The NAD+/NADH ratio reflects the cellular redox state. NAD+ accepts electrons during catabolism; NADH delivers them to the electron transport chain.
Mechanism — A high NAD+/NADH ratio (more NAD+ available) supports sirtuin activity, DNA repair, mitochondrial function, and energy production. Declines with age — total NAD+ pool drops 50% by age 60. Fasting raises the ratio; chronic feeding lowers it.
Use case — Support via NMN/NR precursors, exercise, fasting, sleep. Measurement is research-only — no good clinical test yet.
Caveats — Total NAD+ levels and the ratio are different — supplements can raise levels without changing the ratio. The ratio matters more for sirtuin and energy signaling.