* Complementary proteins are two or more incomplete protein sources, which together provide adequate amounts of all essential amino acids and adequate total protein. It is true that many vegetable sources of protein are not complete, and need to be complemented by other protein sources. A complete protein provides all essential amino acids and sufficient total protein. * An incomplete protein is either low in one or more essential amino acids, or low in total protein. There are eight essential amino acids, and several semi-essential amino acids. But I’ll simply it for you.