The Body maintains Blood Glucose Homeostasis (balance) with the help of Hormones like Insulin, Glucagon & few others (as shown in the pic). When someone eats a diet dominant in Carbohydrates, more glucose is produced & body is forced to overproduce Insulin to maintain Homeostasis. More Insulin production = More Fat storage. This increased Insulin production eventually leads to Insulin Resistance, due to which the action of the Hormone Insulin gets inhibited to an extent. It is basically like Insulin hijacked the whole Glucose regulation process & made Glucagon a minor player.
In a Healthy person, maintaining BG Homeostasis is a piece of cake for the body. When the body has to put serious effort in order to achieve it, that is basically Pre diabetic. When the body is pushed to max & it can’t maintain Homeostasis without external aid (med) that is Diabetic.