Amino acids are the small molecules the body uses to build proteins. They also participate in processes such as making hormones, neurotransmitters, enzymes and other important compounds. Of the 20 amino acids commonly used to make human proteins, nine are considered essential and 11 are generally considered nonessential.
The distinction is straightforward: essential amino acids must come from the diet because the body cannot make enough of them on its own. Nonessential amino acids can be produced by the body in sufficient amounts under normal conditions.
“Essential,” however, does not mean more important. Every amino acid has biological roles, and the body needs all of them to make proteins and maintain normal physiology. The difference is primarily about whether the body must obtain a particular amino acid from food.
What are amino acids?
Proteins are long chains of amino acids joined together. When you eat protein, digestion breaks much of it down into amino acids and smaller peptides, which can then be absorbed and used by the body.
The 20 standard amino acids used to build human proteins can be grouped according to whether the body can synthesize enough of them. The resulting categories are essential and nonessential amino acids, with a third category—conditionally essential amino acids—useful in certain circumstances.
Amino acids are not used only for muscle. Protein synthesis throughout the body depends on them, including the production and maintenance of muscle, skin, connective tissue, enzymes and many other proteins. Some amino acids also serve as starting materials for other molecules involved in metabolism and cell signaling.
The nine essential amino acids
Humans cannot synthesize the nine essential amino acids in adequate amounts, so they need to be supplied by food.
| Essential amino acid | Some important roles |
|---|---|
| Histidine | Protein synthesis; precursor for histamine |
| Isoleucine | Muscle protein metabolism and energy metabolism |
| Leucine | Regulation of muscle protein synthesis and protein metabolism |
| Lysine | Protein synthesis; important in collagen formation |
| Methionine | Methyl-group metabolism and synthesis of other compounds |
| Phenylalanine | Precursor for tyrosine and several signaling molecules |
| Threonine | Protein synthesis; important in structural proteins |
| Tryptophan | Precursor for serotonin, melatonin and niacin-related compounds |
| Valine | Muscle protein metabolism and energy metabolism |
Leucine, isoleucine and valine are often grouped together as branched-chain amino acids (BCAAs) because of their chemical structure. They are all essential amino acids, but they are not a separate nutritional category from the other essential amino acids.
Because essential amino acids cannot be made adequately by the body, inadequate dietary intake can limit the body’s ability to synthesize proteins even when other amino acids are available.
The 11 nonessential amino acids
Nonessential amino acids are not unimportant. The name simply means that, under ordinary physiological conditions, the body can synthesize enough of them to meet its needs.
The commonly classified nonessential amino acids are:
- Alanine
- Arginine
- Asparagine
- Aspartic acid
- Cysteine
- Glutamic acid
- Glutamine
- Glycine
- Proline
- Serine
- Tyrosine
The body makes these amino acids from other metabolic intermediates and, in some cases, by modifying other amino acids. For example, tyrosine can be synthesized from phenylalanine, while cysteine can be produced using sulfur derived from methionine.
This does not mean dietary sources of nonessential amino acids are useless. Foods containing protein provide a mixture of amino acids, and the body can use dietary amino acids directly rather than always having to synthesize them.
What does “conditionally essential” mean?
The essential-versus-nonessential classification is useful, but it is not completely rigid in every physiological situation.
Some amino acids are described as conditionally essential because the body can normally make enough of them, but its ability to do so may become inadequate during particular conditions. These can include periods of serious illness, injury, metabolic stress or, in some cases, stages of growth.
Arginine, cysteine, glutamine, glycine, proline and tyrosine are among the amino acids that may be considered conditionally essential under particular circumstances. The exact classification can vary depending on the physiological context and the nutritional framework being used.
This is why calling an amino acid simply “nonessential” should not be interpreted as meaning that the body never needs to obtain it from food. It means that dietary intake is not normally required to meet the body’s needs because endogenous synthesis can generally provide enough.
Essential does not mean “better” or “more important”
A common misunderstanding is that essential amino acids are somehow superior to nonessential amino acids. They are not.
The body needs an appropriate supply of all amino acids to build proteins. If one essential amino acid is insufficient, protein synthesis can be limited even if the other amino acids are plentiful. Nonessential amino acids are also required as components of proteins and have their own metabolic functions.
The word essential describes a limitation in the body’s ability to manufacture the amino acid, not its biological importance.
How food provides essential amino acids
Protein-containing foods supply amino acids in different proportions. Animal foods such as meat, poultry, fish, eggs and dairy generally provide all nine essential amino acids in substantial amounts. Many plant foods contain essential amino acids as well, but individual plant proteins can differ in their amino acid profiles.
A food or protein source that supplies all nine essential amino acids in adequate proportions is commonly described as a complete protein. The term refers to its amino acid profile, not to whether the food is inherently healthier than other protein sources.
Plant-based diets can provide all essential amino acids when a variety of protein-containing foods are eaten over the course of the diet. Foods do not necessarily have to be combined in the same meal for the overall diet to supply the essential amino acids the body needs.
Why protein quality matters
The body cannot simply treat all dietary proteins as interchangeable. Two foods can contain similar amounts of protein but differ in how much of each essential amino acid they provide and how readily those amino acids can be digested and absorbed.
This is one reason nutrition science evaluates protein quality. A protein source with an adequate balance of essential amino acids can support protein synthesis more effectively than one that provides relatively little of an amino acid needed for that synthesis.
For someone eating a varied diet, this does not mean individual foods need to be scrutinized for every amino acid. Overall dietary intake matters more than whether each meal contains a perfectly balanced amino acid profile.
What happens when an essential amino acid is lacking?
Protein synthesis requires the necessary amino acids to be available. If an essential amino acid is present in too small a quantity relative to the body’s needs, it can become the limiting amino acid for a particular protein-building process.
The body can make some amino acids itself, but it cannot manufacture an essential amino acid from scratch when the relevant metabolic pathway does not exist. That essential amino acid therefore has to come from the diet.
This is also why simply eating a large amount of protein does not automatically compensate for a severely unbalanced amino acid supply. The composition of the protein matters as well as its total amount.
Do you need to take amino acid supplements?
For most people, amino acid supplements are not necessary simply because some amino acids are essential. A normal diet containing adequate protein can provide essential amino acids without requiring individual supplements.
Amino acid supplements can have specific uses in particular circumstances, but they should not be viewed as a general replacement for a balanced diet. Taking one or several isolated amino acids also does not reproduce the nutritional value of eating complete protein-containing foods, which provide a broader mixture of amino acids and other nutrients.
The key distinction is therefore simple: essential amino acids are those the body cannot make adequately and must obtain from food; nonessential amino acids are those the body can normally synthesize in sufficient quantities. Both groups are necessary for normal protein synthesis and human physiology.
