Passive Sensor Identification of Digital Biomarkers to Assess Effects of Orally Administered Nicotinamide Riboside

Inclusion Criteria: * Ability of the participant and/or his/her legally authorized representative to understand the purpose and risks of the study, to provide signed and dated informed consent, and to authorize the use of confidential health information. * Ability to speak and read fluently in English * 18-89 years old (inclusive) * Normal or corrected to normal hearing and vision * Meet clinical diagnostic criteria for MCI or Mild AD, according to the criteria outlined above * Study partner available for duration of trial participation * At least one copy of the APOE ε4 allele * An aggregate risk score \> 4 according to the risk analysis method developed by Sabbagh et al. (2017) * For individuals who are taking niacin (or a vitamin supplement with niacin) of \>200mg, the completion of a two-week wash-out period Exclusion Criteria: * Current serious or unstable medical or neurological condition that could affect cognitive functioning, as determined by study clinician * Clinically unstable mood or anxiety disorder within 6 months prior to screening, as determined by study clinician * Lifetime history of psychotic disorder (i.e. Schizophrenia, Schizoaffective Disorder), as determined by study clinician * Diagnosis of a mitochondrial disorder * Any MRI safety contraindications * History of drug hypersensitivity or intolerance to NR * Transient ischemic attack or stroke within 1 year prior to screening * History of alcohol or substance abuse within prior year, as determined by study clinician and urine toxicology screen * History of head injury rated as moderate or worse, per DSM-5 criteria * History of seizure within prior 10 years * Current use of medication with known adverse effects on cognition (benzodiazepines, barbiturates, opiate analgesics, first generation antipsychotic medication, anticholinergics, sedating antihistamines, tricyclic anti-depressants) * Change in dose of any psychiatric medications within 4 weeks of screening visit * Prior use of L-DOPA, any anti-Parkinsonian medication, or prior treatment with anti-amyloid immunotherapy * Current use of putative mitochondrial enhancers or antioxidants (e.g. carnitine, creatine, Co-Q10, N-acetyl cysteine, pramipexole) * Initiation of treatment or change in dosing of acetylcholinesterase inhibitors (AChEIs) and memantine within 4 weeks of screening * Prior use of prescription narcotics 4 weeks before screening * Female subjects who are pregnant or breastfeeding * The current use of niacin (or a vitamin supplement with niacin) \>200mg within the last two weeks prior to study visit