What are the Essential Amino Acids and What Do They Do?

December 5, 2025 •

3 min read

The human body requires 20 different amino acids to function, yet it can only produce 11 of them. The remaining nine, known as essential amino acids, must be acquired through diet, and understanding what are the essential amino acids and what do they do is key for overall health.

Quick Summary

The nine essential amino acids are vital compounds the body cannot produce, requiring dietary intake to support protein synthesis, tissue repair, energy production, and nerve function.

Key Points

Dietary Necessity: The body cannot produce essential amino acids; they must be obtained from food.
Complete vs. Incomplete Proteins: Complete protein sources, like meat and soy, contain all nine essential amino acids, while incomplete sources, like beans, require combining with other foods.
Muscle Growth & Recovery: Branched-chain amino acids (BCAAs), which include leucine, isoleucine, and valine, are particularly important for muscle protein synthesis and repair.
Mood & Sleep Regulation: Tryptophan is essential for producing serotonin, a neurotransmitter that regulates mood, appetite, and sleep.
Deficiency Consequences: Inadequate intake of essential amino acids can lead to severe issues, including muscle wasting, fatigue, and a weakened immune system.
Holistic Health: EAAs are vital for a wide range of functions, including immune response, tissue repair, and hormone production, not just muscle development.

What are Essential Amino Acids?

Amino acids are the building blocks for proteins, hormones, and neurotransmitters. While the body needs 20 amino acids, only 11 are non-essential, meaning the body can produce them. The other nine are deemed 'essential' because they cannot be synthesized by the body and must be obtained from food. Sometimes, non-essential amino acids can become 'conditionally essential' during periods of illness or stress when the body's demand increases.

The Nine Essential Amino Acids and Their Functions

Each of the nine essential amino acids plays a unique role in the body.

Histidine

Used to create histamine, a neurotransmitter important for immune response, digestion, and sleep-wake cycles.

Isoleucine

A branched-chain amino acid (BCAA) involved in muscle metabolism, immune function, and energy regulation.

Leucine

Another BCAA, crucial for protein synthesis, muscle repair, blood sugar control, and growth hormone production.

Lysine

Important for protein synthesis, calcium absorption, and creating collagen and elastin.

Methionine

Required for metabolism, detoxification, tissue growth, and absorbing minerals like zinc and selenium.

Phenylalanine

A precursor to neurotransmitters like dopamine and epinephrine, and vital for protein structure.

Threonine

A key part of collagen and elastin, involved in fat metabolism, immune function, and blood clot formation.

Tryptophan

Known for its role in producing serotonin, which affects mood, appetite, and sleep.

Valine

The third BCAA, supports muscle growth, tissue regeneration, and provides energy to muscles.

The Role of Essential Amino Acids in the Body

Essential amino acids are fundamental to building new proteins for muscles, skin, hair, and organs. This is especially important for muscle growth and repair. They are also necessary for creating hormones and neurotransmitters that regulate metabolism, mood, and immune response. A deficiency in EAAs can negatively impact these crucial functions.

Food Sources of Essential Amino Acids

Foods containing all nine essential amino acids are called 'complete proteins'. These include animal products and some plant-based options. Foods lacking one or more essential amino acid are 'incomplete proteins'.

Complete Protein Sources

Meat, Fish, and Seafood
Eggs and Dairy
Soy products
Quinoa and Buckwheat

Incomplete Protein Sources

Legumes and Nuts
Whole Grains
Most Vegetables

Vegetarians and vegans can get all essential amino acids by eating a variety of incomplete proteins throughout the day.

Comparison: Essential vs. Non-Essential Amino Acids


Feature	Essential Amino Acids (EAAs)	Non-Essential Amino Acids
Body Production	Must be obtained from diet.	Can be synthesized by the body.
Number	Nine.	Eleven.
Dietary Importance	Crucial to consume through diet or supplements.	Less reliant on diet.
Deficiency Risk	High risk if intake is inadequate.	Low risk in healthy individuals.
Conditional Status	Always essential.	Can become conditionally essential during illness or stress.

What Happens During an Essential Amino Acid Deficiency?

An insufficient intake of essential amino acids impairs protein synthesis and can lead to health issues. Symptoms include muscle atrophy, fatigue, and a weakened immune system. Severe deficiencies can also affect mood, growth, digestion, skin, hair, and nails.

How to Ensure Adequate Intake

A balanced diet with diverse protein sources typically meets EAA needs. Complete proteins provide all nine EAAs in one source. Combining different plant proteins ensures a complete amino acid profile for plant-based diets. Supplements, including BCAAs, can help those with specific needs, but consulting a healthcare professional is recommended.

Conclusion

Understanding what are the essential amino acids and what do they do is vital for health. These nine compounds are crucial for protein building, muscle repair, immune support, and hormone regulation. A varied, protein-rich diet ensures adequate intake to support these processes. For more information, see the Cleveland Clinic on Amino Acids.

Frequently Asked Questions

The nine essential amino acids are histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine.

They are considered essential because the human body cannot synthesize them on its own, meaning they must be consumed through dietary sources like food or supplements.

Complete proteins contain all nine essential amino acids in sufficient amounts. Incomplete proteins lack one or more essential amino acids. Animal products and some plant-based foods like soy are complete proteins.

Yes, by eating a variety of plant-based protein sources throughout the day, you can ensure you get all essential amino acids. Examples include combining legumes with grains.

Yes, essential amino acids, particularly the branched-chain amino acids (BCAAs) like leucine, are critical for stimulating muscle protein synthesis, which is the process of building and repairing muscle tissue.

Common symptoms include muscle atrophy, fatigue, weakened immune function, and mood disturbances. Severe deficiency can lead to stunted growth and other significant health issues.

Most experts recommend obtaining essential amino acids primarily from whole food sources as part of a balanced diet. Supplements can be beneficial for specific goals or in cases of dietary restrictions, but professional consultation is advised.

The three BCAAs—leucine, isoleucine, and valine—are crucial for muscle metabolism, growth, energy production, and repair, especially after exercise.