1. -- That cats can derive all of their nutrition from mice.
There are a number of essential vitamins and minerals which simply aren't present in sufficient quantities in meat.
- It's proposed that cats in the wild eat their prey whole and derive these nutrients from the intestinal contents and organs, but there doesn't seem to be any real substantiation for this idea. To come anywhere near meeting the requirements for vitamins and minerals a meal would need to contain 25% organ meat.
- There's a growing recognition in the scientific community of the importance of the symbiotic bacteria that live in the gut of all mammals. The "GI biome" is now recognized as both a major source of certain essential B-vitamins like biotin and an important part of the immune system.
These intestinal flora need to be fed fiber to thrive, primarily available in plant sources. The only animal source of fiber might be chitin, found in insects and other "bug" type food. None of the advocates of the "prey model" include crickets or earthworms in their formulas and many ignore eggs, all of which undoubtedly were (and are) a very important part of the diet of cats in the wild. The apparent urge to browse on green matter, to eat dirt and to allow raw meat to partially decompose before eating it have also been ignored and left unstudied.
Since there are no detailed studies of wild cats' feeding habits there's no way to tell what else may be missing in an artificial "prey-model" diet.
2. -- That cats can't digest plant matter, particularly carbohydrates.
Cats, like all mammals, retain genes from common herbivorus ancestors enabling them to digest a variety of food sources, including plants. While these genes have been de-emphasized in carnivores they haven't been lost... all of the species who became truly "hypercarnivorus" are now extinct, in large part because they over-adapted to meat.
In fact, current feline species are highly adaptive.
- Trials cited by the National Research Council have demonstrated that diets containing 40% of highly-digestible carbohydrates may be effectively utilized by cats to fulfill energy requirements.
3. -- That cats should only be fed raw food.
Cats don't have opposing thumbs, aren't tool-makers and have a healthy fear of fire, so it's not surprising that they never developed cooking skills. Who's to say, though, that had they been able to they wouldn't have realized the health benefits and increase in longevity enjoyed by humans since this technological leap?
- Data from the USDA Nutrient Database indicates that vitamin losses are relatively low when food is cooked at moderate temperatures and all liquids and fats are retained.
Conjecture about the benefits of feeding raw may be considered purely ideological. Anecdotal testimonies about improvements following a switch to a raw diet don't prove that the raw state was responsible for the change, only that for whatever reason the new diet seems to work better.