Overview
NAD+ is a coenzyme — not a peptide. It is central to redox metabolism and serves as a substrate for enzymes tied to DNA repair, stress responses, and aging biology, including PARPs and sirtuins. Included here for completeness from the source dossier. The "longevity" reputation is well ahead of the controlled clinical evidence.
Mechanism
Mechanistically the case for NAD+ is strong at the biochemical level. The translational case is much weaker. Recent reviews conclude that NAD+ augmentation clearly changes biology, and oral precursors such as NR, NMN, and NAM reliably raise NAD-related biomarkers in humans — but clinical benefits for anti-aging, metabolic, vascular, or performance outcomes remain mixed or inconclusive. Importantly, a recent review found no eligible outcomes trials testing direct IV or IM NAD+ for anti-aging or general wellness, so many popular claims run ahead of the evidence.