I want to add something, it can add some important clue.
I live in Brasil and the pet's food only appeared here about 15 or 20 years ago, the switch from natural to artificial food was slow and in the beginning were very few pet shops.
As the number of people that used pet's food increased so it was the number of cases of kidney problems (what a coincidence!!!!).
It reached the point that all the veterinaries agreeded that the pet food had a component responsable and so appeared in the market new and more expensive products most of them imported that were free of problems and were recomended by the veterinaries. The other products responsable for the problem continued to be sold without any problem!
So it were two braches of products, one "healthy" and expensive and the other that were not recomended by veterinaries.
The important point is that some pet food had a component that created kidney problem and many cases of death, and this has existed from the first moment of artificial food.
Now enters my experience and observation. When I started to use pet's food I wouldn't give them something bad for their health, I used the "healthy" food marks so proclaimed by the veterinaries.
But what I have observed was that many cats began to have kidney and skin problems, so the so "heathy" marks were no so "healthy", so it was not a question of mark or price, all the foods created the same problem.
As the problem was not generalised and only affected some cat and not all, what I did was to alternate artificial with natural food for the problem be minimised.
But there is something very curious that I've found: For about two years I am giving to the pets the most cheapest food that exist and not the most expensive and "healthy" so well spoken by the veterinaries.
The curious thing that I found is that I have not observed any problem with my pets in the last two years and they eat almost artificial food, the same I cannot say about the other more expensive and "healthy" food.
The only explantion that I have why the "unhealthy" is really healthy is that the chemical responsable of the problems is an imported product. As the product is imported it becomes an expensive component (you must take into account all the import process, the resale and all the cumulative taxes of the goverment). So the imported component becomes to expensive to be used in a cheap product, the net result is that this component is not used in cheap food and only used in more expensive pet foods turning the "healthy" food into really unhealthy.
This cannot be applied to the US/Canada because the responsable product is not imported and can be very cheap to be used in any food.

Stupidity also evolves!