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.

All examinations and laboratory tests will be conducted on two occasions. Moreover, two cohorts will be present. That is, one group where participants experiencing a severe flare-up receive medical treatment, and another group where participants with the bowel disease undergo surgery. All samples/examinations and surveys are therefore done before the operation/during the flare-up, and then again one year later during a follow-up assessment

Inclusion Criteria:

• All adult patients experiencing an acute flare-up of their bowel disease (ulcerative colitis or Crohn's disease) admitted to a hospital in the Region Västra Götaland and Stockholm are invited to participate in the study.

Exclusion Criteria:

• Not wishing to participate in the study Inability to understand the language or study procedures

Adults with severe IBD.