Children and adults who are eligible to participate will be identified by medical staff. A member of the clinical team will briefly inform potential participants about the study via the phone, via e-mail, or at the clinic. After the end of the clinical appointment and if the patient and/or the parent/carer are agreeable, a member of the clinical team will introduce the eligible participants to the researcher who will explain the study in full and answer any questions they might have. During this meeting, the researcher will discuss the specific aspects of dietary treatment with CD-TREAT and will inform participants that urine, blood and faecal samples will be collected to try and determine the type of bacteria which may be associated with gut inflammation, and their metabolites, and whether these can predict response to treatment. The researcher will also ask for permission to collect medical information from the participant's medical records. If they are interested in taking part, they will be given detailed written information (including an age-appropriate information leaflet). The information pack will also invite the family to contact the researcher at any time if they require further information.