A balanced diet contains proteins, which are composed of complex amino acids. There are 20 types of amino acids, consisting of about 16 percent of the body weight in a lean individual. A body needs all 20 to be healthy. Amino acids can be divided into two groups: essential and nonessential. There are 9 essential amino acids. These are the proteins that the body cannot produce by itself, so a healthy individual must consume them. The 11 nonessential amino acids, on the other hand, are produced by the body. So it is not necessary to ingest them. Proteins are described as being either high-quality or low-quality, depending on how many of the 9 essential amino acids the food contains. High-quality proteins, typically found in animal meats, are proteins that have ample amounts of the essential amino acids. Low-quality proteins are mainly plant proteins and usually lack one or more of the essential amino acids. Since people who follow a strict vegetarian diet are ingesting only low-quality proteins, they must make sure that their diets contain a variety of proteins, in order to ensure that what is lacking in one food is available in another. This process of selecting a variety of the essential proteins is called protein complementation. Since an insufficient amount of protein in the diet can be crippling, and prolonged absences of proteins can cause death, it is essential that a vegetarian diet contain an ample amount of the essential proteins.