When you are keeping low carbohydrate diet.

 As almost not eat rice, potato, macaronis, bread, sweets, baked food, sweet milk, fruits, juices you organism habit to this , and very benefit for metabolism. But,  on one day you are eaten pie for lunch, thus she is in gastric highway as splitting till glucose and sucking in blood.That  , is needing quantity for glucose signalizing for pancreas gland ,thus sugar level in blood increased and need to remove  from blood vessels and deliver in cells, thus:

As on first , high sugar level in blood not ability keeping longer, that he is damaging blood vessels.

Second on , glucose is source for energy for cells, and in theory a base organism s task is constantly download sugar in cells ,that they will have enough energy.

 And its  a sign from raised sugar in blood stimulates pancreas gland to secreting small quantities of insulin, which entering in blood, attaching to insulin receptors on cellular membrane and help deliver sugar from blood  in   cells. And when abundant glucose distributing on cells, manufacturing insulin lowering till a initial level, and glucose level also fallen .   If sugar level rising is not significantly  ,thus in result of digested meal eaten, quantity of insulin, secreting by pancreas gland also not big, thus cells not needing in stimulating effect of big quantity manufacturing insulin, that for absorbs glucose, and with enjoy using a small quantity offered glucose.