Protein, high-molecular-weight compound that yields amino acid through hydrolysis. Although hundreds of different amino acids are possible, only 20 are found in appreciable quantities in proteins, and these are all alpha-amino acids. Proteins are found throughout all living organisms. Muscle, the major structural material in animals, is mainly protein; the 20% of blood that is not water is mainly protein. Enzymes may contain other components, but basically they, too, are protein. Approximately 700 proteins are known; of these, 200–300 have been studied and over 150 obtained in crystalline form. Some are very stable, while others are so delicate that even exposure to air will destroy their capability as enzymes. The best food sources for proteins are meat, fish, eggs, milk, and cheese. These foods provide all the essential aminoacids and are known as complete proteins. Vegetables and nuts do provide proteins but are not complete proteins. The absence of protein in the diet can diminish growth and reduce energy levels.