The basic structure of protein is a chain of amino acids that contain carbon, hydrogen, oxygen, and nitrogen. The presence of nitrogen differentiates protein from carbohydrate and fat. Proteins are described as essential and nonessential proteins or amino acids. The human body requires approximately 20 amino acids for the synthesis of its proteins. The body can make only 13 of the amino acids -- these are known as the nonessential amino acids. They are called non-essential because the body can make them and does not need to get them from the diet. There are 9 essential amino acids that are obtained only from food, and not made in the body. Protein is the main component of muscles, organs, and glands. Every living cell and all body fluids, except bile and urine, contain protein. The cells of muscles, tendons, and ligaments are maintained with protein.
Children and adolescents require protein for growth and development. All meat and other animal products are sources of complete proteins. These include beef, lamb, pork, poultry, fish, shellfish, eggs, milk, and milk products. Protein in foods (such as grains, fruits, and vegetables) are either low, incomplete protein or lack one of the essential amino acids. These food sources are considered incomplete proteins. Plant proteins can be combined to include all of the essential amino acids and form a complete protein. Examples of combined, complete plant proteins are rice and beans, milk and wheat cereal, and corn and beans.