In humans, there is a large amount of protein turnover. Approximately one pound of protein is degraded and re-synthesized each day. Most of the amino acids liberated by protein degradation are used again in the re-synthesis of new protein but about ten percent of the total amino acid content is lost as it is converted to other important molecules involved in nervous system function, pigments, various hormones, and a variety of other essential activities. Amino acids are also used as fuel and, when present in excess in the diet, can be converted to fat for storage of excess calories.

Because of the universal importance of protein molecules to living cells, both plant and animal tissues can provide dietary protein. During digestion, the long chains of amino acids that make up complex protein molecules are disassembled to produce the twenty different single amino acids. These are taken...