Some diabetes type II does not respond to any kind or brand of insulin - that´s called syndrome X and it´s not exclusive to people. Some cats have it too. Today there are other medications that are successful to some individuals but not to all since Type II has many different causes. There is, however, a diet that has proven successful to all Type II individuals (and Type I alike). This diet is completely carbohydrate (sugar) free. It is very easy to follow such a diet with your cat. Cats are strict carnivores which means they do not eat carbohydrates in their natural diet. To achieve this diet with your cat, all you need to do is switch him to canned-only and make sure the brand you chose has *no* carbohydrates. As a hint, there should be no ingredient of plant origin in the ingredient list. Unfortunately, the only canned product I know that contains no carbohydrates is Wysong´s Au Jus. This food is not complete and needs to be supplemented with Wysong´s Call of the Wild. Two or three years ago there were many canned products without carbs but it seems that all companies are adding carbs to their products to lower their costs. Anyway, most canned cat foods contain very little carbohydrates, some contain nothing at all, as opposed to dry foods which contain 40 to 55% of its weight in carbohydrates. In other words, dry feeds are an absolute no-no! You must however be very careful when you switch your diabetic cat to canned only. This change should be done slowly with veterinary supervision to keep track of the decrease in blood glucose levels. The extra water cats get from canned foods are also very beneficial to their kidneys since diabetic cats are very susceptible to chronic renal failure.
A diet free of carbohydrates is also the best possible for Type I cats. But the change in diet has to be done even more carefully and gradually to avoid a hypoglycemic shock. Keeping track of blood glucose levels during the transition to the new diet is mandatory and should be done with veterinary aid. Once the cat is on canned-only, keep close track of his blood glucose levels until it stabilizes.
Please read the section on Obesity.