NAD⁺ & Sirtuins
The central redox and signaling coenzyme every sirtuin and PARP depends on — and it falls ~50% between ages 40 and 60.
NAD⁺ is the fuel, not the enzyme: sirtuins (SIRT1–7), PARPs, and CD38 all consume it. As NAD⁺ declines with age, mitophagy, DNA repair, and epigenetic maintenance all lose their power source at once — which is why restoring the pool is one of the highest-leverage single targets in longevity biochemistry.