I've had cats all my life, and never knew this information until recently. I'm posting it in hopes that it will save some of you both money and grief.
My cat has recently been diagnosed with diabetes. This means that I will have to inject him with insulin every 12 hours for the foreseeable future. It's possible that eventually we'll be able to wean down to one injection per day but that's a ways off. I love this cat and am willing to spend the time and the money (you do NOT want to know how expensive this cat has become!) However, I wish I'd known how to prevent this from the beginning.
I likely could have prevented my cat getting this disease by feeding him properly. Having now done rudimentary research on the internet and consulting with my vet, I've learned that cats should NOT eat dry cat food of any kind. Dry cat food is full of grains and carbohydrates. Cats are by nature carivores and need a high protein diet, which is best found in canned cat food. The cheapest canned food is a better diet for a cat than the priciest dry food
(yes, that includes "prescription" dry food from the vet and high priced protein rich dry food). My cats are now eating canned Friskies. No more dry food.
Another reason to avoid dry food is that it's full of carbohydrates. As any low-carb eater knows, carbohydrates break down into sugar. Again, cats are carnivores. They don't need sugar in their diets. No wonder my poor kitty ended up with diabetes -- I've been shooting sugar into his system for the past seven years!
Finally, cats need water in their diet. Cats in the wild get lots of their water from the prey that they kill. Dry food obviously has none; canned is a much better option.
Here's the best written article I've found on the topic: http://catinfo.org/