Exercise to ReGain Stamina and Energy (The EXERGISE Study)

Inclusion Criteria: * Consent to participate in the study * Age ≥ 60 years old * Had stage I-III invasive breast cancer * The cancer is diagnosed in 2021 or 2022 * Completed adjuvant therapy for at least 3 months but no more than 1 year * Willingness to participate in all study procedures * Had at least moderate-level fatigue (defined as raw score ≥ 8 on the PROMIS Measure) Exclusion Criteria: * Failure to provide informed consent * Current involvement in rehabilitation program * Absolute contraindications to exercise training * Significant cognitive impairment * Progressive, degenerative neurologic disease * Hip fracture, hip or knee replacement, or spinal surgery within past 4 months * Other significant comorbidities that may impair ability to participate in the exercise intervention * Pregnant * Regular consumption of nicotinamide riboside supplement * Simultaneous participation in other interventional studies * Had no or very mild fatigue (defined as raw score ≤7 in PROMIS) * Diagnosis of any of the following medical conditions in past three years (coronary heart disease, angina, heart attack, heart failure, stroke, high blood pressure, chronic obstructive pulmonary disease, chronic bronchitis, arthritis, diabetes mellitus, and chronic kidney disease), as measured by Behavioral Risk Factor Surveillance System * Receipt of any oral or intravenous antibiotic 4 weeks prior to screening * Receipt of any probiotics within 4 weeks of screening * History of active treatment for HIV, hepatitis B, or hepatitis C infection * Positive stool cultures for enteric pathogens, including Clostridium difficile * Excessive alcohol use (i.e., \> 14 drinks/week) or alcohol abuse (i.e., \> 5 drinks/day for males or \> 4 drinks/day for females) * Other substance abuse within the past 3 years * Smoking history in past 3 years