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While it’s true that wolves and other wild canines primarily eat meat, the domestication process has resulted in dogs having an increased ability to digest starches compared to wolves: https://www.sciencemag.org/news/2013/01/diet-shaped-dog-domestication
You can look at this as: those canines, who had a greater ability to extract nutrients from scraps fed by humans, were more likely to successfully reproduce. Not only do dogs have greater enzymatic ability to extract nutrients from starches, but their digestive systems are longer to allow them to extract those nutrients. This is also breed dependent with more primitive breeds, having less ability to do so.
A dog’s ability to survive on a vegan or even vegetarian diet, doesn’t mean that those are optimal. The only way to test that is with long-term controlled feeding studies, which as far as I’m aware don’t exist to date because they are very expensive to conduct and have ethical concerns. See for review: https://www.mdpi.com/2076-2615/6/9/57/htm
My own personal recommendation would be to at least include eggs- not vegan and to regularly do bloodwork if you switch to a commercial or home-cooked (nutritionally balanced) vegetarian diet. There are certainly dogs who can and do survive on such diets, but it’s not clear if it’s as optimal as a more omnivorous diet. I would not trust a study that only looks at 6 months – 2 years of a diet in a dozen dogs.
There are also a lot of people who say that every human being should be vegan. I know from personal experience that this isn’t the case. Despite taking supplements, it took 5 years for me to become sufficiently nutritionally deficient to see symptoms because I don’t have the genetics to process those nutrients from supplements.
