Updated: Mar 13, 2019
I must admit that it is not a question that ever crossed my mind till I set up my Canine nutritional consulting practice. And it has quickly become a frequently asked question since.
Since dogs, Canis Lupus Familiaris, are preferential scavengers that descended from their close cousins Canis Lupus, the carnivores!
Which means that although they can pretty much eat anything they can scavenge, they are meat eaters and "thrive" on animal protein. Their biology an obvious proof of it. What this means is, they have the teeth, the stomach acids and the digestive tract that shows us that they are meant to tear and rip flesh and chew into bones and digest it. What it also shows us is that they can withstand pretty rotten food. Scavengers usually need this ability as they don't always come by fresh meals. That was the easy part of the answer.
The not so simple part of the history is that dogs have been fed table scraps which included carbohydrates, vegetables, fat and fruits for thousands of years and they have not died out. Having said that, it is important to know why.
Dogs are adaptive and have adapted to the food that was available to them after starting their relationship with humans. And although dogs can use some amount of carbohydrates and fibre from the plant or grain material, they CANNOT extract plant based protein. They simply do not have the enzymes required to do that in the required quantities.
Now for the tricky part of the answer; Dogs when put on a vegetarian diet (also read high carbohydrate & high grain) will not die, or show any immediate signs of ill effect, But slowly and surely it will have a large impact on their health and well being! Unlike humans, dogs do not need carbohydrates as their main source of energy, Dogs can in fact survive on little or no carbohydrates at all. If they are consuming grass eating animal, they get all the nutrients they require indirectly from them.
As humans, we have species specific diet that we require and if we were told to eat food that is appropriate for a farm animal, we may not die, we may adapt over time, but it will NEVER be the best food for us! Similarly, there is a species appropriate diet for dogs and that is to be primarily on an animal protein and animal fat diet with some carbohydrates. As humans, when we choose to share our lives and homes with another species, it is our responsibility to learn about that species and provide the care and nourishment to them based on whats best for them. This applies to birds, rabbits, hamsters, horses, so why do we seem to take the dog and his trust in us for granted? And as we see an ever increasing trend amongst families that share their lives with dogs that are treated at par with human children, it is important to remember that this sentiment can go only as far as love, and dogs are not to be mistaken for humans. Keeping their species specific needs in mind is not only necessary, we are bound by the responsibility of it. Hope this attempt to answer the question has been useful to you. Please share and spread awareness.