sources of the essential ones are on Essential Amino Acids. How protein is broken down into amino acids is explained in How Protein Is Digested, Absorbed and Used.

Amino acids, peptides and supplements

Single amino acids sold as supplements, such as glutamine or arginine, are covered on their own pages in the Amino Acids and Protein library. Whether a single amino acid helps with any condition is a separate question from how it is classified; classification alone is not evidence of benefit. For how amino acids differ from peptides, see Peptides vs Amino Acids.