All patients on the CD-TREAT diet will be supplied with an individually tailored, dietitian-devised diet, consisting of ordinary foods, but custom-designed to recreate the nutritional content and ingredients of EEN. The dietary intervention will be overseen by qualified dieticians including a National Health Service (NHS) research dietician. The energy content will be tailored to adult participants' total energy expenditure and physical activity level calculated by the World Health Organisation (WHO) set of equations or will be calculated for each child to meet their estimated average requirement (EAR) for age. Food and instructions for those patients on CD-TREAT will be supplied directly to them by the researchers on the day following recruitment. Foods which need cooking will be prepared by a sub-contracted (food handling and hygiene certified) catering company and will be delivered to the participants' house every 2-3 days. The rest of the foods will be delivered to the participants' house every week. Participants will initially receive the modified diet for a period of 8 weeks, similar to the duration of an EEN course. The researchers will instruct the participants and their carers on how to follow the diet and how to record their dietary intake during the intervention. Research follow-up visits will coincide with the participants' clinical follow-ups (Week 2: phone review; Weeks 4 & 8: hospital visit). These follow up appointments will include collection of research bloods as an extra aliquot to their routine clinical tests (no additional needle insertion is required), clinical review, quality of life assessment and anthropometry measurements. At these appointments, the researchers will give the participants a stool and urine sample collection kit to take home and will arrange a suitable date/time to collect the faecal and urine sample.