Diabetics are using insulin most of the time because there insulin is not doing its job blood sugar is elevating without having anywhere to go. So the synthetic insulin helps the transport of blood sugar to glucose which in that particular diabetics case is enough insulin to help turn that meal into glucose and sustain some release for a few hours. Where as a bodybuilder with out insulin issues using synthetic insulin has no problem turning glucose into glycogen so much so that blood sugar will begin to drop and thats where you have to compensate by eating excessively