Preliminary clinical data suggest pain reduction through fasting in different diagnoses. This uncontrolled observational clinical study examined the effects of prolonged modified fasting on pain and functional parameters in hip and knee osteoarthritis. Patients admitted to the inpatient department of Internal Medicine and Nature-based Therapies of the Immanuel Hospital Berlin between February 2018 and March 2020, answered questionnaires at the beginning and end of inpatient treatment, as well as 3, 6 and 12 months after discharge. Additionally, selected blood and anthropometric parameters were routinely assessed during the inpatient stay. Fasting was performed as part of a multimodal integrative treatment program, with daily caloric intake of