A French study on 66,000 middle aged women over 14 years that was published in the American Journal of Clinical Nutrition in February of 2013, showed that drinking 12 oz per week of a diet soda increased the risk for type 2 diabetes by 33%. Increasing the amount to 20 oz per week increased the risk to 66%.
It is interesting that the American Diabetes Association and the American Dietetic Association both recommend diet sodas for people who are obese and who have type 2 diabetes.
The likely mechanism is that when a person consumes a diet beverage that the brain anticipates that sugar is coming into the blood stream. When this happens insulin is secreted to handle the load of suger that is expected, but it never comes and consequently blood sugar levels drop and make us hungry.