Contrary to popular belief, adult-onset diabetes isn't initiated by eating too much sugar, or by depletion of insulin. Rather, its onset is due to genetics, expressed earlier or later due to metabolic stress of high glucose diet, which leads to weight gain, which leads to poor aerobic fitness. These factors bring about decreased sensitivity of insulin receptors, reduced glucose utilization, and elevated blood glucose levels. Insulin levels also increase, causing increased fat storage, and worsening of the entire glucose metabolism picture.
Its obviously a vicious cycle.