Most veterinarians do not discuss diet with their clients when pets are taken in for their annual examination. When asked, the majority of veterinarians will tell pet owners that most diets made by large pet food companies and labeled as "complete and balanced", will provide all the proper nutrition to keep the pet healthy well into old age. Unfortunately, this could not be further from the truth.
In order for a pet food manufacturer to label a food as "complete and balanced", they only need to show the food contains ingredients at the levels required by AAFCO (Association of American Feed Control Officials) for the life stage on the label. Unfortunately, those levels on the label are a "best guess". Some vitamin and mineral levels have an extremely wide range between the acceptable minimum and maximum values.
Many vitamins and amino acids are destroyed during the heat processing of canned and dry diets. Manufacturers commonly add a synthetic vitamin/mineral mix to ensure minimum levels are met. Unfortunately, these synthetic mixes have been the source of contamination, excesses, and deficiencies many times, leading to pet illness or death. There have been multiple recalls in recent years for thiamine deficiency, which is a B vitamin that is easily destroyed by heat. Thiamine deficiency can cause seizures, tremors, muscle weakness, and death.
Storage for long periods of time in hot warehouses, during transportation, or after purchase, can result in oxidation of fats which causes pet food to become rancid. Punctures or tears in cans or bags can result in contamination and spoilage. Vitamin levels may be severely low by the time the food reaches the pet's bowl.
A second option for pet food companies is the performance of "feeding trials". In a feeding trial, eight animals must be fed the formula for six months. Only six animals must complete the trial, so two can be eliminated if they become ill or die. The remaining six must not lose more than 15% of their starting body weight. Blood is drawn at the end of the trial, which lasts 26 weeks, to test four parameters: hemoglobin, albumin, serum alkaline phosphatase, and packed cell volume (red cell volume). No testing is required at the beginning of the trial. Unfortunately, many deficiencies or excesses will not appear in the short 26 weeks during the trial.
Interestingly, the AAFCO website states that pet food must be "pure and wholesome, contain no harmful or deleterious substances and be truthfully labeled". Since FDA willfully and knowingly allows maggot-infested, diseased and dead animals that died in the field (not by slaughter) or otherwise, to be used in pet food, they clearly are not ensuring pet food is pure and wholesome. If we can't count on them to regulate and enforce ingredients, we can't count on the food to be "complete and balanced". Recent recalls of food contaminated with pentobarbital, foods with elevated levels of thyroid hormone, and foods with shards of metal, lead the consumer to wonder what is safe to feed. Carcinogenic dyes, propylene glycol, and sugar are also considered "pure and wholesome" by big pet food companies, AAFCO, and FDA.
Personally, I don't trust the big pet food companies to have the best interests of my pets at the forefront. Making my pets' food myself with real food ingredients continues to be the best method to ensure "pure and wholesome food, containing no harmful or deleterious substances".