What amino acids are limiting in beans? A guide to complementary proteins

November 18, 2025 •

4 min read

According to the Food and Agriculture Organization of the United Nations, legumes like beans make a significant contribution to the world's dietary protein intake, yet their nutritional quality is often limited. So, what amino acids are limiting in beans, and how can this be addressed for optimal health?

Quick Summary

Methionine and cysteine are the primary limiting amino acids in most types of beans and legumes. Combining beans with grains, nuts, or seeds creates a complete protein source, ensuring all nine essential amino acids are consumed.

Key Points

Primary Limiting Amino Acid: For most beans and legumes, the limiting amino acid is methionine (and cysteine), the sulfur-containing amino acids.
Complete Protein Pairing: Combining beans with grains, nuts, or seeds creates a complete protein source by complementing each other's amino acid profile.
Beans and Grains Complementation: Beans are high in lysine but low in methionine, while grains are the opposite, making them a perfect nutritional match.
No Same-Meal Rule: Protein complementation does not need to happen in the same meal; eating a variety of plant proteins throughout the day is sufficient.
Soybeans are an Exception: Unlike most other legumes, soybeans are considered a complete protein on their own, containing adequate levels of all essential amino acids.
Dietary Strategy: Understanding limiting amino acids is a key dietary strategy for those on plant-based diets to ensure optimal protein intake and synthesis.

The Importance of Essential Amino Acids

Protein is a crucial macronutrient made up of smaller units called amino acids. The human body can synthesize some amino acids, known as non-essential amino acids. However, nine amino acids are considered "essential" because they must be obtained through the diet. A complete protein source contains all nine essential amino acids in sufficient quantities. Many animal products, such as meat, dairy, and eggs, are complete proteins. In contrast, most plant-based foods are considered incomplete, as they are low in or lacking at least one essential amino acid. This is where the concept of a "limiting amino acid" comes into play—it is the essential amino acid present in the lowest amount relative to the body's needs.

So, What Amino Acids Are Limiting in Beans?

For the vast majority of beans and legumes, the limiting amino acid is methionine, along with the related sulfur-containing amino acid, cysteine. This is a well-established fact across various types of beans, including kidney, pinto, and black beans. While beans provide an excellent source of other essential amino acids, particularly lysine, their relative deficiency in methionine is why they are not considered a complete protein when consumed alone.

The Role of Methionine and Cysteine

Methionine is critical for several bodily functions, including metabolism, growth, and the creation of other molecules like cysteine. Since beans are low in both methionine and cysteine, they are collectively referred to as being limited by the "sulfur-containing amino acids". This deficiency is a natural biological feature of these plant-based foods.

The Concept of Complementary Proteins

Fortunately, it is simple to overcome the limiting amino acid in beans through a dietary strategy known as protein complementation. This involves combining different incomplete protein sources to fill in each other's amino acid gaps. The most classic example is pairing legumes with grains, as they are a perfect match.

Beans and Legumes: High in lysine, low in methionine.
Grains: Low in lysine, but high in methionine.

This complementary pairing ensures all nine essential amino acids are obtained, creating a complete protein source. The good news for vegetarians and vegans is that they do not need to consume these complementary proteins in the same meal. The body maintains a pool of amino acids, so eating a variety of complementary foods throughout the day is sufficient.

Combining Complementary Proteins

Here is a list of popular and delicious complementary protein combinations:

Rice and beans
Pita bread and hummus
Lentil soup with whole-grain bread
Peanut butter on whole-grain toast
Black bean tacos with corn tortillas
Green peas with quinoa

Comparing Amino Acid Profiles: Beans vs. Grains

To illustrate the complementary relationship, consider the typical amino acid profiles of beans and grains. This table provides a simplified comparison of their key amino acid levels.


Feature	Beans (e.g., Kidney, Pinto)	Grains (e.g., Rice, Wheat)
Protein Type	Incomplete	Incomplete
Lysine Level	High	Low (often limiting)
Methionine Level	Low (limiting)	High
Overall Profile	Rich in many EAAs, but deficient in sulfur-containing ones	Rich in many EAAs, but deficient in lysine
Complementation	Pairs perfectly with grains	Pairs perfectly with beans
Common Combinations	Rice and beans, hummus with pita	Rice with dal, peanut butter sandwich

The Exception: Soybeans

While the rule for most legumes is a deficiency in methionine, soybeans are a notable exception. Soybeans and products derived from them, such as tofu and tempeh, are considered complete proteins on their own, meaning they contain adequate levels of all nine essential amino acids. However, even soy has slightly lower concentrations of sulfur amino acids compared to some animal proteins, but it is not considered a limiting amino acid in the same nutritional sense as other legumes.

Conclusion: Strategic Eating for Complete Protein

Knowing what amino acids are limiting in beans, specifically methionine and cysteine, is fundamental for anyone following a plant-based diet. The solution is straightforward and has been practiced in cultures around the world for centuries: protein complementation. By strategically combining beans with grains, nuts, or seeds, you can easily obtain a complete protein, regardless of whether these foods are eaten at the same meal or throughout the day. This simple dietary practice ensures you receive a full spectrum of essential amino acids, supporting your body's needs for growth, repair, and overall health. A varied and balanced diet that includes a mix of these plant-based protein sources is key to optimal nutrition.

For more detailed information on protein quality, a resource from the American Society for Nutrition offers a good overview of protein complementation and incomplete protein sources.

Frequently Asked Questions

Methionine is a limiting amino acid in beans because beans naturally contain lower concentrations of this sulfur-containing amino acid compared to the body's requirements. This imbalance means protein synthesis can be limited by the supply of methionine, even if other amino acids are abundant.

Protein complementation is the practice of combining two or more incomplete protein sources to create a complete protein. For example, pairing beans (low in methionine) with rice (high in methionine) ensures all essential amino acids are provided.

No, you do not need to eat complementary proteins like beans and rice in the same meal. The body maintains a pool of amino acids, so consuming a variety of plant-based protein sources throughout the day is enough to ensure you get all nine essential amino acids.

While most beans have similar amino acid profiles and limitations, soybeans are the exception, as they are a complete protein source. Other beans like lentils, kidney beans, and black beans are excellent protein sources but are incomplete and require pairing with complementary proteins.

Vegans and vegetarians can get enough methionine by regularly incorporating a variety of complementary protein sources into their diet. This includes combining legumes with grains, nuts, and seeds. Examples include rice and beans, hummus with pita bread, and nut butter with whole-grain toast.

Foods high in methionine that can complement beans include grains such as rice, oats, and wheat, as well as nuts and seeds like pumpkin seeds and sesame seeds.

While most legumes are limited by methionine and cysteine, some variations exist. For example, while methionine is typically the most limiting amino acid in black beans, studies have also identified deficiencies in leucine and tryptophan, though to a lesser degree.