Inclusion Criteria:

1. Males and females, ages 10-30 years at enrollment
2. Able to understand and speak English
3. Diagnosis of acute leukemia (myeloid, lymphoid), aplastic anemia, or myelodysplastic syndrome requiring allogeneic HCT
4. 6-48 months from allogeneic HCT
5. 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.
6. Parental/guardian permission (informed consent) and if appropriate, child assent
7. Minimum weight of 24 kg

Exclusion Criteria:

1. Known sensitivity to NR
2. Concurrent use of any medications, including statins, likely to increase risk of NR toxicity
3. Active malignancy, investigational agent(s) within 4 wks, or supraphysiological glucocorticoids at the baseline visit
4. Currently meeting public health exercise guidelines1
5. Use of NAD+ precursors (supra-physiologic) within 4 weeks
6. Hemoglobin \< 10 g/dL
7. Platelets \< 50K
8. Diabetes Mellitus requiring insulin or insulin secretagogue
9. HbA1C ≥ 8% or fasting glucose \> 125 mg/dL; However if the participant is diabetic, both fasting glucose level and HbA1C must meet the criteria to be deemed eligible.
10. Kidney disease (eGFR \< 60 ml/min/1.73 m2)
11. Liver disease (ALT/AST \> 3 x ULN)
12. Limitations in physical function preventing exercise testing/training
13. Unstable angina or history of acute myocardial infarction (\<5 days of planned study procedures)
14. Recurrent syncope
15. Symptomatic severe aortic stenosis
16. Uncontrolled arrhythmia causing symptoms
17. Pulmonary embolus \<3 months of study procedures
18. Thrombosis of lower extremities
19. Symptomatic moderate or severe persistent asthma based on FEV from pre-HCT pulmonary function testing
20. Room air desaturation at rest ≤85%
21. Females: Pregnant or planning pregnancy
22. Non-cardiopulmonary disorders that may affect exercise performance or be aggravated by exercise (e.g. infection, renal failure, thyrotoxicosis, \>moderate GVHD resulting in physical or functional impairment)
23. Parents/guardians or subjects who, in the opinion of the Investigator, may be non-compliant with study schedules or procedures.
24. For MRI procedure: Contraindications to MRI. Individuals will be screened to ensure they can safely complete the MRI. \*participants who cannot complete the MRI will not be excluded from participation in the remainder of the study procedures if they meet those inclusion and exclusion criteria.
25. Current weight precludes safe completion of study procedures