I meant those various "schools" or at least some exponents of them that take their feeding plan up like some religion in pointless discussions as to the ultimate, one and only healthy and/or natural dog diet. Mainly it's as to whether any vegetable matter must/can/had better/must never be added (delete as appropriate).
I very much doubt that there is a study around that an apple and some lettuce or even a sprinkle of grain will either lengthen or shorten a dog's life, and anyway, experience shows that dogs can live a healthy and long life on a variety of different diets, so that tells us only they are more adaptable than say a Big Panda Bear.

If I remember right, the oldest dog on record was even fed a vegetarian diet, which certainly cannot be called natural, but obviously could not have been that unhealthy.
Reading here what some Springers will and do eat if given the chance sometimes has me wondering.... Rhian once ate the better part of a plastic water bowl - did she not read that she's supposed to be a carnivore?