What are the 9 amino acids we must eat?

November 25, 2025 •

4 min read

There are 20 different amino acids required for the human body to function correctly, but the body can only produce 11 of them. This leaves a crucial gap of nine amino acids that we must eat through our diet for proper growth, repair, and overall health.

Quick Summary

The nine essential amino acids—histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine—are vital for functions like muscle repair, immune response, and mood regulation. They must be obtained from food since the body cannot synthesize them, making dietary intake essential.

Key Points

Essential, not just important: Unlike nonessential amino acids, the nine essential amino acids cannot be produced by the body and must be obtained through your diet.
The nine building blocks: These are histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine, each with specific functions.
Complete vs. Incomplete: Animal products typically provide complete protein (all nine EAAs), while most plant sources are incomplete, requiring dietary variety.
Vegan and vegetarian strategies: Combining different plant-based proteins throughout the day, like rice and beans, ensures a complete essential amino acid profile is achieved.
Health consequences of deficiency: Inadequate intake can lead to a range of issues including muscle wasting, weakened immunity, mood changes, and other systemic problems.
Not needed at every meal: You don't need all nine essential amino acids in one sitting; the key is consuming a balanced and varied diet throughout the day.

The Foundational Role of the Nine Essential Amino Acids

Amino acids are the building blocks of protein, which in turn are the building blocks of life. They are responsible for a wide range of bodily processes, from repairing tissue and growing new cells to creating hormones and neurotransmitters. While the body can synthesize some of these amino acids, there are nine that it cannot produce on its own. These are known as essential amino acids (EAAs) and must be sourced from the foods we consume.

The List of Nine Essential Amino Acids

The nine amino acids that you must obtain from your diet are:

Histidine: This amino acid is critical for making histamine, a neurotransmitter that plays a role in immunity, digestion, and sleep. It is also important for maintaining the myelin sheath, the protective barrier around nerve cells.
Isoleucine: As one of the three branched-chain amino acids (BCAAs), isoleucine is heavily concentrated in muscle tissue and is involved in muscle metabolism and growth. It is also important for immune function, hemoglobin production, and energy regulation.
Leucine: Another BCAA, leucine is crucial for protein synthesis and muscle repair. It helps regulate blood sugar levels, promotes wound healing, and aids in the production of growth hormones.
Lysine: This amino acid plays major roles in protein synthesis, hormone and enzyme production, and calcium absorption. It's also vital for energy production and immune function.
Methionine: Essential for metabolism and detoxification, methionine also supports tissue growth and the absorption of important minerals like zinc and selenium.
Phenylalanine: The body uses phenylalanine to produce key neurotransmitters like dopamine, epinephrine, and norepinephrine. It also plays an integral role in the structure and function of proteins and enzymes.
Threonine: A principal component of structural proteins like collagen and elastin, threonine provides structure to your skin and connective tissues. It also plays a part in fat metabolism and immune function.
Tryptophan: This amino acid is famously associated with its role as a precursor to serotonin, which regulates mood, sleep, and appetite. It also helps maintain your body's nitrogen balance.
Valine: The final BCAA on the list, valine is involved in muscle growth, tissue regeneration, and energy production. It is essential for mental focus and emotional calmness.

Sources of Essential Amino Acids: Complete vs. Incomplete Proteins

To get the nine essential amino acids, you must consume foods that contain them. Proteins are classified as either 'complete' or 'incomplete' based on their EAA content.

Complete Proteins: These foods contain all nine essential amino acids in sufficient quantities. Animal products are a reliable source, but some plant-based foods also qualify.
Incomplete Proteins: These foods contain some, but not all, of the essential amino acids. Many plant-based sources fall into this category, but combining different types of incomplete proteins throughout the day can provide a complete amino acid profile.

Comparison Table: Complete vs. Incomplete Proteins


Feature	Complete Proteins	Incomplete Proteins
Definition	Contains all nine essential amino acids.	Lacks one or more essential amino acids.
Primary Sources	Animal products (meat, fish, eggs, dairy), soy, quinoa, buckwheat.	Most plant foods like legumes, grains, nuts, and seeds.
Dietary Requirement	Can provide all EAAs from a single source.	Must be combined with other protein sources throughout the day to meet all EAA needs.
Common Examples	Eggs, beef, chicken, fish, milk, soy products (tofu, tempeh).	Beans, lentils, peanuts, wheat, rice, corn.
Flexibility	Highly efficient for ensuring EAA intake.	Requires variety and careful meal planning for complete coverage.

Ensuring You Get Your Essential Amino Acids

For omnivores, a varied diet that includes animal products usually covers all essential amino acid needs. However, for vegetarians and vegans, thoughtful meal planning is key. Combining complementary proteins is a time-tested strategy to ensure full EAA coverage. The old notion that you must combine these proteins in the same meal is outdated; studies show that consuming a variety of plant-based protein sources throughout the day is sufficient to provide all the essential amino acids.

For example, pairing beans (rich in lysine) with rice (rich in methionine) helps create a complete protein profile over the course of a day. Other complementary pairings include hummus with whole-wheat pita bread or a lentil soup with whole-grain toast. Incorporating a variety of legumes, nuts, seeds, and whole grains ensures you cover all your nutritional bases.

What Happens if You Don't Get Enough?

Deficiency in essential amino acids can negatively impact nearly every system in the body. The specific symptoms depend on which amino acid is lacking, but common signs include digestive issues, depression, weakened immunity, and slowed growth, especially in children. For instance, a lack of tryptophan can cause mood issues due to its role in serotonin production, while inadequate leucine can lead to hair loss and skin rashes. In severe cases of malnutrition, such as kwashiorkor, protein deficiency in children can lead to permanent cognitive deficits. For adults, insufficient EAA intake over time can lead to muscle wasting, weakness, and fatigue.

Conclusion

The nine essential amino acids—histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine—are non-negotiable for human health. As the building blocks of protein, they support critical functions from immune response to muscle repair. While animal products provide complete protein sources, a well-planned plant-based diet can also provide all essential amino acids by combining a variety of legumes, grains, and other protein-rich plants. Ensuring adequate daily intake of these essential nutrients through a balanced and varied diet is fundamental to maintaining optimal health and well-being. For further reading on the biological mechanisms, a review published by the National Center for Biotechnology Information offers detailed insights into protein and amino acid metabolism.

Frequently Asked Questions

The nine essential amino acids are histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. They must be obtained from food because the human body cannot produce them on its own.

Complete proteins contain all nine essential amino acids in adequate amounts, while incomplete proteins lack one or more of them. Animal-based foods are typically complete, whereas most plant-based foods are incomplete.

Vegans and vegetarians can get all nine essential amino acids by eating a varied diet that includes different protein sources throughout the day, such as combining legumes with grains (e.g., rice and beans). Foods like soy (tofu, tempeh) and quinoa are also complete plant-based proteins.

No, it is not necessary to consume all essential amino acids in a single meal. As long as you eat a varied diet that includes different sources of protein throughout the day, your body will accumulate all the necessary EAAs.

Essential amino acids are crucial for muscle growth and repair, immune system function, energy production, tissue repair, and the creation of hormones and neurotransmitters. A balanced intake supports overall health and physical performance.

An amino acid deficiency can lead to various health problems, including muscle wasting, weakened immunity, skin and hair issues, fatigue, mood changes, and problems with digestion. The specific symptoms depend on which EAA is insufficient.

Excellent sources of complete protein include meat, fish, eggs, dairy, and soy products like tofu and edamame. Quinoa and buckwheat are also notable plant-based complete protein sources.