There were no patients recruited during the trial period. This was a result of: 1) COVID-19 pandemic; 2) MRIs were done prior to patients being seen in consultation, which made them ineligible for enrolment.

This pilot study will aim to determine the feasibility, safety, and cost associated with a preoperative VLED for obese rectal cancer patients. Ultimately, the investigators seek to provide evidence that may inform the development of a standardized preoperative weight loss protocol in obese rectal cancer patients.

This study is a single center, single-arm prospective cohort pilot study with a primary aim of determining the feasibility, safety, and cost associated with a preoperative VLED for obese rectal cancer patients. Efficacy of preoperative VLED in this study population will be assessed through measures of mesorectal fat volume with MRI at two separate time points, prior to commencing a pre-operative VLED diet and immediately prior to patients undergoing laparoscopic low anterior resection for rectal cancer, to objectively quantify the proportion of mesorectal fat loss in response to VLED. Secondary aims include an assessment of whether preoperative weight loss contributes to improved intraoperative outcomes including operative time, blood loss, and rate of conversion to open procedure. Furthermore, postoperative outcomes including rates of 30-day complication rate, hospital stay, among others will be evaluated.

Optifast 900 will be administered to enrolled patients over a 3-week period immediately prior to their scheduled elective operation date. Compliance will be measured with a food diary. Two MRIs of the rectum will be obtained: one prior to the commencement of VLED and one following the completion of the VLED before the scheduled operation date. The first MRI will be a clinically indicated preoperative staging investigation, while the second MRI will be used purely for research purposes. Patients will be enrolled in an enhanced recovery after surgery (ERAS) for colorectal surgery program. All patients will be followed by their surgeon as per that individual surgeon's postoperative surveillance schedule. At the routine one-month follow-up visit, patients will see the surgeon as well as a study investigator who will complete a standardized follow up questionnaire, focusing on adverse events and functional independence. Research personnel and study investigators will follow patients throughout their postoperative course in hospital and note any secondary outcomes including laboratory tests or adverse events through the electronic patient chart.

Ultimately, the investigators seek to provide evidence that may inform the development of a standardized preoperative weight loss protocol in obese rectal cancer patients.

Inclusion Criteria:

• 18 years of age or older
• BMI greater than or equal to 30kg/m2 and less than or equal to 55kg/m2
• Patients undergoing elective surgery
• Documented pathological diagnosis of rectal adenocarcinoma
• Preoperative staging investigations consistent with stage I, II, and III disease
• Laparoscopic surgery occurring at St. Joseph's Healthcare Hamilton

Exclusion Criteria:

• BMI less than 30kg/m2
• Contraindications to magnetic resonance imaging, general anesthesia, or major colorectal surgery
• Patient undergoing open surgery
• Patients undergoing emergency or palliative surgery
• Preoperative staging investigations consistent with metastatic disease
• Allergy or contraindication to the use of Optifast
• Prior pelvic colorectal surgery or bariatric surgery
• Enrolled in any other clinical trials or prospective studies where similar outcomes are measured
• Severe, chronic cardiovascular disease (i.e. recent myocardial infarction, unstable angina), renal disease, or hepatic dysfunction
• Patients with pharmacologically managed diabetes mellitus
• Pregnant and/or breastfeeding patients
• Inability to provide informed consent