I suppose the idea of keeping pets is a bit like not going to zoo's but cats were domesticated for a reason to keep down the mice and rats in cereal silo's in Egypt,so even though they were domesticated they were useful as well and the Egyptians treated cats like gods (hmmmmmm don't we all)even mummified them.
I do believe in allowing cats to act naturally (but have sometimes allowed one that had been stray and over 5 years to become an indoor cat,for which I might say they were eternally grateful)and sometimes acting naturally they will eat veggies,the odd bit in pouch food or straight out of the garden but its not natural for them to do this give a cat a plate of rough old meat or a plate of beautifully cooked veg I know which I would put my money on
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We have to much of people trying to change other peoples/animals diets they fed cows to cows and look where that got us BSE and CJD some of the original dry food for cats I am sure has bought about kidney problems and maybe even diabetes but thankfully that has been corrected,I also think that if manufacturers thought there was a profit in veggie or vegan cat food they would have it in TESCO's maybe thats to come but not in this decade.
You can only give your cats the best balanced diet you can and unless you know all of the kittens history you are taking pot luck and if you take a cat that is already set in its ways it already has baggage and there is not a thing you can do to change that other than to love the cat and adapt to its needs......................................
all because we love them.