Methods for treating impaired glucose tolerance and early and late stage diabetes in mammals, for prophylactically sparing β-cell function, aiding in preventing β-cell death, preventing the onset of overt diabetes in a mammal with type 2 diabetes, treating the current level of glycemic control dysfunction of a mammal with impaired glucose tolerance or diabetes, comprising orally administering insulin and a delivery agent that facilitates insulin absorption from the gastrointestinal tract at the time of or shortly before mealtime, e.g., within about 10 minutes prior to ingestion of a meal, on a chronic basis. The methods also comprise, in addition to administering a rapid-acting insulin to provide a first insulin peak, administering a slow acting insulin to provide a second insulin peak occurring at a later time but of a longer duration. These methods achieve improved glycemic control without the risks of hypoglycemia, hyperinsulinemia and weight gain and the need for frequent blood glucose monitoring that are normally associated with insulin therapy.