Which of the following correctly defines essential amino acids?

January 12, 2026 •

3 min read

Of the 20 different amino acids required by the human body to function properly, nine are classified as essential, meaning the body cannot produce them on its own. Knowing which of the following correctly defines essential amino acids is foundational to building a nutritionally sound diet for optimal health and bodily function.

Quick Summary

This article explains that essential amino acids are the nine amino acids the body cannot synthesize, making dietary intake mandatory for critical functions like protein synthesis and tissue repair. Learn about the individual roles of each EAA, their food sources, and the difference between complete and incomplete proteins.

Key Points

Dietary Necessity: Essential amino acids cannot be produced by the human body and must be obtained from food sources.
Nine Vital Amino Acids: There are nine essential amino acids: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine.
Comprehensive Function: EAAs are crucial for protein synthesis, hormone and neurotransmitter production, immune support, muscle repair, and energy metabolism.
Complete vs. Incomplete: Complete proteins (typically animal-based and some plant sources) contain all nine EAAs, while incomplete proteins (most plant foods) lack one or more.
Varied Diet is Key: Vegans and vegetarians can acquire all necessary EAAs by consuming a variety of complementary plant-based proteins throughout the day.
Deficiency Risks: A lack of essential amino acids can lead to muscle wasting, fatigue, immune dysfunction, and impaired wound healing.

The correct definition of essential amino acids is that they are organic compounds that the body cannot synthesize itself and must therefore acquire through dietary intake. These vital building blocks of protein are indispensable for numerous bodily functions, from repairing tissue and fueling muscles to producing hormones and neurotransmitters. Unlike non-essential amino acids, which the body can produce from other compounds, essential amino acids (EAAs) are a required component of a healthy diet. A deficiency in even one EAA can disrupt protein synthesis and negatively impact overall health.

What are the Nine Essential Amino Acids?

There are nine essential amino acids, each playing a specific and important role within the body. A well-rounded diet ensures you receive all of them in adequate amounts. They are:

Histidine: Used for producing histamine, vital for immune response, digestion, sleep, and sexual function. It also supports the protective myelin sheath around nerve cells.
Isoleucine: A branched-chain amino acid (BCAA) crucial for muscle metabolism, immune function, hemoglobin production, and energy regulation.
Leucine: Another BCAA, critical for protein synthesis, muscle repair, blood sugar regulation, wound healing, and growth hormone production.
Lysine: Important for protein synthesis, calcium absorption, and producing hormones and enzymes; essential for energy and collagen/elastin formation.
Methionine: Vital for metabolism, detoxification, tissue growth, and absorbing minerals like zinc and selenium.
Phenylalanine: Converted into tyrosine, dopamine, epinephrine, and norepinephrine; integral to protein and enzyme structure and function.
Threonine: A principal part of structural proteins like collagen and elastin, important for skin and connective tissue; also involved in fat metabolism and immune function.
Tryptophan: Precursor to serotonin, regulating appetite, sleep, and mood; key for maintaining nitrogen balance.
Valine: The final BCAA, helps stimulate muscle growth, aids tissue regeneration, and is involved in energy production.

Functions of Essential Amino Acids

EAAs are involved in many vital processes for health and optimal function.

Supporting Muscle Growth and Repair

Amino acids are the building blocks of muscle. BCAAs are particularly important for triggering muscle protein synthesis to repair and rebuild tissue after exercise. This process is crucial for muscle growth and recovery.

Regulating Hormones and Neurotransmitters

EAAs are precursors for hormones and brain chemicals that regulate mood, sleep, and appetite. Tryptophan forms serotonin, and phenylalanine forms dopamine.

Boosting Immune Function

Adequate EAA intake is critical for a healthy immune system, as immune cells require them to function properly and produce antibodies. A deficiency can weaken immune response.

Complete vs. Incomplete Proteins and Food Sources

Understanding complete and incomplete proteins is important for obtaining all essential amino acids.


Feature	Complete Proteins	Incomplete Proteins
Definition	Contains all nine essential amino acids in sufficient quantities.	Lacks one or more of the nine essential amino acids.
Typical Sources	Primarily animal-based products (meat, fish, eggs, dairy).	Predominantly plant-based foods (legumes, nuts, seeds, grains).
Plant-based Exceptions	Soybeans, quinoa, buckwheat, and chia seeds are also complete proteins.	Most plant sources, although combining them can provide a complete profile.
Combining for Completeness	Not necessary, as they already contain all EAAs.	Essential for vegans/vegetarians; combinations like rice and beans create a complete protein.

Sourcing Essential Amino Acids

Animal-Based: Good sources include meat, poultry, fish, eggs, and dairy. These are easily absorbed.
Plant-Based: Vegetarians and vegans can get all EAAs by eating a variety of plant foods daily, including legumes, nuts, seeds, whole grains, soy, quinoa, and buckwheat.

Potential Consequences of Essential Amino Acid Deficiency

Insufficient dietary EAAs can lead to negative health effects.

Muscle Wasting: The body may break down muscle for necessary amino acids.
Fatigue and Weakness: Deficiency can cause persistent tiredness and weakness.
Impaired Wound Healing: A shortage slows recovery from injuries.
Compromised Immune System: Deficiency weakens defense against illness.
Mood and Cognitive Issues: Lack of EAAs can negatively impact mood, sleep, and cognition due to their role in neurotransmitter production.

Conclusion

The correct definition of essential amino acids refers to the nine amino acids not synthesized by the body that must come from the diet. They are fundamental for human health, serving as the raw material for proteins, hormones, and neurotransmitters needed for muscle growth, tissue repair, immune function, and mood regulation. A balanced diet with complete proteins or complementary incomplete proteins is crucial for physiological function and avoiding deficiency consequences. For further reading, consult the National Institutes of Health.

Frequently Asked Questions

Essential amino acids cannot be produced by the body and must come from the diet, while non-essential amino acids can be synthesized internally from other amino acids and raw materials.

Yes, while many plant foods are incomplete proteins, some notable exceptions that contain all nine essential amino acids include soy products (tofu, tempeh), quinoa, buckwheat, and chia seeds.

BCAAs are a group of three essential amino acids—leucine, isoleucine, and valine—that are particularly important for muscle metabolism, growth, and recovery after exercise.

Yes, pairing different incomplete protein sources, such as rice and beans or peanut butter on whole grain bread, can create a complete protein profile over the course of a day.

A deficiency can lead to various health problems, including muscle loss (wasting), decreased immune function, fatigue, and issues with mood regulation and tissue repair.

Certain EAAs, like tryptophan and phenylalanine, are precursors to neurotransmitters such as serotonin and dopamine, which play critical roles in regulating mood, sleep, and motivation.

Most people can get all the essential amino acids they need from a varied and balanced diet. Supplements are generally only considered for athletes, individuals with certain medical conditions, or those on restrictive diets, and should be discussed with a healthcare provider.