Fasting and Exercise for Women with Advanced Breast Cancer

Inclusion Criteria: 1. Women 2. Able to provide written informed consent 3. Able to speak, read, and understand English or Spanish 4. Postmenopausal (including concurrent use of ovarian suppression) 5. Diagnosis of ER+ and/or PR+, hormone estrogen receptor-2 (HER-2)- or HER-2+ metastatic or locally advanced unresectable breast cancer 6. Initiation of first-line or second-line endocrine therapy in combination with a CDK4/6 inhibitor (e.g., palbociclib, ribociclib, or abemaciclib), with or without hormone estrogen receptor-2 (HER2)-directed therapy (e.g., trastuzumab ± pertuzumab), or in combination with both a CDK4/6 inhibitor and a Phosphoinositide 3-kinase (PI3K) inhibitor, within the past 90 days. 1. Prior use of a CDK4/6 inhibitor in the adjuvant setting is permitted; participants must be enrolled within 90 days of starting a CDK4/6 inhibitor for the first or second time in the metastatic or locally advanced setting. 2. Participants who switch from one CDK4/6 inhibitor to another due to toxicity or insurance constraints remain eligible, provided enrollment occurs within 90 days of initiating the first or second-line CDK4/6 inhibitor in the metastatic or locally advanced setting. 7. Having not consistently (not equal to or more than 50% of the time) engaged in more than 90 minutes of moderate or 45 minutes of vigorous physical activity per week over the past 3 months 8. Having not consistently engaged in resistance training 2 or more days per week over the past 3 months 9. Having not consistently (not equal to or more than 50% of the time) engaged in structured fasting (i.e., not regularly fasting for ≥12hr/night) for the past 3 months 10. Approval from a medical oncology provider to participate. Exclusion Criteria: 1. Unable to provide consent 2. Unable to read or understand English or Spanish 3. Oxygen dependent 4. Unstable cardiac disease 5. Insulin-dependent diabetes 6. Unable to walk 2 blocks without assistance (excluding canes) 7. Regularly maintaining a shiftwork schedule, i.e. working evening shifts, night shifts, or rotating shifts in the last 3 months 8. History of a clinical eating disorder 9. Unstable bone metastases 10. More than 90 days since initiation of first- or second-line endocrine therapy in combination with 1) a CDK4/6 inhibitor, with or without HER2-directed therapy, or 2) a CDK 4/6 inhibitor in combination with a PI3K inhibitor, in the metastatic or locally advanced. 11. Presence of a severe medical condition or psychiatric condition or medications that would preclude participation of the study intervention.