Most of the nutrients we eat cannot be used in their existing form. They must be broken down into smaller components, such as amino acids and monosaccharides, that are universal to all species. Consider what happens if you eat a piece of beef, for example. The myosin of the beef differs very little from that of your own muscles. Yet the two are not identical, and even if they were, beef myosin could not be absorbed, transported in the blood, and incorporated into your muscles. Dietary protein must be broken down into amino acids before it can be used. Since all proteins are made of a limited "alphabet" of the same 20 amino acids, those of beef proteins might indeed become part of your own myosin but could equally well wind up in your insulin, fibrinogen, collagen, or any other protein. The primary purpose of the digestive system is to break nutrients down into forms that can be used by the body and absorb them so they can be distributed to the tissues.

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