Background & Rationale:

Breast cancer (BC) is the most commonly diagnosed malignancy in women worldwide (2.1 million diagnoses in 2018, 25% of new cancer cases). In Canada, early stage BC mortality rates have decreased by 48% over the past 30 years as a result of advances in prevention, detection, and treatment. However, competing risks for mortality from non-cancer causes have emerged, where cardiovascular disease (CVD) is now a leading cause of death for BC survivors. The direct toxic effects of BC treatment on the heart (cardiotoxicity) are well characterized by the investigators and many others, as a contributor to elevated cardiovascular risk. However, BC treatment and the associated lifestyle changes (i.e. physical inactivity, poor diet quality, stress) are increasingly recognized to also strongly affect metabolism negatively manifesting as insulin resistance, dyslipidemia and adipose tissue (fat) accumulation. These adverse metabolic changes are strongly linked to CVD risk and represent a currently underappreciated contributor to the elevated CVD risk among BC survivors. Preliminary data and recent publications demonstrate that regional fat accumulation occurs during BC treatment and that the fat burden in key locations is associated with poor cardiorespiratory health. A trigger of these adverse metabolic and inflammatory effects is excess fat specifically within ectopic fat (viscera, intermuscular, or hepatic) regions. In 2019, a member of the study team found that the volume of visceral and intermuscular but not subcutaneous fat at BC diagnosis were linearly associated with CVD events within 6 years, even among those with normal BMI and after adjustment for pre-existing CVD risk factors and for BC treatment type. Using MRI, investigators found that ~1 year after chemotherapy, BC survivors had significantly larger depots of visceral fat (49% larger) and thigh intermuscular fat (41% larger) compared to age and sex-matched controls, despite similar BMI and subcutaneous fat volumes in the two groups. Investigators also showed that the fat fraction within the thigh muscle and visceral fat volumes independently explained ~50% of the variation in cardiorespiratory fitness (measured by peak VO2). In particular, peak VO2 is one of the most powerful predictors of all-cause and CVD mortality and health care costs, and is the most consistently reported negative sequelae after treatment for BC. Unfortunately, there are no known therapies to recover long-term myocardial damage (i.e. cell death, fibrosis) from cancer therapies. There are several reasons to target fat as a therapeutic target in BC patients: 1) The study team have compelling preliminary data showing accelerated formation of ectopic fat during BC treatment. 2) Investigator's recent data showed that high fat content in key fat pools was associated with reduced peak VO2. 3) The burden of fat and the associated metabolic abnormalities are dynamic and malleable, and thus highly treatable.

Research Question & Objectives:

The primary purpose of this study is to evaluate the effect of a behavioural intervention involving supported time-restricted eating (TRE), diet quality improvements, and reduced sedentary time versus usual cancer and nutrition care in BC patients receiving chemotherapy treatment on ectopic fat, cardiometabolic profile, and chemotherapy outcomes. The investigators hypothesize that the intervention will attenuate the growth of ectopic fat during chemotherapy and reduce chemotherapy symptoms.

Inclusion Criteria:

• Female biological sex at birth
• >18 years
• Diagnosis of stage I, II, or III breast cancer
• starting neoadjuvant or adjuvant intravenous chemotherapy
• ECOG <3;
• Oncologist approval to participate;
• English speaking (all study materials and study staff will be in English)
• Willing and able to adhere to study intervention

Exclusion Criteria:

• Individuals who do not have access to a smart phone with Bluetooth capability (required for Fitbit and for responding to intervention text messages) or at least a shared cell phone with someone in the same household (i.e., some couples may share a phone).
• Type 1 or type 2 diabetes who require exogenous insulin (due to the potential need to adjust insulin dosing with TRE) or with hemoglobin A1c >10%
• Research MRI contraindications (e.g., pacemaker, magnetic implants, pregnancy)
• Uncontrolled thyroid disorder
• Self-reported eating disorder history
• Body mass index <18.5 kg/m2 or clinical signs of cachexia (discretion of treating oncologist)
• ≥5% body weight loss within last 6 months
• Those who are currently working night/rotating shifts, eating within ≤10-hour window or consistently eating less than 3 meals/day in the past 3 months.
• patients who meet the criteria for medical clearance prior to exercise using the Physical Activity Readiness Questionnaire+ and are not cleared by their treating oncologist or family physician to perform maximal exercise testing.

Female biological sex at birth.

Information collected during the study will be used in analyses and will be published/ presented to the scientific community at meetings and in journals. In addition, study data without any identifying information will be shared with the research team at the University of Toronto, where the study is also taking place.