By T. J. Dunn, Jr., DVM
It is common knowledge and generally agreed upon by experts that dogs (and cats) are meat eaters and have evolved through the ages primarily as meat eaters. Although now "domesticated," our pets have not evolved rumens along their digestive tracts in order to ferment cellulose and other plant material, nor have their pancreases evolved a way to secrete cellulase to split the cellulose into glucose molecules, nor have dogs and cats become efficient at digesting and assimilating and utilizing plant material as a source of high quality protein. Herbivores do those sorts of things. That’s how nature is set up at this time.
On the other hand, some plant material such as rice, soybean meal and corn have some, although limited, usefulness in the meat eater's diet. Corn, wheat, soy, rice and barley are not bad or harmful to dogs and cats. These plant sources are simply not good choices (we do get to choose what we feed our pets, don't we?) for the foundation of a diet to optimally nourish animals what are, have been, and for the foreseeable future will be meat eaters.
What is the difference between grain based and meat based foods for pet dogs and cats? If you don't believe that dogs and cats are primarily meat eaters, you might as well click away now because you certainly won't believe what follows. Most of what is presented next has been derived from two excellent references on small animal nutrition: Canine and Feline Nutrition by Case, Carey and Hirakawa and Small Animal Clinical Nutrition, III by Lewis, Morris, Jr., and Hand.
There are 22 different alpha amino acids that mammals need for various metabolic and energy activities. Dogs and cats are able to synthesize twelve of these internally and, therefore, are required to ingest the other ten in their diets. Because these ten amino acids are necessarily acquired only through food acquisition, they are termed essential amino acids. (Refer to the list in Table 1.)
However, the word "essential" is misleading because all of these are essential for good health. Somebody a long time ago started referring to the amino acids that are not formed internally, and need to be eaten, as the "essential amino acids". Who says science is exact?!
"Non essential" (not required in the diet)
"Essential" (is only obtained via the diet)
Herbivores conveniently have amino acids produced to a great extent by billions of microorganisms along their multi-stomached and lengthy gastrointestinal tracts. Our furry friends, with their relatively short and simple gastrointestinal tracts, are unable to capitalize on microbe amino acid synthesis and require preformed (i.e., they can't make it themselves) amino acids (in the form of larger protein molecules) to be present in sufficient diversity in ingested food.
Note that cats have a few special needs that dogs to not have, such as a dietary source of a beta amino acid called taurine (to read more about this see "Cats Are Different").
Fats required are easy to acquire from both plant and animal origins and are easily mixed into the diet. Everyone agrees that linoleic and (for cats) arachidonic fatty acids are necessary. (Linolenic is synthesized from linoleic both by dogs and cats). High quality fats are readily available, can be stabilized with vitamin E and vitamin C, and are fairly consistent in cost. There, that takes care of the fat in the diet. Nothing complicated to this.