This is the part of NAD biology that is genuinely settled, so it is worth setting out properly.
The problem cells solve
Food contains energy in chemical bonds. Cells cannot use that directly. They convert it into ATP, the molecule that powers most cellular work.
Where NAD fits
Getting from food to ATP takes many steps. At several of them, electrons are stripped from molecules and must be carried elsewhere. NAD+ accepts those electrons, becoming NADH.
NADH then delivers them to the electron transport chain in the mitochondria, where the energy is used to generate ATP. Having handed off its cargo, the molecule returns to NAD+ and repeats.
The scale of it
This cycle runs constantly, in essentially every cell, throughout your life. NAD appears in hundreds of reactions.
Where the textbook stops
All of the above describes normal function in a healthy cell. It does not describe what changes if you consume additional NAD or a precursor. That is a separate question, studied separately, and the answers are considerably less settled than the pathway itself.
We think both halves are worth knowing, which is why we publish them together.
