Time-Restricted Eating and Cancer: Clinical Outcomes, Mechanisms, and Moderators

Inclusion Criteria:

* Any sex/gender of any ethnic/racial background
* Age greater than or equal to 18 years
* Histologically-confirmed rectal cancer stage II, III, or IV (if curative) or human epidermal growth factor receptor 2-positive (HER2+) or triple negative breast cancer stage I, II, or III (only if definitive intent) per American Joint Committee on Cancer (AJCC) criteria
* BMI 18.5 kg/m2 or greater
* Receiving either neoadjuvant therapy with curative intent (breast cancer patients) or total neoadjuvant therapy with a 5-fluorouracil-based regimen and curative intent (rectal cancer patients)
* Has completed ≤ 4 weeks of neoadjuvant treatment prior to study enrollment
* Willing and able to adhere to the assessments, visit schedules, prohibitions, and restrictions

Exclusion Criteria:

* History of cytotoxic chemotherapy less than or equal to 12 months prior to rectal or breast cancer diagnosis
* Allergic reaction to any of the treatment agents
* Any prior pelvic radiotherapy
* Currently active second malignancy other than non-melanoma skin cancers or cervical carcinoma in situ
* History of GI perforation ≤12 months prior to enrollment
* History of predisposing colonic or small bowel disorders with severe or rapidly worsening symptoms (not related to current cancer symptoms)
* Receiving any parenteral nutrition or enteral (tube) feeding or using similar nutritional supplement during the study period
* History of uncontrolled congestive heart failure defined as ew York Heart Association Class (NYHA) Class III or greater
* Pre-existing grade ≥3 neuropathy
* Currently participating in or has participated in a study of an investigational agent or investigational device ≤4 weeks of the first dose of treatment
* Pregnant or breastfeeding
* Currently perform overnight shift work more than one day/week on average
* Strictly adhering to a \<10-hour eating window on most days
* Known psychiatric or substance abuse disorders that would interfere with adhering to the requirements of the trial
* Medical condition or laboratory abnormality that could impact participant safety or data validity, in the opinion of the medical investigators.