Vitamins and minerals are inexpensive, well documented as to types and amounts, and can be added conveniently to any food product. No problem here.
Carbohydrates are useful to dogs and cats for readily burnable fuel for all kinds of muscular and metabolic activities. Cheap and easily produced sources of carbohydrates are such items as rice, corn, wheat, barley and soy. Hmmmmmmm ... sounds like what some pet food manufacturers are commonly using as their first choice for a diet’s foundation. Some even claim these plant products to be an excellent sources of protein!
In their book on nutrition Case, Carey and Hirakawa list seventeen plant products including ground rice, corn, wheat, oats, barley, alfalfa and others as sources of carbohydrates. In fact, one of the benefits of carbohydrates, so say the experts, is that they are protein sparing. That is, the animal will utilize inexpensive carbohydrate sources for energy if available to the animal before the animal will utilize more expensive (a human concept!) protein.
So, let's add some plant material to our ideal food for the carbohydrate benefits and not confuse anybody by implying (or worse, stating) that the corn, rice or wheat is primarily a protein product. (The same authors list nineteen pet food ingredients used as protein sources. . . and ground corn, wheat, rice, oats, barley and alfalfa are NOT on that list.)
That takes care of the carbohydrates in our diet; we know we will use some inexpensive grains, however our diet will NOT use grain as the foundation or primary ingredient. And just so you know, dogs and cats do very well on diets with minimal carbohydrates and a preponderance of fats and high quality protein. Dogs and cats differ from humans in this respect. Remember, all aspects of human nutrition do not necessarily correspond to canine and feline nutrition.
Having stated that proteins can readily be used as a source of energy for dogs and cats, that carbohydrates are of much less importance than in human diet andmetabolism, we should place a major responsibility on the protein content of our ideal diet. Since we know that ten amino acids are required from dietary sources, it only makes sense that we pick a protein source that has a full spectrum of amino acids.
We know for sure we are not going to pick corn as a protein source since it contains only four of the ten essential amino acids and contains no taurine, plus nutrition experts didn't even include corn on the list of protein sources in pet foods. Corn was on the carbohydrate list!
Canine and Feline Nutrition lists substances that provide protein, including beef, chicken, eggs, fish, lamb and meat by-products. (Just so you know… the meat by-products in pet foods as defined by the Association of American Feed Control Officials do NOT contain hair, hide, hooves or feathers, but actually refer to organ meats like liver, kidney, stomach, heart, blood, spleen, etc.) Meat by-products are a great source of protein for a meat eating animal.
Therefore, for our diet to contain a wide spectrum of amino acids, we will choose to have it contain the best source of protein for mammals -- eggs, or more precisely the egg whites. This substance has a wide amino acid profile and is highly digestible. In fact, egg white is considered a standard against which other protein sources are measured. Other really good choices would be meat, poultry or fish.
So for dogs (and cats) our custom diet will contain vitamins and minerals, some grain for readily available energy, a proper amount and ratio of fat sources, and as a foundation, a high quality MEAT source.
Pet food manufacturers know very well how to make a great diet just like the one we put together. The problem is that it would be expensive to produce, especially if eggs and beef and fish were in it. And to be competitive with other pet food producers, the price of the food dictates what the foundation (primary ingredients) of the diet will be.