Protein” — a polypeptide or molecule made up of polypeptides. A complex, nitrogen-containing substance that is found in food and is essential for the functioning of the human body. Protein molecules consist of long chains of building blocks called amino acids. Some of these amino acids can be manufactured in the human body. Others must be supplied by the diet. The body breaks down food proteins into their amino acid constituents and then reassembles the amino acids into the proteins needed for normal functioning.