A single-center, randomized, placebo-controlled study in community dwelling, healthy, regularly exercising, highly physically fit men and women, 19 to 40 years. Eligible participants will be randomized to receive either 1000 mg NMN or placebo twice daily for 10 weeks.

The study participants will be screened for potential participation and those meeting eligibility criteria on screening will be offered participation in the study. 120 subjects will be block randomized in a 1:1:1:1 ratio, stratified by sex (male, female) and baseline VO2 max. Participants will be randomly assigned to one of four groups using a 2 by 2 factorial design: Group 1 - MIB-626 (MIB-626 - UPA) plus usual physical activity; Group 2 - standardized, progressive, high intensity, multidimensional exercise plus placebo (PL-Ex); Group 3 - MIB-626 plus standardized, progressive, high intensity, multidimensional exercise (MIB-626-Ex); Group 4 - placebo plus usual physical activity (PL-UPA).

Inclusion Criteria:

1. Highly physically fit and familiar with high intensity exercise training, as indicated

   1. Engages in vigorous exercise regularly (4 or more days each week) for 30 minutes or longer and which includes elements of resistance exercise training
   2. Physically fit ascertained using Daniels' equation https://runsmartproject.com/calculator/, 5K time: ≤ 23.0 min, and/ or 10K time: ≤ 48.0 min These criteria will maximize the likelihood of including individuals with VO2max of at least 40 mL/kg/min.

2. Has a body mass index (BMI) between 18.5 and 32 kg/m2, inclusive

3. Willingness to engage in 10-weeks of intensive exercise training designed to improve aerobic capacity and endurance, muscle strength, power, and fatigability, anaerobic capacity, and body composition, and to reduce fatigue

4. Willing to not engage in additional exercise training if randomized to the progressive exercise group and willing to not change usual physical activity if assigned to the usual physical activity group

5. Is free from clinically significant medical problems as determined by the Investigator

6. Is capable of providing written informed consent.

7. Is willing and able to provide authorization for the use and disclosure of personal health information in accordance with Health Insurance Portability and Accountability Act (HIPAA).

In addition, female participants must:

1. Not be pregnant and not planning to become pregnant over the next 6 months

Exclusion Criteria:

1. Has AST or ALT > 2 times the upper limit of normal

2. Hematocrit < 36% or > 50% for men, or < 35% to > 48% for women

3. Has a diagnosis of diabetes or is using diabetes medications or has an A1C > 6.4%

4. Serum creatinine > 2.0 mg/dL or eGFR <60 mL/min

5. Prohibited medications and substances:

   a. Use of any performance enhancing substance (anabolic steroids, rhGH, DHEA, androstenedione, or any other performance enhancing drug

6. Current use of opiates, amphetamine, cannabinoids and cocaine

   1. Use of any other dietary supplement during the course of the trial. Subjects who are using a supplement containing nicotinamide or niacin or nicotinamide riboside may be included if they agree to stop the supplement at least 2 weeks prior to randomization and agree not to use such supplement/s during the entire duration of the study.
   2. Initiation of a new pharmaceutical product during the preceding 3 months

7. Known allergy to niacin or nicotinamide mononucleotide

8. In the judgment of the study physician, the participant is unlikely to comply with the study protocol for any reason or it may not be safe to administer the study.

9. Competing in organized athletics (e.g., collegiate sports) or training in preparation for competition

10. Contraindications to magnetic resonance imaging (MRI) and magnetic resonance spectroscopy (MRS). These include: a) electrical implants such as cardiac pacemakers b) ferromagnetic implants such as aneurysm clips, surgical clips, artificial hearts valves with steel parts, shrapnel or steel implants c) ferromagnetic objects such as jewelry or metal clips in clothing d) pre-existing history of claustrophobic reactions. (Some participants who cannot undergo MRI/MRS but can undergo other study procedures might be considered on a case-by-case basis)

For women only:

1. Pregnant or planning to get pregnant over the next 6 months
2. Use of oral contraceptives is allowed provided the contraceptive regimen was initiated at least 3 months before randomization and the participant agrees not to change the regimen during the course of the study. 6.3 Excluded Medications and Treatments

1. The ingestion of the following is prohibited during 14 days prior to Day 0 and for the duration of the intervention period:

• Multivitamin preparations that contain niacin or one of its metabolites
• Products that contain niacin, nicotinic acid, nicotinamide in any form, other than natural foods
• Herbal supplements 2. Treatment with another investigational drug, investigational device, or approved therapy for investigational use within 3 month is prohibited.

Individual participant data will not be shared

The study is double-blind in that the study participants and the study staff involved in outcomes assessments will be unaware of the intervention assignment. The randomization schedule will be masked from all study personnel except those specifically designated below.

The unblinded study biostatistician The staff of the Investigational Drug Pharmacy Services. The DSMB, if requested