Proteins, are large, complex molecules made up of amino acids. Twenty major amino acids make up most proteins. Proteins differ in size and function and in the relative proportions of the amino acids they contain. Some proteins lack certain amino acids while others contain all 20. Proteins are the major organic materials in some animal tissues, constituting about 65 to 75 percent of the total on a dry-weight basis. Animals must consume dietary protein to obtain a continual supply of amino acids. After ingestion, it is digested or hydrolyzed to release free amino acids that are absorbed from the intestinal tract of the animal. These are then distributed to the various organs and tissues. Amino acids are used by the tissues to synthesize new protein. Thus, animals do not necessarily require protein, but do require the amino acids which comprise proteins (Akiyama et al., 1992).

Since proteins are continually being used by the animal for growth and repair of tissues, a continuous supply of proteins or amino acids is needed in the diet. Inadequate protein in the diet results in a reduction or cessation of growth, followed by weight loss due to withdrawal of protein from tissues to maintain the vital functions. On the other hand, if too much protein is supplied in the diet, only part of it will be used to make new protein and the remainder will be converted to energy or excreted (Akiyama et al., 1992).

• Protein level
  
• Amino acids