- any specific central nervous system disease history (e.g. major clinical stroke, brain tumor, normal pressure hydrocephalus, etc); - clinically significant unstable medical condition that could affect safety or compliance with the study; - was intubated due to covid-19; - major active or chronic unstable psychiatric illness (e.g. depression, bipolar disorder, obsessive compulsive disorder, schizophrenia) within the previous year; - history of alcohol or other substance abuse or dependence within the past two years; - any significant systemic illness or medical condition that could affect safety or compliance with study; - current use of medications with psychoactive properties that may be deleteriously affecting cognition; - any known hypersensitivity to nicotinamide riboside, or its principal metabolite, nicotinamide mononucleotide; - use of other investigational agents or interventions one month prior to entry and for the duration of the trial; - if participating in the optional magnetic resonance imaging (mri) sub-study: any contraindication to undergo mri; - pregnant women or women who are planning to become pregnant within 7 months from study entry.

- any specific central nervous system disease history (e.g. major clinical stroke, brain tumor, normal pressure hydrocephalus, etc); - clinically significant unstable medical condition that could affect safety or compliance with the study; - was intubated due to covid-19; - major active or chronic unstable psychiatric illness (e.g. depression, bipolar disorder, obsessive compulsive disorder, schizophrenia) within the previous year; - history of alcohol or other substance abuse or dependence within the past two years; - any significant systemic illness or medical condition that could affect safety or compliance with study; - current use of medications with psychoactive properties that may be deleteriously affecting cognition; - any known hypersensitivity to nicotinamide riboside, or its principal metabolite, nicotinamide mononucleotide; - use of other investigational agents or interventions one month prior to entry and for the duration of the trial; - if participating in the optional magnetic resonance imaging (mri) sub-study: any contraindication to undergo mri; - pregnant women or women who are planning to become pregnant within 7 months from study entry.