Are Beans an Incomplete or Complete Protein?

January 14, 2026 •

4 min read

Protein is a crucial macronutrient made up of amino acids. There are nine essential amino acids that the human body cannot produce on its own and must obtain from food. So, are beans an incomplete or complete protein? Most beans, with the notable exception of soybeans, are considered an incomplete protein, meaning they lack or are low in one or more of these essential amino acids.

Quick Summary

Most beans are incomplete proteins, but can be paired with other plant foods like grains, nuts, or seeds to form a complete protein source. This strategy ensures your body receives all nine essential amino acids. The myth that protein must be complemented within the same meal is outdated; your body can combine these amino acids over the course of a day.

Key Points

Incomplete Protein: Most common beans are incomplete proteins, lacking sufficient amounts of the essential amino acid methionine.
Complementary Proteins: Combining beans with grains, nuts, or seeds, such as rice and beans, creates a complete protein.
24-Hour Rule: The body maintains a pool of amino acids, so complementary proteins do not need to be eaten in the same meal, only within the same day.
Soy is Complete: Soybeans and soy products like tofu and tempeh are among the few complete proteins found in the plant kingdom.
High in Fiber: Beyond protein, beans are an excellent source of dietary fiber, promoting digestive and heart health.
Rich in Micronutrients: Beans provide essential vitamins and minerals, including iron, magnesium, and B vitamins.

What Makes a Protein Complete?

Protein is vital for building and repairing tissues, producing enzymes, and supporting overall health. It is made up of 20 different amino acids. Of these, nine are considered essential, as the body cannot synthesize them on its own and must get them from dietary sources. A "complete" protein contains all nine of these essential amino acids in sufficient quantities. Animal-based products, such as meat, poultry, fish, and eggs, are typically complete proteins. However, for those following vegetarian or vegan diets, or for anyone looking to increase their plant-based protein intake, it is crucial to understand which plant foods provide complete proteins and how to combine them effectively.

The Verdict: Beans Are Mostly Incomplete Proteins

Most common varieties of beans, including black beans, kidney beans, and pinto beans, are considered incomplete proteins. They are relatively low in the sulfur-containing essential amino acid, methionine. While beans contain all 20 amino acids, the quantity of methionine is the limiting factor that prevents them from being classified as complete proteins on their own.

The Exception: Soybeans

The most significant exception to the rule is the soybean. Soy is one of the few plant-based sources of protein that contains all nine essential amino acids in the right proportions to be considered a complete protein. Foods made from soy, such as tofu, tempeh, and edamame, are excellent complete protein options for plant-based eaters.

The Power of Complementary Proteins

The good news is that you do not need to rely solely on complete protein sources to meet your dietary needs. By combining different plant-based foods, you can create a complete protein profile. This is known as protein complementation. The classic example of this is combining beans with a grain. Beans are typically rich in lysine, an essential amino acid that is often limited in grains. Grains, on the other hand, are higher in methionine, the amino acid that beans lack. By pairing these two food groups, you get a complete protein source.

The Timing Myth: You Don't Need to Combine at Every Meal

For many years, it was believed that complementary proteins had to be eaten within the same meal to be effective. However, modern nutritional science has debunked this myth. The body is remarkably efficient and maintains a pool of amino acids throughout the day. As long as you consume a variety of protein sources over a 24-hour period, your body will assemble the essential amino acids it needs. This flexibility makes it much easier to plan a balanced and healthy vegetarian or vegan diet.

How to Create Complete Proteins with Beans

Combining beans with other plant foods to create a complete protein is both simple and delicious. Here are some popular and nutritious combinations:

Beans and Rice: A staple in many cuisines, this combination creates a complete protein and is a great source of fiber and complex carbohydrates.
Hummus and Pita Bread: Hummus is made from chickpeas, a type of bean, while pita bread is a grain. Together, they form a complete protein snack or meal.
Bean Soup and Crackers: A warm bowl of lentil or bean soup served with whole-grain crackers is a comforting and complete protein meal.
Pasta with Beans: Add chickpeas or white beans to your favorite pasta dish for an extra boost of complete protein.
Mixed Bean Salad with Nuts or Seeds: Mixing beans with nuts or seeds provides a complete amino acid profile while also offering healthy fats.

The Nutritional Benefits of Beans

Beyond their role in protein complementation, beans offer a wide array of health benefits. They are rich in soluble fiber, which can help lower cholesterol levels and regulate blood sugar. This high fiber content also promotes better digestive health and increases satiety, helping with weight management. Additionally, beans are packed with important micronutrients like iron, magnesium, and B vitamins. These nutrients contribute to better heart health and a lower risk of chronic diseases.

Incomplete vs. Complete Protein Sources: A Comparison


Feature	Beans (Incomplete Protein)	Animal Products (Complete Protein)
Essential Amino Acids	Low or insufficient in one or more (e.g., methionine).	Contains all nine essential amino acids in sufficient quantities.
Saturated Fat	Typically low in saturated fat.	Can be high in saturated fat.
Fiber Content	Excellent source of dietary fiber.	Contains no dietary fiber.
Micronutrients	Rich in iron, magnesium, and B vitamins.	Varies, but good source of B12 and iron.
Environmental Impact	Generally lower environmental footprint.	Higher environmental footprint.

Conclusion: Beans Are a Vital Part of a Healthy Diet

In summary, most beans are incomplete proteins due to their low levels of the amino acid methionine. However, this fact should not diminish their value in a healthy diet. By pairing them with complementary protein sources like grains, nuts, or seeds over the course of a day, you can easily obtain a complete protein profile. The long-held belief that these foods must be consumed in the same meal is a misconception. With the exception of soy, all beans are incomplete proteins, but their versatility and nutritional benefits make them an indispensable part of a balanced diet, whether you are a meat-eater or a plant-based food enthusiast.

For Further Reading:

For more detailed information on protein and its role in human health, you can visit the Cleveland Clinic website.

Frequently Asked Questions

No, you do not need to combine complementary proteins at every meal. As long as you eat a varied diet with different protein sources throughout the day, your body will get all the essential amino acids it needs.

While most plant foods are incomplete, some notable exceptions include soy (tofu, tempeh, edamame), quinoa, buckwheat, and hemp seeds, which all contain a sufficient amount of all nine essential amino acids.

If you eat an unbalanced diet that only includes incomplete protein sources, you could risk a deficiency in certain essential amino acids. However, by simply eating a variety of plant foods, such as beans, grains, and nuts, you can ensure your body receives all the necessary amino acids.

The simplest and most classic way is to combine beans with a grain. For example, a meal of black beans and rice, or a bowl of lentil soup with whole-grain bread, will provide a complete protein.

Yes, black beans are considered an incomplete protein because they are low in the amino acid methionine. Pairing them with rice or another grain will form a complete protein source.

While it's a common misconception, the protein needs of vegans and meat-eaters are generally the same. The key for vegans is to ensure they eat a variety of protein sources throughout the day to get all the essential amino acids.

Yes, beans are great for muscle building. When combined with complementary proteins, they provide a complete amino acid profile necessary for muscle repair and growth. They also offer fiber and micronutrients that support overall athletic performance.