Increased Intestinal Permeability is detectable in clinically active Crohn's disease, in stable Crohn's disease and is predictive of clinical relapse. Maintenance of the epithelial barrier integrity is therefore essential for intestinal homeostasis. Vitamin D may have a critical role in improving barrier function.

Inclusion Criteria:

• All patients must fulfill the clinical criteria for diagnosis of CD as defined by Lennard-Jones et al. (1976)
• All patients must be in clinical remission for at least 1 month at study entry as defined by a Crohn's Disease Activity Index (CDAI) of <150
• 18-65 years
• Patients on stable drug therapies for at least 1 month pre-enrolment
• Sufficient English language ability to carry out the study requirements

Exclusion Criteria:

• Symptomatic CD at study entry including CDAI >150
• Pregnancy
• Previous extensive small bowel resection (less than 200 cm of viable small bowel or a loss of 50% or more of the small intestine)
• Presence of an ileostomy or colostomy
• Known hypersensitivity to vitamin D
• Hypercalcaemia (corrected serum calcium > 2.66 mmol/L)
• Those currently using supplemental vitamin D >800 IU/D
• Diagnosis of any of the following: active tuberculosis, sarcoidosis, hyperparathyroidism, renal failure, pseudohyperparathyroidism, malignancy, active, lymphoma, short bowel syndrome
• Antibiotic use in the 4 weeks prior to enrolment
• Current use of bisphosphonates
• Renal Impairment, Diabetes Mellitus
• Patients participating in a concurrent RCT
• Alcohol dependency