What is the main topic of this talk?

A balanced diet contains proteins, which are composed of complex amino acids. There are 20 types of amino acids, comprising 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 nine essential amino acids. These are the proteins that the body cannot produce by itself, so a healthy individual must ingest 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. This refers to how many of the nine essential amino acids the food contains. High-quality proteins, typically found in animal meats, are proteins which 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 insure that what is lacking in one food is available in another. The 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 absence of proteins can cause death, it is imperative that a vegetarian diet contains an ample amount of the essential proteins.