March 26th, 2007.

"Consumers should look for the nutritional statement on the label of any dog  or cat food that they purchase. There are three types of nutritional  statements  that consumers will find.

One type says that the food is not intended as a  balanced diet and should not be the sole source of nutrition for the pet.  

The  second type says that the diet is formulated to AAFCO standards for  maintenance  diets. 

The third type says the diet is formulated to AAFCO standards for  maintenance diets and tested in feeding trials. AAFCO is the organization  that  determines the types and amounts of vitamins and minerals that dogs and cats  need  to stay healthy. Companies that produce diets that are intended as the sole  food source for dogs and cats must meet these requirements. But companies  can  use whatever raw material satisfies the requirement for each vitamin and  mineral. Sometimes companies use raw materials that have low  "bio-availability, meaning that the vitamin or mineral canŐt be absorbed efficiently through the intestine."

Robert Jay Russell, Ph.D. Anatomy and Zoology,  Duke University.