Participants will be asked to use intermittent fasting to reduce calorie intake. Participants will perform a modified fast three days per week for the duration of the 6-month intervention. On the 3 modified fast days per week, participants will restrict calories significantly (eat only 25% of energy needs, about 500 calories per day). On the 4 non-fast days per week, participants will be asked to eat a sensible, healthy diet but will not have to restrict calorie intake. Participants will attend regular weight loss group meetings for 6 months. These group meetings are held on a weekday evening in-person at the Anschutz Health and Wellness Center or virtually via Zoom and last 60-90 minutes. The meetings are held weekly for the first 3 months and then monthly during months 4-6. These group meetings are taught by a registered dietician and will provide support for intermittent fasting. Participants will be enrolled in the AHWC BfitBwell oncology exercise program and asked to gradually progress your physical activity level to 150 minutes per week of moderate intensity exercise. Participants will attend BfitBwell exercise sessions both in-person at the Anschutz Health and Wellness Center Fitness Center and virtually through True Coach. Training sessions will be 45-60 minutes in length and will focus on aerobic, resistance, and flexibility exercises. During the 6-month intervention, participants will also undergo periodic assessments of body weight, body composition, laboratory values, and diet and exercise behaviors (2-3 study assessment visits, occurring approximately every 3 months for a total of 6-9 visits). At 3 months, participants will also be asked to participate in a focus group to give feedback on the intervention.

Age 18 and 65; Body mass index (BMI) between 25-45kg/m2; Have a diagnosis of stage 1-3 breast cancer within the past 10 years and have completed standard of care definitive surgery, adjuvant chemotherapy, radiation, immunotherapy and/or targeted therapy as appropriate; Live or work within 30 miles of the Anschutz Health and Wellness Center. Inclusion Criteria 1. Age 18-65 years 2. Body Mass Index 25-45 kg/m2 3. Diagnosis of stage 1-3 breast cancer within the past 10 years with completion of standard of care definitive surgery, adjuvant chemotherapy, radiation, immunotherapy and/or targeted therapy as appropriate > 3 months prior to the intervention start date. Current use of anti-endocrine directed therapy for breast cancer is acceptable if participants have been on therapy for 3 months prior to study start date 4. Live or work within 30 miles of the AHWC 5. Capable and willing to give informed consent, willing to attend weekly group sessions, and attend one in person exercise session/week 6. Have a primary care provider (PCP) or are willing to establish care with a PCP. 7. Obtain a signed medical clearance from their oncologist or PCP for participation 8. Own or willing to purchase a smart phone or computer for study procedures Exclusion Criteria 1. Eastern Cooperative Oncology Group (ECOG) Performance Status 2 or higher 2. Actively undergoing chemotherapy, radiation, immunotherapy and/or targeted therapy (other than anti-endocrine therapy) or are within 3 months of completion of treatment (other than anti-endocrine therapy) 3. Plans to relocate in the next 12 months 4. Plans for extended travel within the next 12 months 5. Major surgery within the past 3 months or planned major surgery during the timeframe of the 6 month study intervention 6. For Females: Currently pregnant or lactating Pregnant within the past 6 months Planning to become pregnant in the next 12 months 7. Recent acute coronary event, unstable angina, coronary revascularization, stroke, pulmonary embolism, or hospitalization for heart failure or cardiac arrythmia 8. Symptoms suggestive of cardiovascular disease or unstable angina 9. Uncontrolled hypertension 10. Diabetes (Type 1 or 2) unless on metformin or DPP-V inhibitor monotherapy and well-controlled with Hemoglobin A1C <8%. 11. Undiagnosed hypo- or hyper-thyroidism or history of uncontrolled thyroid disorder. History of thyroid disease or current thyroid disease treated with a stable medication regimen is acceptable 12. Stage 4 or 5 chronic kidney disease 13. Triglycerides >500 mg/dL or LDL cholesterol >200 mg/dL 14. Serious arrhythmias 15. Clinically significant abnormalities in hematocrit/hemoglobin, white blood cell count, platelets, serum sodium, potassium, or bicarbonate or presence or history of other metabolic or chronic health problems including significant cardiac valvular disease or heart failure, significant gastrointestinal, pulmonary, renal, musculoskeletal, neurologic, hematologic, orthopedic, or psychiatric disease 16. Sustained use of prescription or over-the-counter medications known to significantly impact appetite, weight, or energy metabolism with the exception of anti-endocrine directed treatment for breast cancer and standard of care anti-emetic or anti-diarrheal agents 17. Sustained use of systemic glucocorticoids (current or past 6 months) unless physiologic replacement therapy for adrenal insufficiency. 18. Previous obesity treatment with metabolic bariatric surgery or weight loss device, except liposuction and/or abdominoplasty if performed > 1 year before screening, or endoscopically placed weight loss device if the device has been removed > 1 year before screening 19. Nicotine use (current or past 6 months) 20. Current alcohol or substance abuse 21. History of clinically diagnosed eating disorders 22. Current severe depression or history of severe depression within the previous year or history of other significant psychiatric illness 23. Currently participating in or planning to participate in any formal weight loss programs or clinical trials over the next 12 months. 24. Previous participation in the Anschutz Health and Wellness Center (AHWC) BfitBwell Oncology Exercise Program 25. Previous participation in a weight loss, exercise, or nutrition research study