NAD+ metabolism: what the human data actually shows

NAD+ is the central redox cofactor of metabolism and the substrate for sirtuins and PARPs. Tissue NAD+ declines with age in multiple models, which is why precursors such as nicotinamide riboside and nicotinamide mononucleotide became a research focus. Human trials consistently show one thing: oral precursors raise whole-blood NAD+ in a dose-dependent way, and tolerability has been good across the published studies. The evidence is thinner and more mixed for the outcomes people care about — insulin sensitivity, muscle function, inflammatory markers and vascular measures. Some trials report signals, others report none, and sample sizes are usually small with short follow-up. That is a normal position for an early field. The honest summary is: pharmacokinetics established in humans, functional outcomes still open.