I suspect that more or less all mammals can digest all shorts of macronutrients, the question could be how good use they make of them. For example in the vet I go there is an enormous number of dogs suffering of diabetis, obviously fed on kible, treats and leftovers from starches and no exercise at all. So, again, a study presents some results, the interpretation of these results is a completely different story. I would be less worried for a hard trained dog than a pet one. Maybe the level of performance wouldn't be the optimal but possibly less health issues?