I am an animal nutritionist and have formulated my share of dog foods. Including high performance working dog diets. The basic science is pretty well known and is not mysterious. But the commonly seen marketing hype in dog foods is way over the top. People will tell you all kinds of things that are not true in order to reinforce their beliefs or to sell you expensive food that's no better for your dog. As an example, people who say a dog can't properly utilize some "byproducts" or rendered ingredients are full of ... crap. But, Soy is not a good source of protein for dogs. Some grain is OK, even essential to a balanced dry diet. I'd avoid buying foods that have wheat though. Dogs fed wheat can have some allergic sensitivities develop. Rice and corn and oats are decent ingredients, but not if they're too much of the total. Fat content is quite important, especially for puppies and working dogs. 18-22% or so total fat is good for those categories. And proper antioxidant protection is extremely important in any dry dog food containing fat. If the fat is not protected with a good antioxidant, such as ethoxyquin, rancidity quickly robs the food of fat soluble vitamins ( A,D,E) and the free radicals produced by rancid fats have many other bad effects. So be wary of foods that say " no artificial additives" They may be using expensive vitamin E as an antioxidant, which works, but is inefficient and you're paying for a vitamin to do the job of a preservative. Silly and expensive and no better for the dog. I'm all for supporting local, and Canadian, and wholesome ingredients. But be wary of the hype. Buying reasonably fresh stock is important. Some small town stores have dog food that is months and months old. Not good for vitamin and fat stability.
The big name proven brands are OK. You don't need to buy an expensive, specialty or fashionable dog food. Purina Pro Plan is a good place to start, or Eukanuba, Iam's etc. are all good. Even Costco Nature's domain is OK for adult dogs. You should feed hunting dog puppies large breed puppy formula while they are under one year of age, and a high fat "active" formulation for adults in the late summer and fall working season.