Organ homeostasis in humans is maintained by the "auto-regulatory system." Although this biological regulatory system is regarded as complicated and difficult to analyze, with the aid of control theory it has become possible for us to construct a mathematical model. To date we have created models for water-electrolyte balance and glucose metabolism. The validity of these models has been evaluated by simulation study. As a clinical application of the model, we have successfully developed an artificial pancreas system for the treatment of diabetic patients. Through this system, we have made many contributions to the study of clinical nutrition and we feel that this has initiated a new era in this field. In order to make further progress in this field, it will be necessary to take a new dynamic and systemic approach.