What Is NAD+ and Why Does It Decline With Age?

Nicotinamide adenine dinucleotide — NAD+ — is one of the few molecules present in every living cell on earth, from bacteria to humans. It is not a vitamin, a hormone, or a drug. It is a coenzyme: a helper molecule that other proteins cannot function without. Its job is deceptively simple. NAD+ carries electrons. In doing so, it makes possible almost every energy transaction your body performs.

The energy currency behind the energy currency

Most people know ATP as the body's energy currency. Fewer know that producing ATP depends on NAD+. When your cells break down glucose or fat, the chemical energy released has to be captured and shuttled to the mitochondria, where it is converted into ATP. NAD+ is the shuttle. It accepts electrons (becoming NADH), delivers them to the mitochondrial electron transport chain, and returns to its oxidized state to do it again — thousands of times per second in a single cell.

Because of that turnover, the ratio of NAD+ to NADH functions as an internal readout of metabolic state. Cells use it to decide whether to build or break down, store or burn.

Repair, signaling, and the second job

NAD+ has a second role that has drawn most of the recent research attention. A family of enzymes called PARPs uses NAD+ to repair damaged DNA. Another family, the sirtuins, consumes NAD+ to regulate gene expression, mitochondrial quality control, and inflammatory signaling. Both families are consumers: every repair event and every regulatory adjustment spends NAD+ rather than recycling it.

This matters because it means NAD+ is not just a metabolic middleman but a shared budget. When DNA damage is high — from UV exposure, inflammation, or general metabolic stress — PARP activity rises and NAD+ availability for everything else falls.

Why levels fall with age

Measurements in human tissue consistently show NAD+ concentrations declining across the adult lifespan, with commonly cited estimates suggesting roughly half of youthful levels by midlife, though figures vary substantially by tissue and measurement method. The decline appears to be driven from both ends.

On the supply side, the salvage pathway that recycles NAD+ from its breakdown products becomes less efficient, partly because a rate-limiting enzyme called NAMPT is expressed at lower levels in older tissue. On the demand side, consumption rises: accumulated DNA damage keeps PARPs busier, and chronic low-grade inflammation activates CD38, an NAD+-degrading enzyme that becomes markedly more abundant with age.

Lower supply, higher demand. The result is a shrinking pool available for ordinary cellular work — which is one proposed explanation for why mitochondrial function, recovery capacity, and repair efficiency all decline together rather than separately.

What is and isn't established

The decline itself is well documented. What remains an active research question is how much of age-related dysfunction it causes versus reflects, and how effectively NAD+ levels can be restored in humans. Animal studies raising NAD+ have produced striking results in muscle, metabolic, and neurological measures. Human trials are smaller, shorter, and more mixed — showing reliable increases in blood NAD+ markers, with less consistent effects on downstream clinical outcomes.

The honest summary: NAD+ is unambiguously central to cellular function, its decline with age is real, and the therapeutic question is still being answered.