The goal of this exploratory, prospective, monocentric randomized crossover study is to investigate the influence of a blueberry-rich diet compared to a blueberry-poor diet on the microbiome, the intestinal barrier and the inflammatory process in IBD patients with chronic colitis. The main question the study aims to answer is: * Does a diet rich in blueberries compared to a diet low in blueberries have an influence on disease activity, intestinal inflammation and symptoms in IBD patients with chronic colitis? Ulcerative Colitis (UC) and Crohn's disease (MC) patients are recruited during an inpatient stay at the Clinic for Internal and Integrative Medicine in Bamberg, Germany. During their routine inpatient stay, patients are treated by an interdisciplinary team using a comprehensive multimodal integrative inpatient therapy concept that combines conventional medicine with a wide range of integrative non-pharmacological treatments, phytotherapy, lifestyle-modification and nutritional therapy. During this time, it is verified whether the patients can participate in the study. The study consists of two study arms, which are conducted in a cross-over design: * Intervention arm: in this study arm, participants receive 12 weeks of blueberry therapy in addition to the regular two weeks of inpatient care at the Clinic for Internal and Integrative Medicine in Bamberg. The blueberry therapy includes drinking around 35 g of blueberry powder stirred into water once a day for 12 weeks. * Control arm: in this study arm, participants receive the standard therapy (integrative medical therapy concept described above) during the regular two-week hospital stay. There is no additional blueberry intake. Due to the crossover study design, all participants undergo both study arms. This means that each participant takes the supplementary blueberry powder (intervention) for a period of 3 months and the control therapy for 3 months. The order in which the participants receive the forms of therapy is determined randomly. Thus, half of the study participants start the supplementary blueberry intake directly after the inpatient stay, the second half three months later (cross-over principle). UC and MC patients will: * initially receive a clinically indicated colonoscopy with confocal laser endomicroscopy (CLE) during their routine inpatient stay at the Clinic for Internal and Integrative Medicine in Bamberg * return to the clinic for an optional sigmoidoscopy with CLE after 12 and 24 weeks * answer questionnaires at study start as well as after 12 and 24 weeks to analyze their quality of life, the severity of their symptoms and their disease activity * give samples of blood, urine, stool and bile fluid as well as intestinal biopsies during the colonoscopy/sigmoidoscopy at study start as well as after 12 and 24 weeks * document their intake of blueberry powder as well as their symptoms and stool frequencies in a diary throughout the study participation