One of the three main nutrients in food. Foods that provide protein include meat, poultry, fish, cheese, milk, dairy products, eggs, and dried beans. Proteins are also used in the body for cell structure, fighting infection, and other functions. The stomach, small intestine, and pancreas break down proteins into amino acids. After the body's cells use protein, it is broken down into waste products containing nitrogen that must be removed by the kidneys. The blood absorbs amino acids and uses them to build and mend cells.