What Is a Complement Protein?

Proteins perform a wide range of functions, from muscle repair to immune function to fluid balance. While you probably know that getting the right amount of protein is important, you may not be aware that not all protein sources are created equal. Complement, or complementary, proteins are those that come together to form a high-quality protein source.

Protein Structure

Proteins are made up of structures called amino acids. There are a total of 20 amino acids, some of which are essential and some of which are nonessential. Essential amino acids are those that you must consume in your diet because your body cannot make them. Nonessential amino acids are those that your body can produce, so they are not as vital in the diet.

The way the amino acids come together determines what type of protein is made. Proteins that contain all of the essential amino acids are called complete proteins, whereas proteins that are missing or low in one or more of the essential amino acids are called incomplete proteins.

Complement Proteins

Complement proteins are two or more incomplete protein sources that together form a complete protein. If a protein is low in a certain amino acid, like lysine, a complement protein is one that provides the missing protein, in this case lysine. Together, these two proteins provide you with all of the essential amino acids that your body needs to function properly.

Because most plant proteins are incomplete, finding complementary proteins is especially important for vegetarians.

Complementary Protein Examples

Beans and rice are an example of complementary proteins. Beans are low in specific essential amino acids that rice provides.


Previously, researchers thought that complementary proteins had to be eaten at the same meal to reap the benefits. After further research, however, the American Dietetic Association noted that complementary proteins provide benefits if eaten in the same day, not just at the same meal 1.