To understand NAD you need one piece of background: what a coenzyme is.
Enzymes first
Enzymes are proteins that speed up chemical reactions in the body. Without them, most of the chemistry keeping you alive would happen far too slowly to be useful.
Where coenzymes come in
Many enzymes cannot work alone. They need a non-protein helper molecule to complete the job — often to carry something from one reaction to another. That helper is a coenzyme.
NAD is one of the most widely used coenzymes in biology. Its particular job is carrying electrons.
The useful consequence
Coenzymes are not consumed the way fuel is. They cycle — used, regenerated, used again. This matters because it means the body maintains a pool of the molecule rather than burning through a supply.
Why this frames the whole category
Once you understand that NAD is a recycled pool rather than a fuel tank, marketing that talks about "topping up" or "refuelling" starts to sound less precise than it did. The biology is more interesting, and more complicated, than the metaphor.
