Protein, along with fats, carbohydrates and water make up the macronutrients of the diet. Protein is composed of amino acids. There are 20 amino acids that bind to form large molecules of many individual amino acids called proteins. Although protein can be used as a source of energy, it is largely used as a structural component in organs and skeletal muscle as well as in bones. In skeletal muscle as well as in organs such as the heart and the gastrointestinal tract, protein forms the machinery that results in contractions. Proteins also make up a large portion of the blood and different hormones and enzymes in the body. Protein must be taken in to repair and replace these structures as they are worn out. It is generally accepted that the average person requires one gram of protein for every kg of body weight. Bodybuilders and those exercising strenuously are believed to need one gram per pound of body weight or even more. As mentioned, proteins can be used for energy and provide four calories per gram just as carbohydrates do.
Proteins can be complete or incomplete. Complete proteins contain all of the essential amino acids and usually come from animal sources. Incomplete proteins do not contain all of the essential amino acids and usually come from plants. Biological value is the efficiency of a protein at supporting the bodies needs. Protein synthesis in the body stops when an essential amino acid is missing – any remaining amino acids are broken down and excreted. Biological value is expressed as a percentage of the nitrogen absorbed that is retained. Supplied in adequate quantity, a protein with a biological value of 70 or greater can support human growth as long as energy intake is adequate.