Patients with colorectal cancer will be recruited to undergo vitamin D supplementation in the peri-operative period. Vitamin D levels (25OHD) will be measured to assess response to supplementation.

Patients with a new diagnosis of colorectal cancer will be recruited from colorectal outpatient clinics after confirmation of eligibility. Potential participants will be identified by their healthcare team comprising of clinical nurse specialist or surgeon and co-enrolment with portfolio studies including ALLEGRO and Prepare-ABC will be permitted. Patients undergoing neo-adjuvant therapy, already established on supplementary Vitamin D or with contra-indications to vitamin D supplementation will be excluded. After informed consent participants will undergo blood sampling at baseline (for 25OHD level and DNA) and given 3200IU cholecalciferol (Fultium) per day to take until the time of surgery (median time from pre-operative clinic 4 weeks). Further blood sampling will be performed on the day of surgery, in the early post-operative period (1-2 days, 3-5 days, 6-9 days) and at the first post-operative clinic appointment. Where possible surplus serum from clinically indicated blood tests will be collected to assay 25OHD and reduce need to additional venesection.

Inclusion criteria -

• All participants will be aged 16 years or over.
• Participants must be resident in the United Kingdom.
• Undergoing bowel cancer surgery

Exclusion criteria

• Patients who may be at increased risk from Vitamin D supplementation including:
• Kidney disease
• High levels of calcium in the blood
• Atherosclerosis
• Sarcoidosis
• Histoplasmosis
• Over-active parathyroid gland (hyperparathyroidism)
• Lymphoma
• Currently taking thiazide diuretics, digoxin or other cardiac glycosides
• Known allergy to nuts (as peanut oil contained within vitamin D preparations)
• Female subjects of child bearing age who are not taking effective contraception during the period of the trial