Which essential amino acids are likely to be lacking in beans?

November 19, 2025 •

3 min read

While pulses are an excellent source of protein, ranging from 16 to 30% depending on the variety, they are generally not considered 'complete' proteins on their own. This is because the essential amino acids that are likely to be lacking in beans, primarily the sulfur-containing ones, limit the body's ability to utilize their full protein potential.

Quick Summary

Beans are a rich plant-based protein source but are typically low in sulfur-containing amino acids, primarily methionine and cysteine. Combining them with other foods, like grains, is a simple and effective way to achieve a complete amino acid profile.

Key Points

Lacking Methionine: Beans are most notably lacking in the sulfur-containing essential amino acid methionine.
Cysteine Deficiency: As a related sulfur amino acid, cysteine is also typically found in low levels in beans.
Complement with Grains: Pairing beans with grains like rice or wheat effectively provides all essential amino acids, since grains are typically higher in methionine.
Protein Combining Over Time: You don't need to combine complementary proteins in a single meal; the body can pull from its amino acid pool throughout the day.
Soy is an Exception: Soybeans are unique among legumes as they are a complete protein source, though still relatively lower in methionine than animal proteins.
Cooking Improves Digestibility: Proper cooking methods can increase the bioavailability and absorption of amino acids from beans.
Varied Diet is Key: The most effective strategy for a plant-based diet is consuming a variety of protein sources to ensure all essential amino acids are met.

The Limiting Amino Acids in Legumes

The protein quality of any food is determined by its ability to provide all nine essential amino acids (EAAs) in sufficient quantities. While beans offer a substantial amount of protein, they have a reputation for being 'incomplete' because they contain low levels of specific EAAs. The primary amino acid that is likely to be lacking in beans is methionine. Methionine, along with cysteine, is a sulfur-containing amino acid that beans are consistently low in across many different types, including kidney, black, and navy beans.

Some research also indicates that certain legumes may have a lower content of tryptophan. The concept of a 'limiting amino acid' refers to the EAA present in the smallest quantity relative to the body's needs. Because methionine is the limiting amino acid in most legumes, it governs how efficiently the body can use the other amino acids present in beans for protein synthesis and tissue repair.

The Nine Essential Amino Acids

Your body cannot produce the following nine essential amino acids, so they must be obtained through your diet:

Histidine
Isoleucine
Leucine
Lysine
Methionine
Phenylalanine
Threonine
Tryptophan
Valine

Combining Foods for a Complete Amino Acid Profile

For many years, it was thought necessary to consume complementary proteins in the same meal to get all the EAAs. However, this theory has been largely debunked. The body maintains an 'amino acid pool' from which it can draw the necessary components to synthesize proteins throughout the day. This means you do not need to eat rice and beans together in one sitting, as long as your diet includes a variety of protein sources over the course of a day.

Pairing beans with grains, nuts, or seeds is an excellent strategy to create a complete protein profile. Grains, for example, are typically low in lysine but high in methionine, which directly complements the amino acid profile of beans.

Some classic complementary protein pairings include:

Rice and beans
Hummus (chickpeas) and pita bread (grain)
Peanut butter (a legume) on whole-grain bread
Lentil soup with whole-grain crackers
Tortilla chips and bean dip

Amino Acid Profile: Beans vs. Grains

To illustrate the complementarity, here is a simplified comparison of the amino acid content of beans versus grains. While the exact amounts vary widely by specific type, this table highlights the general pattern of their protein content.


Feature	Beans (Typical)	Grains (Typical)
Primary Limiting Amino Acid	Methionine and Cysteine	Lysine and Threonine
High in	Lysine, Fiber, Folate	Methionine, Tryptophan
Excellent for Combining With	Grains, Nuts, Seeds	Legumes, Dairy, Seeds

The Exception: Soybeans

It is important to note that soybeans are an exception within the legume family. Soybeans, and products made from them like tofu and tempeh, are considered complete proteins because they contain all nine essential amino acids in adequate amounts. While soy is still lower in methionine than animal protein, its overall profile is more balanced than other beans. This makes soy a very reliable protein source for those on a vegetarian or vegan diet.

Maximizing Protein Quality and Digestibility

Beyond simply consuming the right amino acids, factors like digestibility can also affect protein utilization. Plant proteins are generally less digestible than animal proteins due to the presence of fiber and other compounds. However, proper cooking and processing can significantly improve the digestibility and bioavailability of nutrients from beans. For example, studies have shown that cooking pulses can increase the level of methionine available for absorption.

Eating a varied diet is the best way to ensure you meet all your nutritional needs, including a complete profile of amino acids. Pairing different plant-based protein sources, even across different meals throughout the day, effectively bridges any nutritional gaps. For a detailed scientific look at complementary amino acids in various plant foods, see the research presented by the National Institutes of Health.(https://pmc.ncbi.nlm.nih.gov/articles/PMC8541063/)

Conclusion: Beans Are Still an Excellent Choice

Despite the fact that they are typically low in methionine, beans remain a powerhouse of nutrition. They are a fantastic source of plant-based protein, fiber, and essential minerals like iron, folate, and potassium. Understanding their amino acid profile simply provides the knowledge to strategically pair them with other foods. By consuming a varied diet that includes beans, grains, nuts, and seeds, anyone can easily get all the essential amino acids they need without over-relying on a single food group. The traditional and widespread combination of rice and beans across many cultures is a testament to this simple and effective nutritional strategy.

Frequently Asked Questions

A limiting amino acid is the essential amino acid present in a food protein in the smallest amount relative to the body's needs. This amino acid limits the body's ability to synthesize new proteins from the food consumed.

Beans and most other legumes are generally low in the sulfur-containing amino acids, methionine and cysteine. Some research also suggests lower levels of tryptophan in certain varieties.

No, it is not necessary to combine complementary proteins in the same meal. The body maintains a pool of amino acids from which it can draw over the course of a day to create complete proteins.

The most widely recognized complete plant protein is soy (edamame, tofu, tempeh). Other plant foods like quinoa, hemp seed, and chia seeds also contain a balanced profile of all nine essential amino acids.

Excellent food pairings include beans with grains (rice and beans), nuts, or seeds. For example, hummus (chickpeas and tahini) or a peanut butter sandwich on whole-grain bread are classic complementary protein combinations.

No, while methionine is the most prominent limiting amino acid, beans also contain other compounds that can reduce the bioavailability of certain minerals like iron. However, they are rich in fiber, folate, and potassium.

Yes, proper cooking of beans is essential. It not only eliminates naturally occurring toxins in raw beans but can also improve the digestibility of their proteins and the availability of amino acids like methionine.