Summary In preclinical models, the upregulation of the sirtuin-NAD+ pathway by increasing intracellular NAD+ through administration of NAD precursors, such as β nicotinamide mononucleotide (βNMN), engages the mechanisms of aging and prevents or attenuates Alzheimer's disease (AD) pathology. In contrast to many AD drugs in development that target one mechanism, NAD precursors may prevent AD pathology by multiple mechanisms: by improving mitochondrial energetics; inducing a switch to non-amyloidogenic processing of amyloid precursor protein (APP) due to increased α-secretase activity; preventing microglia-dependent Aβ toxicity; attenuating neuroinflammation; promoting neuronal regeneration; and improving insulin action. NAD- precursors are sold over the counter, and despite widespread claims of their benefits and growing use of NAD precursors, carefully controlled studies of their pharmacology, efficacy, and safety have been limited. In response to the Reviewers' recommendations, we have extensively revised the study and now propose a smaller, single-site, 90-day randomized, placebo-controlled, parallel group, double-blind, trial in 24 participants with mild AD dementia to demonstrate: 1) CNS penetration across the blood-brain barrier; and 2) the ability of oral NMN to increase brain levels of NAD, using an innovative ultra-high field 7T magnetic resonance spectroscopy (MRS) method. To determine whether βNMN penetrates the blood-brain barrier in humans, we will measure the cerebrospinal fluid (CSF) concentrations of βNMN and its key metabolites at baseline and on day 90 (Aim 1). We will assess whether βNMN engages the target mechanism by measuring the abundance of NAD+ (a SIRT1 substrate) in the brain using ultra-high field 7T MRS and in peripheral blood mononuclear cells using a validated LC-MS/MS assay (Aim 2). We will determine the effects of βNMN on circulating biomarkers of aging that the geroscience experts have recommended (HbA1C, IGF1, T3, IL6, TNFα, and urinary F2- isoprostane) (Aim 3). This trial is neither long enough nor large enough to determine the effects of βNMN on clinical outcomes and AD biomarkers. However, we have included CSF and serum biomarkers of AD as well as measures of global cognition, instrumental activities of daily living, and neuropsychiatric symptoms as exploratory outcomes to estimate design parameters such as bias and precision of changes in outcomes, and pre-post correlation to guide the design of subsequent efficacy trials. The scientific rigor and innovation in the proposed proof-of-principle trial is underscored by the use of a high quality crystalline formulation of βNMN; a dose-regimen informed by carefully performed pharmacokinetic studies; rigorous sample collection procedures to ensure preanalytical stability; and evaluating the engagement of the sirtuin-NAD+ pathway in the brain using ultra-high field 7T MRS. The trial will provide important information on the engagement of target mechanisms that is nece...