The overall working hypothesis is that intake of dietary fiber during radiotherapy can mitigate or hinder end states or the triggering of long-lasting pathophysiological processes that decreases intestinal health in the cancer survivor. If correct, there is a dose-effect relationship to be documented. Moreover, if correct, different kinds of dietary fibers may have different effects. Mechanisms for the mitigatory effects may be that dietary fiber helps to preserve the two protective mucus layers and hinder gut-wall starvation. Lack of protection, as well as gut-wall starvation, may decrease the gut walls' integrity. That, in turn, may enhance bacterial migration from the lumen into the gut wall, causing unnecessary inflammation. This inflammation may, in turn, lead to a number of different pathophysiological processes, including a chronic self-propagating low-grade inflammation. Clinical experience suggests the intake of dietary fiber during radiotherapy may increase acute intestinal side effects. Our own data suggest a modest, if any, increase by dietary fiber.

Through recipes of tasty meals by price-winning chefs and general advice, the investigators guide the participant to try to consume at least 16 g of dietary fiber per day via food. The guidance takes place through telephone calls, calls via video link, and text on a website or paper material. The participant gets access to a mobile application that measures the daily intake of dietary fiber. Dietary fiber is ingested in 15 capsules with psyllium husk which contains a total of 5.5 g of dietary fiber. The investigators ask the participant to take the capsules two weeks before radiotherapy, during radiotherapy, and to stop four weeks after the end of radiotherapy. Placebo capsules (maltodextrin) are taken in the same way. To document the frequency of acute side effects, and what symptoms they cause, the participant is asked to report patient-reported outcomes once a week via a mobile application. They are also welcome to report side effects to the study office.

One month after the end of radiotherapy, the degree of inflammation is measured via markers in the blood and feces. One year after the end of radiotherapy, intestinal health is measured via patient-reported outcomes. Blood and feces are collected and patient-reported outcomes are reported in questionnaires and a mobile application before, during, and at least one year after the end of radiotherapy. This data will be a source of in-depth analysis. Radiotherapy gives rise to increased intensity of five different syndromes, fecal-leakage syndrome, urgency syndrome, uncontrolled flatulence, excess mucus discharge, and anal blood discharge. Damage of nerves and small vessels, weakening the anal-sphincter function by muscle fibers turning into the connective tissue (fibrosis), may explain some of the intensity of the fecal-leakage syndrome. An ongoing self-propagating low-grade inflammation, small-vessel and nerve damage in the gut wall may be related to urgency. Reasonably the microbiome, and the communication between the microbiome and the gut wall, is related to uncontrolled flatulence and excess mucus discharge. It is not known to what extent telangiectasias on a fibrotic inner gut wall explains anal blood discharge. Ad hoc studies in FIDURA will explore suggested mechanisms.

An interim analysis will be performed for the primary outcome to identify if there is any effect of the intervention, to decide whether to continue or terminate the inclusion of patients in the study. The concentration of c-reactive protein will be analyzed for all included patients who have donated blood to date (May 2023). To ensure that the blinding is not revealed, all analyses (biochemical and statistical) will be made by external personnel not included in the research group. The capsules are marked with X and Y, and the code for which is intervention or placebo will not be revealed even for the external personnel performing the interim analysis.