The Nine Essential Amino Acids: Functions, Requirements, Sources (2024)

There are nine essential amino acids that we require from our diet.

Since the human body cannot make these amino acids, we need to derive them from the foods we eat.

In this article, we examine what the nine essential amino acids are, their functions, how much we require, and the best food sources.

The data for recommended intake amounts are sourced from the World Health Organization’s expert consultation report entitled ‘Protein and Amino Acid Requirements in Human Nutrition’.

What Are Essential Amino Acids?

We can often hear amino acids referred to as the ‘building blocks’ of protein.

Amino acids are organic compounds that contain the elements carbon, hydrogen, oxygen, nitrogen, and sometimes sulfur (1).

There are twenty primary amino acids, but not all of them are essential. We can classify these amino acids in the following categories;

• Essential amino acids: the human body cannot synthesize these amino acids, so we need to obtain them from our diet. There are nine essential amino acids (2).
• Conditionally essential amino acids: generally speaking, the human body can produce these amino acids when in good health. However, they are ‘çonditionally essential’ because the body cannot synthesize them in certain medical conditions or in times of stress (3). Arginine is an example of a conditionally essential amino acid (for more information refer to: foods high in arginine).
• Nonessential amino acids:the body can produce these amino acids, so we do not have a dietary requirement for them. However, this does not mean they hold no nutritional value.

The nine essential amino acids are as follows;

• Histidine
• Isoleucine
• Leucine
• Lysine
• Methionine
• Phenylalanine
• Threonine
• Tryptophan
• Valine

What Do They Do?

Essential amino acids play a multitude of roles within the body.

Among these varied roles, amino acids support gene expression, immune response, protein formation, digestion, tissue repair, and enzyme production (4).

We will look into the functions of each amino acid in more detail later on.

Where Can We Find Them?

Generally speaking, the best sources of essential amino acids include:

• Dairy
• Eggs
• Meat
• Poultry
• Seafood
• Soy

These foods contain adequate amounts of all nine essential amino acids. For this reason, we can refer to them as ‘complete proteins’ (5).

Although plant foods are still a good source of protein, the amino acids arenot quite as bioavailable as proteins from animal foods.

Histidine

Histidine has a wide variety of roles. Firstly, it is a precursor to histamine, which is a compound involved in modulating immune response (6).

The various functions of histidine include (7);

• Blood cell formation
• Protecting nerve cells
• Growth and repair of tissues
• Precursor for the synthesis of histamine

Requirement

The current recommendations advise that healthy adults should get at least 10 mg of histidine per kilogram of body weight per day (8).

Best Food Sources

The table below shows the foods with the highest concentration of histidine per 100 grams (9);

Isoleucine

Isoleucine is an essential branched-chain amino acid (BCAA), and it plays various roles in the human body.

The functions of leucine include (10).

• Helps with blood sugar regulation
• Important for optimal energy levels
• Supports immune response
• Supports wound healing processes

Requirement

According to the current guidelines, adults should aim for 20 mg of isoleucine per kilogram of body weight per day (8).

Best Food Sources

The table below shows the foods with the highest concentration of histidine per 100 grams (11);

Leucine

Leucine is another branched-chain amino acid (BCAA), and among its various functions, leucine contributes to (12);

• Blood sugar regulation
• Growth and repair of tissues
• Production of growth hormone
• Wound healing

Requirement

The current essential amino acid recommendations suggest that an adult requires 39 mg per kg of body weight per day (8).

Best Food Sources

The following table lists some of the best dietary sources of leucine (13);

For a complete guide to leucine and why it is important, see this article here.

Lysine

Lysine is often the limiting essential amino acid in protein foods.

When protein synthesis cannot continue due to the unavailability (depletion) of one amino acid, we can refer to this as the ‘limiting amino acid’ (14).

Among its different functions, lysine supports (15);

• Growth and tissue repair
• Immune function
• Production of carnitine and collagen
• Wound healing

Requirement

Healthy adults require 30 mg of lysine per kilogram of body weight per day (8).

Best Food Sources

Here are some foods with the highest lycine concentrations (16);

Methionine

Methionine is a sulfur-containing essential amino acid that has a vast array of roles within the body.

Among these roles, methionine assists in the following processes (17).

• Maintaining healthy hair and skin
• Proper absorption of the minerals selenium and zinc
• Protecting cells against damage
• Regulating liver fat
• Removing heavy metals from the body

Requirement

Adults require 15 mg of methionine and cysteine per kilogram of body weight each day (8).

Best Food Sources

Lean meats and fish tend to be the best sources of methionine. Here are some foods with high concentrations (18);

Phenylalanine

Phenylalanine is an essential amino acid that is a precursor for dopamine, epinephrine, norepinephrine, and tyramine.

Additionally, phenylalanine plays a vital role in producing other amino acids (19).

Requirement

Adults require 25 mg of phenylalanine per kilogram of body weight per day (8).

Best Food Sources

Here are some of the best dietary sources of phenylalanine (20);

Threonine

Threonine is an essential amino acid that has a key role in the nervous system.

Additionally, threonine’s may have some potential benefits. Research shows the amino acid (21);

• Helps to regulate liver fat
• May help to treat anxiety and other mental health symptoms

Requirement

According to present estimates, healthy adults need 15 mg of threonine per kilogram of body weight per day (8).

Best Food Sources

Some of the best dietary sources of threonine include the following options (22);

Tryptophan

Although it is an essential amino acid, tryptophan occurs in lower concentrations than other amino acids.

However, it has several important functions and potential benefits (23);

• Important for growth and repair
• Precursor for serotonin synthesis
• Through its role in serotonin production, it can help with improving mood and sleep

Requirement

Per kilogram of body weight per day, healthy adults require only 4 mg of tryptophan (8).

Best Food Sources

Here are the best dietary options for tryptophan. As shown in the table, the concentrations of tryptophan in food are relatively low (24);

Valine

Valine is the third of the three essential branched-chain amino acids alongside leucine and isoleucine.

The various functions of valine include (25);

• Supporting muscular growth and tissue repair
• Important for the nervous system and cognitive health
• May help with relaxation and mental calmness

Requirement

According to the recommendations, healthy adults require 26 mg of valine per kilogram of body weight each day (8).

Best Food Sources

Some valine-rich foods include the following options (26);

Summary of Essential Amino Acid Requirements

The following table provides an at-a-glance overview of the essential amino acid requirements for healthy adults.

The data is the result of joint consultation between the World Health Organization, Food and Agriculture Organization, and United Nations University (8).

Essential Amino Acid Requirements In Infants

There are some differences between the estimated essential amino acid requirements for adults and infants. These differences depend upon the specific stage of life and growth phase.

Therefore, the table below shows the published estimated requirements for infants (8).

Essential Amino Acid Deficiency

Should there be a deficiency in any of the essential amino acids, this will affect protein synthesis since all nine essential amino acids are required.

Deficiency in an essential amino acid may also lead to further deficiency symptoms dependent upon the particular amino acid.

Final Thoughts

Ensuring sufficient intake of essential amino acids is vital for our overall health.

However, we don’t need to overthink each specific essential amino acid.

In other words; consuming enough high-quality protein foods should provide sufficient amounts of each amino acid.

As previously mentioned, animal foods like dairy, meat, seafood, and eggs are all high-quality protein sources.

Among plant-based foods, soy is the best provider of all essential amino acids. This includes both soybeans themselves and soy-based products, such as tofu, fermented soy curd, and tempeh.