NAD+ research, past the sirtuin headline.

If you've followed the longevity conversation since roughly 2013, NAD+ has been positioned almost entirely as a sirtuin story: NAD+ levels decline with age, sirtuins need NAD+ to function, restore NAD+ and you restore sirtuin activity, slow down aging. That framing has done a lot of work commercially. It also flattens what the actual research has been studying for the last decade.

Where the sirtuin pitch came from

Sirtuins are a family of NAD+-dependent deacetylases first characterized in yeast, where SIR2 was shown to extend replicative lifespan. The mammalian homolog SIRT1 became the focus of a generation of longevity research because of its links to caloric restriction phenotypes. Because sirtuins consume NAD+ as a co-substrate, the argument was elegant: declining NAD+ during aging starves sirtuins of fuel, and restoring NAD+ restores their activity.

Rajman, Chwalek, and Sinclair's 2018 Cell Metabolism review is the canonical version of this pitch, and it's still a useful starting point ^[1]. The paper synthesizes a lot of the supporting work — NAD+ decline in aged tissue, mechanistic NAD+ supplementation experiments in mice, biomarker rescue in humans.

What the research actually pivoted to

The sirtuin lens turned out to be one of several. Three other NAD+-consuming pathways have been increasingly central:

• PARP signaling — poly(ADP-ribose) polymerases consume NAD+ during DNA-damage repair. Aging tissue accumulates DNA damage; PARP activity goes up; NAD+ pool gets depleted. PARP-driven NAD+ consumption is now thought to be a larger contributor to age-related NAD+ decline than the sirtuin pathway alone.
• CD38 — a transmembrane glycohydrolase that consumes NAD+ as part of immune signaling. CD38 expression rises with age ("inflammaging"), and CD38 knockout in mice partially preserves NAD+ levels and rescues some age-related metabolic phenotypes.
• Mitochondrial bioenergetics — NAD+/NADH redox state directly drives oxidative phosphorylation. Declining NAD+ levels reduce mitochondrial efficiency. This is the bioenergetics-first view of NAD+ aging, with sirtuin and PARP effects layered on top rather than as the primary mechanism.

The supplementation question

Most of the human-trial work in NAD+ supplementation has used precursors — nicotinamide riboside (NR) and nicotinamide mononucleotide (NMN) — rather than NAD+ itself. Both reliably raise blood NAD+ levels. Whether that translates to functional outcomes depends on the population and the endpoint.

Yoshino et al. 2018 conducted one of the better-controlled NMN trials, demonstrating dose-dependent NAD+ elevation and modest improvements in muscle insulin sensitivity in postmenopausal women ^[2]. Other trials have been smaller and less controlled. The aggregate signal is real but smaller than the original sirtuin-restoration narrative implied.

Direct NAD+ supplementation (intravenous infusion or intramuscular) sidesteps the precursor-conversion question but adds others — bioavailability of intact NAD+ is low, the molecule is large and charged, and most of the cellular NAD+ pool is replenished from precursors anyway. Whether direct NAD+ outperforms NR or NMN at equivalent NAD+ exposure is an open question with conflicting trial data.

Where the field is now

The current consensus, as much as there is one, is that NAD+ is genuinely important for cellular bioenergetics and that age-related NAD+ decline is real. The mechanism through which restoration improves outcomes is more distributed than the sirtuin-only model suggested — sirtuin activity, PARP rescue, CD38 modulation, and direct mitochondrial effects all contribute.

The next wave of research is moving toward selective intervention: targeting CD38 inhibition to preserve NAD+ pools rather than supplementing precursors broadly, modulating PARP activity in specific tissues, characterizing the relationship between NAD+ and senescent-cell load. Whether any of this maps to a clinically meaningful intervention in healthy aging adults is the open question that the next decade of trials will answer.

The honest read

NAD+ is real, the decline is real, the supplementation work is real but more modest than the marketing. The sirtuin story is one chapter, not the book. Anyone selling you NAD+ as the longevity hack is either oversimplifying or selling. We carry NAD+ because the in-vitro and pre-clinical research on the broader pathway network is active and important. We're not telling you what it does in your body.