Protein definition: Any one of a group of complex organic compounds containing nitrogen formed from various combinations of amino acids.

Protein is necessary for the development of bodily tissues. It is the major source of building material for internal organs, blood, hair, teeth, nails, and, of course, muscle. Each gram of protein equals approximately 4 calories. Protein is made up of essential and non-essential amino acids. Since the essential amino acids are not formed in the body, they have to be ingested through diet. Foods that contain the 9 essential amino acids are called complete proteins. Examples include lean beef, poultry, fish and seafood. Dairy products also contain complete proteins.

Vegetarians get their protein from non-animal food sources, which are usually incomplete proteins. Plant proteins like soybeans, nuts, or beans usually lack one or more essential amino acids in sufficient quantity. However, if certain plant foods are eaten together, the vegetarian can get all the essential amino acids necessary. For example, beans and rice, when eaten together, provide a complete protein source.