Inclusion Criteria:  Male and postmenopausal females aged 18 - 75 years Body mass index (BMI) 20 - 35 kg/m2 With protective ileostomy due to benign or malignant rectal surgery  Exclusion Criteria:  History of postoperative adjuvant chemotherapy Other significant illness, including epilepsy, cardiovascular or respiratory disease Impaired renal or liver function (as assessed by calculated creatinine clearance < 90 mL/min or abnormal liver function tests (> 2 times upper limit of normal range)) Donation of blood within the previous 3 months Participation in any other research studies within the previous 3 months Females who are pre-menopausal Inability to give informed consent Vegetarians