There is limited research on the impact of the vegan diet on the feline companions in comparison to canines. As it is not as well known in the general public that cats can be healthy on a plant-based diet provided with taurine, B12, vitamin A.
Vegan diet:
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https://www.magonlinelibrary.com/doi/abs/10.12968/vetn.2022.13.6.252
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https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0253292
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https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0284132
Vegetarian diet:
The main thing I’ve read (not a cat owner) is that unlike a dog, you’re walking a fine line when providing a vegan diet to a cat. It can be done, and this is reflected in robust scientific literature, but you need to seriously know what you’re doing. As a critical part of that, I think consulting a vet about getting them on a specific, well-planned diet is something any vegan cat owner ought to be doing. Mainly because while you can probably get away with feeding your cat a suboptimal meat-based diet and have them still be healthy, the risks with a suboptimal plant-based one mean that I don’t think DIY research cuts it.
Nonetheless, I think the best thing for a vegan if possible (not your case) is simply not to own a cat, not because it’s impossible to accomplish this but moreso because statistically it’s much, much harder to accomplish, and normalizing it among vegans is liable to create situations where well-meaning but incompetent owners simply cannot meet the challenge and either give up partway through and go with meat-based food or slowly destroy the cat’s health.