Proteins form the body's main structural elements and are found in every cell and tissue. Your body uses proteins for growth and to build and repair bone, muscles, connective tissue, skin, internal organs and blood. Hormones, antibodies and the enzymes that regulate the body's chemical reactions are all made of protein. Without the right proteins, blood won't clot properly and cuts won't heal. And if carbohydrates and fat can't meet your energy needs, proteins can be broken down and used as a source of emergency energy. Each protein is a large complex molecule made up of a string of building blocks called amino acids. The 20 amino acids the body needs can be linked in thousands of different ways to form thousands of different proteins, each with a unique function in the body. Both the amino acids manufactured in the liver and those derived from the breakdown of the proteins we eat are absorbed into the blood stream and taken up by the cells and tissues to build new proteins as needed.
Your body can't use food protein directly, even though the amino acids in food and in your body are the same. So after protein is ingested, digestive enzymes break the protein down into shorter amino acid chains (polypeptides and then peptides) and finally into individual amino acids. The amino acids then enter the blood stream and travel to the cells where they are incorporated into proteins the body needs.
The quality of a food protein is in part measured by its amino acid content, and there are two types: Nine of the 20 amino acids required by human beings are considered "essential" because they come only from the diet; the other 11 are considered "nonessential" because the body can make them. A complete protein contains all the essential amino acids in amounts your body needs. Animal proteins from eggs, meat, fish, poultry, cheese and milk are generally complete. Plant proteins from fruits, vegetables grains and beans are often low in one or more essential amino acids and are considered incomplete. A well-balanced vegetarian diet, however, can provide the body with all the needed amino acids.