What Amino Acids Do Peas Lack? A Complete Nutritional Guide

November 27, 2025 •

4 min read

Though often called a "complete" protein, studies show that pea protein is notably low in the sulfur-containing amino acids methionine and cysteine. This information is crucial for those relying on plant-based protein, helping to address what amino acids do peas lack and how to create a nutritionally complete profile.

Quick Summary

Peas are deficient in the sulfur-containing amino acids methionine and cysteine. This can be easily remedied by pairing them with complementary protein sources like rice to create a full amino acid profile.

Key Points

Low in Methionine: Peas are deficient in the essential amino acids methionine and cysteine, which are crucial for metabolism and protein synthesis.
Not a Complete Protein (on its own): Due to low methionine content, pea protein is not considered a complete protein in the same way as animal proteins like whey.
Combine with Grains: The deficiency is easily remedied by pairing peas with a complementary protein, such as grains like brown rice, which are rich in methionine.
Rich in Other Amino Acids: Despite the deficiency, peas are an excellent source of other essential amino acids, including lysine, and are rich in BCAAs.
Beneficial for Plant-Based Diets: When combined correctly, pea protein is a valuable and sustainable source of protein for vegans, vegetarians, and omnivores.
High in Iron and Arginine: Beyond their amino acid profile, peas provide significant amounts of iron and arginine, supporting blood flow and overall health.

Understanding the Basics of Protein and Amino Acids

Protein is a crucial macronutrient made up of building blocks called amino acids. There are 20 different amino acids that the human body uses, but only nine of these are considered "essential". This means your body cannot produce them on its own, and you must obtain them through your diet.

Proteins can be categorized as complete or incomplete based on their amino acid profile. A complete protein contains all nine essential amino acids in sufficient amounts. While animal products like meat, eggs, and dairy are classic examples of complete proteins, many plant-based options, including peas, can fall short in one or more essential amino acids. The amino acid that is present in the lowest amount relative to the body's needs is called the "limiting amino acid".

The Limiting Amino Acids in Peas: Methionine and Cysteine

Peas technically contain all nine essential amino acids, but they are particularly low in methionine. This low level is what prevents pea protein from being classified as a complete protein by some nutritional standards. Since cysteine is synthesized from methionine, a deficiency in methionine also affects the body's cysteine levels. The low content of these sulfur-containing amino acids is a characteristic of many legumes, though peas are rich in other essential amino acids, especially lysine.

The Role of Methionine in the Body

Methionine is an essential amino acid with several important physiological functions. It is involved in protein synthesis, metabolism, and the synthesis of other important compounds like cysteine and taurine. For individuals focused on muscle growth and repair, ensuring adequate methionine intake is particularly important.

Complementary Proteins: Creating a Complete Profile

Fortunately, it is simple to achieve a complete amino acid profile when consuming peas, especially for those on a plant-based diet. The solution lies in the concept of complementary proteins, which involves combining two or more plant-based foods that, together, provide all the essential amino acids. A classic combination for peas is pairing them with a food rich in methionine.

For example, rice protein has a higher methionine content and can effectively compensate for the lower levels found in peas. Many commercial plant-based protein powders combine pea and rice protein for this very reason.

Foods High in Methionine

To balance the amino acid profile of peas in a whole-foods diet, consider incorporating the following methionine-rich foods:

Grains: Brown rice, oats, quinoa, and wheat.
Nuts and Seeds: Brazil nuts, sesame seeds, and sunflower seeds.
Animal Products (for omnivores): Eggs, meat (like beef, chicken, or fish), and cheese.

Peas vs. Other Protein Sources: A Comparison

To put the pea protein profile into perspective, comparing it to a common and readily complete protein source like whey is useful. This table highlights some key differences in their amino acid and nutritional characteristics.


Feature	Pea Protein	Whey Protein
Source	Yellow split peas (Plant-based)	Milk (Animal-based)
Amino Acid Profile	Contains all 9 essential amino acids, but low in methionine and cysteine.	Contains all 9 essential amino acids in high and balanced amounts.
Completeness	Considered incomplete due to low methionine content, but can be complemented.	A complete, high-quality protein source.
Limiting Amino Acid	Methionine and Cysteine.	None.
BCAA Content	Good source, but generally lower in leucine compared to whey.	High content of Branched-Chain Amino Acids (BCAAs), particularly leucine.
Digestibility	Highly digestible for many, especially those with lactose intolerance.	Highly digestible and rapidly absorbed.
Allergens	Hypoallergenic, dairy-free, and gluten-free.	Contains lactose and dairy protein, which can cause issues for some.
Sustainability	Generally more sustainable due to plant-based origin.	Less sustainable due to resource-intensive dairy farming.

Conclusion: The Final Word on Pea Protein

Peas are an excellent and versatile source of plant-based protein, offering a wealth of nutrients and health benefits. While it's true that peas lack sufficient levels of the sulfur-containing amino acids methionine and cysteine, this does not diminish their value as a nutritional food. The simple act of combining peas with a complementary protein source, like brown rice or other grains, easily overcomes this limitation, ensuring a complete and well-rounded amino acid intake. For vegans and omnivores alike, incorporating peas into a varied diet is a smart and healthy choice for boosting protein intake and overall wellness. Read more about pea protein benefits from Healthline.

Beyond Amino Acids: The Other Benefits of Pea Protein

In addition to their amino acid profile, peas offer several other notable health benefits. They are a great source of iron, especially important for those on a plant-based diet, as well as fiber, which supports digestive health. Pea protein is also rich in the non-essential amino acid arginine, which promotes healthy blood flow and heart health. Its hypoallergenic nature makes it a safe and gentle alternative for those with common food allergies or sensitivities.

Frequently Asked Questions

Peas are low in the sulfur-containing amino acids methionine and cysteine. This is the main reason they are often not considered a complete protein source on their own.

Pea protein technically contains all nine essential amino acids, but due to its low level of methionine, it is not considered a complete protein in the same class as animal proteins. However, when combined with a complementary protein source, it can form a complete profile.

To compensate for the low methionine content in peas, you can combine them with grains, which are high in this amino acid. Common combinations include pea and rice protein blends, or eating peas alongside foods like brown rice or oats.

Excellent food sources of methionine include nuts (like Brazil nuts), seeds (sesame, sunflower), grains (brown rice, oats), and for omnivores, animal products like meat, eggs, and dairy.

While both provide a good amount of protein, whey is a complete protein with higher levels of all essential amino acids, especially BCAAs. Pea protein is lower in methionine and leucine but is an excellent plant-based, dairy-free alternative when complemented with other foods.

Yes, vegans can get all essential amino acids from a pea-based diet, but it requires balancing intake with other protein sources. For example, combining peas with grains throughout the day ensures a complete amino acid profile is met.

Most pea protein powders contain methionine, but often not in sufficient quantities to meet the definition of a complete protein on its own. Many manufacturers blend pea protein with a source high in methionine, like brown rice protein, to create a more balanced profile.