I am a former veterinarian. Science Diet, Eukanuba and Iams claim to make some of the best dog foods and they do make a very good product. The price is higher but the digestibility of their products are higher. This means that they have less fiber and undigestible products. More digestible means less stool out the other end. Therefore, they claim it requires less food to feed your dog which offsets the higher price.
There is no proof that dog food causes cancer. Certain breeds are more prone to certain illnesses and cancers but food is not the reason. To my knowledge, all food companies use the waste/leftover products from dead animal and other by products of food processing. The animal products are pressure cooked to kill bacteria and then reprocessed. Corn and other plant products are added to make a balanced diet. Dogs fed primarily meat diets will do poorly. Dog food diets are actually well balanced and the process is fairly scientific especially in the higher quality products such as Iams, Science Diet etc. However, both Iams and Science diet cannot make any claims that there product is actually better for you dog than Purina or other companies in terms of longer or healthier life. They have tried but can't prove it. They do have some data that suggest there products are better for puppies and growing dogs. I do believe that even the very cheap dog food products meet the minimum requirement required by federal law for protein, fat and nutrients. The digestibility and absorption of these products can be marginal. Fiber is one of the cheapest products to add and of marginal value other than producing a lot of stool in animals. Fat is one of the most expensive ingredients in dog food and therefore many companies meet the minimum requirement. Adding a teaspoon or very small amount of corn oil does add some essential fatty acids will help some dogs with skin and poor coat problems. Animal fat such as bacon is not a good source of these fatty acids.
Overall, the average dog food is a much better diet than the average person consumes. It is much more controlled and regulated process and has government set minimum standards. The requirements for protein, fat and other nutrients is actually similar to the ideal human diet even though dogs are considered carnivores.
