That is because there must be perfect equalibrium between the body. It's like the fosbury flop, which was a why of high jumping that would allow high jumpers to "cheat" physics and leave equalibrium for short periods of time. If something goes up, something else must go down, this is a general rule becuase if it isn't true you would start floating out of the water
The American Diabetes Association recommends aiming for a blood sugar level between 70 to 130 mg/dl before meals and less than 180 mg/dl one to two hours after a meal.