Everyday the human body repairs or replaces damaged and dead cells. In order to do this, the body needs to manufacture different proteins out of amino acids. Some amino acids can be generated by the body, while others have to be imported via an outside food source. A complete protein is a food source that provides the body with all the amino acids it needs to function. In contrast, an incomplete protein is a food source that does not have all the essential amino acids.
There are about twenty amino acids that the body needs to create the various proteins it requires. Of the twenty, an adult human can manufacture all but nine. These nine essential amino acids include histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. Without these substances, the body may be hard pressed to repair itself. Since they are not able to be manufactured by the body, these nine have to come from an outside food source.
Since they are so essential to human function, the proper amount of essential amino acids must be consumed every day. If a food source has all nine of these amino acids, it is called a complete protein. Should it lack one or more of the nine essential amino acids, it is an incomplete protein. Commonly, plant proteins are classified as an incomplete protein. Different types of plant proteins can be combined, however, to make up a complete protein.
Complementary proteins are two or more incomplete proteins from different food sources that combine to make up a complete protein. While one food may be lacking in a certain amino acid, another may have it in abundance. For example, many beans are rich in protein but are still considered an incomplete protein. Combining it with rice, however, creates a complete protein because what was missing in the beans is supplied by the rice. This is another reason why it is generally wise to eat a variety of foods daily.
While plants often contain incomplete proteins and animals complete proteins, this isn't always the case. There are some plants that have complete proteins and some animal sources that do not. For example, soy beans and the products made from them are usually complete proteins. Gelatin, on the other hand, comes from an animal but is an incomplete protein. A balanced diet typically will provide all the essential amino acids required.