Are Beans Considered a Complete Protein? A Comprehensive Look

January 12, 2026 •

2 min read

Most beans are not considered a complete protein on their own, primarily due to their low levels of the essential amino acid methionine. A complete protein must contain all nine essential amino acids, which are the building blocks of protein. However, this does not diminish their crucial role in a healthy, balanced diet.

Quick Summary

Beans are incomplete protein sources, lacking sufficient methionine, but are rich in lysine. They can be combined with grains, nuts, or seeds, which provide the missing amino acids, to form a complete protein over the course of a day.

Key Points

Incomplete Protein: Most beans lack sufficient levels of the essential amino acid methionine to be considered a complete protein on their own.
Soy is Complete: Soybeans and soy products like tofu and edamame are complete protein sources, containing all nine essential amino acids.
Protein Complementation: Combining beans with grains, nuts, or seeds allows you to create a complete protein over the course of a day.
Dietary Variety: Eating a variety of plant-based foods is a reliable way to ensure you get all essential amino acids without needing to plan every meal for combination.
Rich in Nutrients: Beyond protein, beans are packed with fiber, vitamins, and minerals that offer significant health benefits.
Health Benefits: Including more plant-based protein in your diet can lower the risk of chronic diseases like heart disease and diabetes.
Traditional Wisdom: Many traditional cuisines naturally combine complementary plant proteins, such as rice and beans.
Flexibility: The concept of needing to combine specific foods in a single meal for complete protein has been largely debunked as long as overall dietary variety is maintained.

What Defines a Complete Protein?

To understand why most beans are considered an incomplete protein, it's essential to define what makes a protein "complete." Our bodies require 20 different amino acids to build proteins, with nine being deemed "essential" because the body cannot produce them. These nine essential amino acids—histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine—must be obtained through diet. A food that contains all nine in adequate amounts is classified as a complete protein. While animal products like meat and dairy are common examples, several plant foods also qualify.

The Amino Acid Profile of Beans

Most beans, including kidney, black, and pinto beans, are rich in several essential amino acids but are typically limited in the sulfur-containing amino acids methionine and cysteine. This makes them incomplete protein sources individually. The good news for those following a vegetarian or vegan diet is that you don't need to consume all essential amino acids in a single meal. The human body maintains an amino acid pool, allowing you to combine different plant-based foods throughout the day to meet your needs. This practice is known as protein complementation.

The Exception: The Mighty Soybean

It's important to note the soybean as a significant exception. Soybeans are one of the few plant-based foods that contain all nine essential amino acids, making them a complete protein. This is why soy products like tofu, tempeh, and edamame are incredibly popular and valuable protein sources in vegetarian and vegan diets. Tempeh, for example, is a fermented soybean product with a dense, nutty texture that provides a powerful protein punch.

Examples of Complete Protein Combinations

Traditional dishes often combine beans with other ingredients like rice or grains to create a complete amino acid profile. Other complementary pairings include hummus with pita and peanut butter on whole wheat bread.

The Benefits of Protein Complementation

Focusing solely on whether a single food is a "complete" protein can be misleading. A varied diet of plant foods offers many benefits beyond just protein, including dietary fiber. Plant-based diets are associated with a lower risk of chronic conditions like heart disease, diabetes, and certain cancers. The environmental impact of plant-based protein production is also significantly lower than that of animal products.

Conclusion

In summary, while most beans are not considered complete protein sources on their own due to a limitation in methionine, they are still a highly nutritious and essential component of a healthy diet. By embracing the principle of protein complementation and consuming a wide variety of plant-based foods throughout the day, you can easily obtain all the essential amino acids your body needs. The key takeaway is to focus on dietary diversity, not on creating the perfect protein combination in every single meal.

For more in-depth information on plant-based nutrition, refer to the resources provided by the National Institutes of Health (NIH).

Note: The NIH link serves as an authoritative source on the biochemistry of essential amino acids.

Frequently Asked Questions

Beans are most often limited in the sulfur-containing amino acids, methionine and cysteine.

Yes, combining beans (rich in lysine) with rice (rich in methionine) creates a meal with a complete amino acid profile, a classic example of protein complementation.

No. While many are, some plant-based proteins like soybeans, quinoa, buckwheat, and hemp seeds are complete proteins, containing all nine essential amino acids.

No, it's not necessary. Your body maintains a pool of amino acids, so consuming a variety of different protein sources throughout the day is sufficient to meet your needs.

Yes, tofu is made from soybeans, which are a complete protein source containing all nine essential amino acids.

Protein complementation is the practice of combining two or more incomplete protein sources to ensure you get all nine essential amino acids. The combination can be over the course of a day, not necessarily in one sitting.

Plant-based proteins are often rich in fiber and offer a lower risk of chronic diseases like heart disease and diabetes. They also tend to be lower in saturated fat compared to animal proteins.