I am not sure why the dose of insulin would have to be lowered. The kidney is responsible for much of the elimination of insulin from the body. The estimates are that the kidney is responsible for eliminating about 50% of the insulin dose from the body daily. If the creatinine was to fall and the muscle mass of the individual did not change significantly, this would mean that kidney function would have improved and more insulin would be necessary to accomplish the same effect. Hence, I have no explanation for your suggestion that insulin dose should be decreased.