Morbidly obese patients are generally in a malnutritional state with an extremely high operative risk. In order to reduce the operative risk, surgeons should make every effort before the operation. A protein sparing modified fast (PSMF) may exhibit its high effectiveness when taken at least for 2 weeks preoperatively. In this study, PSMF prescribed with natural food products was compared with prepared PSMF having an ideal composition from the clinical standpoint. Though it is natural for patients to favor the former, the latter proved to have a superior effect in clinical practice. By performing a PSMF for 2 weeks preoperatively, however, even the PSMF with natural food products seemed to fulfill its purpose sufficiently.