According to the Wikipedia Entry on Diabetes in cats and dogs-
Diabetes mellitus strikes 1 in 400 cats and a similar number of dogs. Symptoms in dogs and cats are similar to those in humans. Generally, most dogs and about half of cats experience type-1 (insulin-dependent) diabetes, rather than the type-2 that's now becoming common in obese humans. The condition is definitely treatable, and need not shorten the animal's life span or life quality. In cats, prompt effective treatment can even lead to diabetic remission, in which the cat no longer needs injected insulin. Untreated, the condition leads to blindness in dogs, increasingly weak legs in cats, and eventually malnutrition, ketoacidosis and/or dehydration, and death.
According to research at Cornell University-
Diabetes mellitus-also known as "sugar" diabetes-is a complex but common disease in which a cat's body either doesn't produce or doesn't properly use insulin. During digestion, the fats, carbohydrates, and proteins that are consumed in the diet are broken down into smaller components that can be utilized by cells in the body. One component is glucose, a fuel that provides the energy needed to sustain life.
Insulin is a hormone produced in the pancreas, is responsible for regulating the flow of glucose from the bloodstream into the cells of the body. When insulin is deficient or ineffective, the cat's body starts breaking down fat and protein stores to use as alternative energy sources. As a result, the cat eats more yet loses weight. Additionally, the cat develops high levels of sugar in the bloodstream, which is eliminated in the urine. In turn, sugar in the urine leads to excessive urination and thirst. Cat owners often notice these four classical signs of diabetes mellitus: ravenous appetite, weight loss, increased urination, and increased water consumption.