What are the 20 Amino Acids and Their Functions?

January 12, 2026 •

3 min read

The human body requires 20 different amino acids to function, serving as the fundamental building blocks of life. This article explores what are the 20 amino acids and their functions, shedding light on their diverse roles in metabolism, tissue repair, and overall health.

Quick Summary

A comprehensive guide to the 20 standard amino acids, their classification as essential or non-essential, and their specific biological roles in the human body. Includes a detailed table and addresses common questions.

Key Points

Essential vs. Non-Essential: 20 amino acids are classified by whether the body can synthesize them. Nine are essential (from diet), eleven are non-essential (produced internally).
Building Blocks of Proteins: All 20 amino acids are necessary for forming proteins vital for body growth, repair, and maintenance.
Diverse Biological Functions: Amino acids are precursors for molecules like hormones (Tyrosine for thyroid) and neurotransmitters (Tryptophan for serotonin).
Energy and Metabolism: Amino acids can provide energy and are involved in metabolic and detoxification processes.
Dietary Importance: A balanced intake of all amino acids, especially essential ones from complete proteins, is crucial for proper body function.

What are Amino Acids?

Amino acids are organic compounds that combine to form proteins, each featuring a central carbon atom bonded to an amino group, a carboxyl group, a hydrogen atom, and a unique side-chain (R-group). This R-group determines the amino acid's specific properties and functions. When proteins are digested, they break down into amino acids, which the body uses to build new proteins, hormones, enzymes, and other essential compounds.

Classifying the 20 Amino Acids

The 20 standard amino acids are classified primarily as essential or non-essential based on the body's ability to synthesize them.

Essential Amino Acids

There are nine essential amino acids that the body cannot produce and must be obtained through diet. Their functions include:

Histidine: Involved in histamine production.
Isoleucine: Supports muscle metabolism, immune function, and energy.
Leucine: Critical for protein synthesis, muscle repair, and blood sugar regulation.
Lysine: Key for hormone and enzyme production, and collagen synthesis.
Methionine: Involved in metabolism, detoxification, and tissue growth.
Phenylalanine: Precursor for brain chemicals like dopamine and norepinephrine.
Threonine: Component of structural proteins and involved in fat metabolism.
Tryptophan: Precursor to serotonin and Vitamin B3.
Valine: Works with leucine and isoleucine for muscle growth and energy.

Non-Essential Amino Acids

The body can synthesize the remaining eleven non-essential amino acids. Their roles include:

Alanine: Helps remove toxins and is used in the glucose-alanine cycle.
Arginine: Promotes protein/hormone synthesis, immune function, and wound healing; can be conditionally essential.
Asparagine: Facilitates nitrogen transport and is involved in DNA/RNA formation.
Aspartic Acid: Important for metabolism and synthesis of other amino acids.
Cysteine: Acts as an antioxidant and is vital for collagen synthesis.
Glutamic Acid: Excitatory neurotransmitter in the central nervous system.
Glutamine: Supports brain function and nitrogen transport.
Glycine: Aids cell growth, wound healing, and acts as a neurotransmitter.
Proline: Crucial for collagen formation and tissue repair.
Serine: Supports muscle growth and immune protein synthesis.
Tyrosine: Precursor for thyroid hormones and melanin.

Comparison of Amino Acid Types

The table below outlines the key differences between essential and non-essential amino acids.


Feature	Essential Amino Acids	Non-Essential Amino Acids
Source	Must be obtained through diet.	Produced by the human body.
Number	There are 9 standard essential amino acids.	There are 11 standard non-essential amino acids.
Metabolic Pathway	Often require complex pathways not present in humans.	Synthesized through simpler pathways.
Deficiency Risk	Higher risk if diet is inadequate.	Deficiency is rare but possible during severe illness.

The Role of Amino Acids Beyond Protein Synthesis

Amino acids are vital for many biological processes beyond protein formation. Some serve as precursors for other molecules, such as tryptophan for serotonin and Vitamin B3, and tyrosine for thyroid hormones and melanin. They also support the immune system, provide energy during strenuous activity, and aid in detoxification.

The Importance of Dietary Intake

A balanced intake of all 20 amino acids is crucial for health. Consuming complete proteins—foods containing all nine essential amino acids—is vital as the body cannot store amino acids. Complete protein sources include meat, eggs, dairy, soy, and quinoa. Plant-based diets require consuming a variety of plant proteins to ensure all essential amino acids are acquired. Proper amino acid availability supports growth, repair, and maintenance.

Conclusion

Understanding what are the 20 amino acids and their functions highlights their crucial role in human health beyond building proteins. From hormone and neurotransmitter synthesis to tissue repair and immunity, a balanced intake of these compounds is fundamental for life. Both essential and non-essential amino acids work together to support the body's functions, emphasizing the need for a balanced diet.

For further reading, the MedlinePlus Medical Encyclopedia offers more details on amino acids.

Frequently Asked Questions

Essential amino acids are the nine the body cannot produce and must come from the diet. Non-essential amino acids are the eleven the body can synthesize internally.

Amino acids form proteins, which are fundamental for almost all biological functions, including growth, repair, and metabolism.

No, a balance throughout the day is important, ensuring adequate complete proteins over time.

These are typically non-essential but become essential during stress, illness, or growth when the body's need exceeds its production. Examples are arginine and glutamine.

Animal proteins (meat, eggs, fish, dairy) are complete sources. Plant sources like soy and quinoa are also complete. Combining other plant proteins can provide all essential amino acids.

Deficiency can cause poor growth, weakened immunity, fatigue, and metabolic issues. Severe deficiencies can lead to kwashiorkor and marasmus.

Some amino acids are precursors for neurotransmitters vital for brain function and mood. Tryptophan makes serotonin, which regulates mood, sleep, and appetite.