Sarcopenia is a condition characterized by significant muscle loss resulting in impaired muscle function. This condition is likely associated with a biological deviation leading to reduced reserves to withstand stressors, resulting in a poorer prognosis. The incidence of sarcopenia among patients with inflammatory bowel diseases (Ulcerative Colitis and Crohn's disease) is currently approximately 40-60%. This is likely a consequence of the preference for drug treatment over surgery, while many patients have continuous inflammation in their intestines leading to muscle loss and subsequently increased morbidity and mortality. The purpose of the study is to identify the prevalence of sarcopenia among patients experiencing a severe flare-up of their bowel disease and to evaluate whether the removal of the colon results in improved nutritional status. Furthermore, the investigators aim to investigate whether there is a specific microbiota composition related to an unfavorable course. The participants are longitudinally monitored with measurements taken at the flare-up/before surgery and one year afterward, including body composition, function, dietary recording, quality of life, blood tests, fecal samples, and intestinal biopsies that will be analyzed. The investigators plan to correlate body composition with a biological profile and then evaluate if there is an association. Additionally, the investigators aim to analyze if these markers are linked to different outcomes after colectomy. This study will contribute to an enhanced understanding of patients with advanced IBD and possibly change the perspective on how the clinicians should prioritize these patients for surgery. The investigators believe this study will lead to an improvement in healthcare quality and an enhanced understanding of how these disease processes function.