Re: why is gelatin a poor protein?

The quality of proteins is determined by the essential amino acid content 
of the protein relative to the essential amino acid needs of the body.  The 
best quality proteins are eggs, milk and meats.  Gelatin is considered a 
poor quality protein because it contains a limited number of amino acids, 
particularly the essential ones, such as phenylalanine and tryptophan.  A 
better way to increase muscle mass would be to eat slightly more protein 
than usual, making sure that that protein comes from eggs, milk, cheese, 
yogurt, meats, and a variety of grains, beans, and vegetables.  

Reference:  Marie A. Boyle, Personal Nutrition, 4th edition, 2000.