All proteins are not equal. Proteins are characterized by their digestibility and their biological value. Digestibility is a measure of how much (what percent) of a protein is absorbed from the digestive tract. Undigested protein breaks down to produce ammonia, an irritant that can compromise the intestinal lining leading to various problems.

Biological value is a measure of how effectively a protein is utilized by the body once it has been absorbed. Protein with a high biological value yields less waste for the kidneys to process (so long as the food contains sufficient fat that protein is not burned for energy). Protein with a low biological value has less benefit and produces more waste.

Animal-based protein generally has greater digestibility and biological value than plant-based protein. A common feature of dog food labels is the attempt to make it look as though the food contains a lot of (expensive) animal-based protein, while padding the protein percentage with plant sources. Remember this is an extraordinarily competitive market. Egg has the highest biological value, with poultry and meats close behind. The protein casein (milk protein-found in cottage cheese) is highly digestible, as is the protein in white meat of chicken-the reason boiled white meat of chicken (accompanied by highly digestible boiled white rice) is often prescribed for digestive upsets.

Order of ingredients
Ingredients are listed on the label in decreasing order of weight, so if the label says, "chicken, ground yellow corn,..." you know that there is more chicken than there is ground yellow corn, and more ground yellow corn than any subsequent ingredient.

But if the next few ingredients are "corn gluten meal, wheat gluten, white rice, brewer's rice,..." it is easily possible that the total quantity of grain far outweighs the chicken. Gluten is protein, so when you see it on the label you know that a portion of the protein content is from plant sources.

The "Meal" Argument
"Meal" is desiccated--reduced in water content--relative to whatever it was made from. Water is heavy, and a given weight of meal thus contains much more protein than the same weight of meat or poultry. A food with "chicken meal" followed by a bunch of grains is likely to contain more chicken protein than one listing "chicken" followed by a bunch of grains.

Some people argue that the heat treatment used to make meal degrades the ingredients, and that meal should be avoided. Note also that some AAFCO definitions, such as "meat meal," allow for use of carcasses several days old. Although heat treatment kills bacteria, bacterial toxins may remain in the product. Other definitions, such as "lamb meal," specify that fresh lamb is to be used.

By-Products
According to the AAFCO definition, "chicken by-product meal" is mostly smooth muscle and is required to be made from freshly slaughtered chickens. We are not aware of any evidence that it contains beaks, feet, and feathers in violation of AAFCO standards, as detractors of by-products suggest.