NAD+ decline impairs DNA repair not by depleting energy, but by disabling sirtuin signaling

NAD+ drops with age, and DNA repair suffers. The traditional explanation: less NAD+ = less ATP = less energy for repair. But this doesn't fit the evidence.

The real mechanism: NAD+ is the fuel for sirtuins (SIRT1, SIRT6), which coordinate DNA repair. When NAD+ declines, sirtuins lose activity, disrupting the signaling networks that direct repair machinery to damage sites.

The energy supply is sufficient—it's the coordination that fails.