 Diets are the mainstay of many obesity treatment programs, yet they fail for a variety of reasons. Patients find it difficult to comply with caloric restriction during active weight loss, and to adhere to long-term changes that promote weight loss maintenance. Underlying these difficulties are a number of physiologic changes enacted by the body in response to energy restriction. These changes promote increased hunger, decreased satiety and slowed metabolic rate, making diet compliance and sustained weight loss very difficult. From the bodys perspective, a rapid and substantial drop in caloric intake may represent a metabolic emergency, and it responds accordingly, in an attempt to maintain body weight. 