Inclusion Criteria:  Males and females, ages 15-30 years at enrollment Able to understand and speak English Diagnosis of acute leukemia (myeloid, lymphoid) requiring allogeneic HCT 6-24 months from allogeneic HCT Females must have a negative urine/serum pregnancy test and must use an acceptable method of contraception, including abstinence, a barrier method (diaphragm or condom), Depo-Provera, or an oral contraceptive, for the duration of the study. Parental/guardian permission (informed consent) and if appropriate, child assent  Exclusion Criteria:  Known sensitivity to NR Concurrent use of any medications, including statins, likely to increase risk of NR toxicity Active malignancy, investigational agent(s) within 4 wks, or systemic glucocorticoids within 12 wks Currently meeting public health exercise guidelines Use of NAD+ precursors (supra-physiologic) within 4 weeks Hemoglobin < 10 g/dL Platelets < 50K Diabetes Mellitus requiring insulin or insulin secretagogue HbA1C ≥ 8% Kidney disease (eGFR < 60 ml/min/1.73 m2) Liver disease Alanine aminotransferase/Aspartate aminotransferase(ALT/AST) > 3 x ULN Limitations in physical function preventing exercise testing/training  Contraindications to MRI. Including:  ANY intra-luminal implant, filter, stent or valve replacement ANY type of life assist device, pump, or prosthetic ANY vascular clip or clamp ANY surgically placed clips or clamps or bands on visceral organs ANY intracranial implants of any type other than dental fillings ANY non-removable piercings, jewelry, or medicinal patch ANY personal history of intraocular injury or fragment in or around the orbit that cannot be cleared through radiologic examination. ANY personal history of bullet, shrapnel, or stabbing wounds that cannot be cleared through radiologic evaluation. Inability to lie flat in the MRI scanner for 60-90 minutes Unstable angina or history of acute myocardial infarction (<5 days of planned study procedures) Recurrent syncope Symptomatic severe aortic stenosis Uncontrolled arrhythmia causing symptoms Pulmonary embolus <3 months of study procedures Thrombosis of lower extremities Symptomatic moderate or severe persistent asthma based on forced expiratory volume (FEV) from pre-HCT pulmonary function testing Room air desaturation at rest ≤85% Females: Pregnant or planning pregnancy Non-cardiopulmonary disorders that may affect exercise performance or be aggravated by exercise (e.g. infection, renal failure, thyrotoxicosis, >moderate graft versus host disease (GVHD) resulting in physical or functional impairment) Parents/guardians or subjects who, in the opinion of the Investigator, may be non-compliant with study schedules or procedures.