Sirtuins, SIRT1, and Why NAD+ Is the Key That Unlocks Them

If you have spent any time in the longevity conversation, you will have met the same cast of characters: cold plunges and ice baths, intermittent fasting, caloric restriction, hard exercise, and a handful of plant compounds from red wine and blueberries. The hype around each tends to travel on its own, and on the surface they look unrelated. Yet when researchers trace what these interventions actually do inside a cell, the paths keep converging on the same family of enzymes: the sirtuins.

This convergence is not a coincidence. Sirtuins appear to function as a kind of molecular bridge between the signals that indicate a cell is under mild stress (fasting, caloric restriction, cold exposure, exercise, oxidative challenge) and the cellular programs that respond to that stress in health-promoting ways (DNA repair, inflammation suppression, metabolic efficiency). In a real sense, longevity researchers now regard sirtuin activation as one of the central mechanisms through which healthy habits translate into cellular lifespan extension. ¹ One honest qualifier is worth setting down at the start: caloric restriction has the deepest evidence behind it, while the case for some of the trendier interventions, cold and fasting in particular, is more suggestive than settled.

The story of how to activate them, however, turned out to be more complicated than it first appeared. Early excitement was enormous. A celebrated 2003 paper ignited a decade of pharmaceutical interest. That interest collided with a replication controversy that seemed to derail the whole field. And then, quietly, the science resolved in a more nuanced direction that has significant practical implications for anyone thinking about supplementation.

This article walks through what sirtuins actually do, why NAD+ is the non-negotiable substrate they require, what the resveratrol controversy revealed about how they are activated, and why the combination of NAD+ precursors with pterostilbene addresses both sides of the equation.

One detail makes this feel less exotic. Sirtuins are most active during fasting, calorie restriction, and exercise, the states of mild energy stress. That is not a coincidence: they evolved as the cell's response to lean times, the signal to switch into repair-and-conserve mode. It is why the same lifestyle habits people already trust map onto the very machinery a supplement is trying to support. The honest limit is worth stating up front too. Restoring NAD+ refills the fuel tank, but the engine still has to be tuned. Raising NAD+ is a mechanistically coherent target, not a proven guarantee of longer human healthspan.

Mammals have seven sirtuin proteins, designated SIRT1 through SIRT7. Each has a distinct location inside the cell and its own set of targets, but three carry the clearest ageing stories and are the ones worth meeting first: SIRT1, SIRT3, and SIRT6. The remaining four play more specialised roles, which we touch on at the end.

Almost everything sirtuins do comes down to one chemical move: deacetylation. Cells decorate many of their proteins, including the histone spools that DNA winds around, with small chemical tags called acetyl groups. Adding a tag (acetylation) and removing it (deacetylation) work like an on/off switch that changes how a protein behaves or how tightly a stretch of DNA is packed. Sirtuins are the enzymes that remove those tags.

SIRT1 is primarily nuclear and cytoplasmic. Its most significant function in the ageing context is probably its role in DNA repair: it strips acetyl tags off histones at the sites of DNA damage, loosening the tightly wound chromatin so the repair machinery can physically reach the break. This matters because DNA damage accumulates with age, and slowed repair is one of the defining features of the ageing cell. The consequences of letting that damage go unrepaired are not abstract: unrepaired breaks drive mutations, push cells into senescence (a worn-out, non-dividing state that leaks inflammatory signals into surrounding tissue), raise the risk that a cell turns cancerous, and in many cases end in the cell simply losing function or dying. Efficient repair is what holds that cascade in check, which is why an enzyme that accelerates repair earns its place in the ageing story. Beyond DNA, SIRT1 suppresses the inflammatory transcription factor NF-kB. Chronic low-grade inflammation, sometimes called "inflammaging," is now understood as a driver of age-related diseases, and SIRT1 acts as a direct brake on this process by deacetylating key NF-kB subunits. ² SIRT1 also deacetylates PGC-1alpha, the master regulator of mitochondrial biogenesis. By activating PGC-1alpha, SIRT1 stimulates the production of new mitochondria, maintaining energy capacity in ageing tissue. ³

Here is the core constraint that determines everything else in this space. Sirtuins are NAD+-dependent deacylases. This means they do not simply use NAD+ as a cofactor in the conventional sense. They consume it. Every deacetylation reaction catalysed by a sirtuin requires one molecule of NAD+, which is cleaved in the process. Without adequate NAD+, sirtuins cannot catalyse reactions at all, regardless of how much of the enzyme protein is present.

This was articulated with particular clarity by Imai and Guarente in a landmark 2014 review: NAD+ levels regulate sirtuin activity directly, and the age-associated decline in cellular NAD+ is therefore a proximal cause of reduced sirtuin function in ageing tissue. ⁸ The decline in NAD+ with age is well-documented. Multiple routes contribute: increased activity of CD38 (an NAD+-consuming enzyme that climbs with age and inflammation, and the reason we pair a CD38 inhibitor, apigenin, with NMN in The Repair), reduced biosynthesis, and the simple accumulation of DNA damage (which activates PARP enzymes that deplete NAD+ in repair reactions).

Raising NAD+ via precursors is therefore a rational strategy to restore the substrate availability that ageing cells have lost. NMN (nicotinamide mononucleotide) enters the NAD+ biosynthesis pathway one step further along than NR (nicotinamide riboside), directly joining as a substrate for the enzyme NMNAT, which converts it to NAD+. Both precursors raise intracellular NAD+; they do so via slightly different biochemical routes, and the clinical evidence base for each is at different stages of development. ¹

What this substrate dependency means practically is that raising NAD+ is necessary but may not be sufficient. A cell with restored NAD+ has the fuel sirtuins need. Whether the enzyme is also in an activated conformation, positioned at the right genomic sites, and responding appropriately to cellular signals is a separate question. This is where the story of resveratrol becomes instructive.

In 2003, Howitz and colleagues published a paper in Nature reporting that resveratrol, a polyphenol found in red wine, extended the lifespan of yeast by activating Sir2, the yeast homolog of human SIRT1. ⁹ The finding received extraordinary media attention. The idea that a molecule in red wine might directly activate a longevity enzyme was compelling, and it drove years of pharmaceutical investment in SIRT1 activators.

The backlash came in 2009, when Beher and colleagues published data showing that the apparent direct activation of SIRT1 by resveratrol in the original assays was a fluorophore artifact. The biochemical assays used a fluorescent peptide substrate, and resveratrol was interacting with the fluorophore, not with SIRT1 itself. When the assay was repeated with a native peptide substrate, the direct activation disappeared. ¹⁰ For a period, this appeared to invalidate the entire resveratrol-sirtuin hypothesis.

The resolution came from a 2012 study by Price and colleagues, using animal models. Resveratrol still produced measurable metabolic benefits: improved mitochondrial function, increased energy expenditure, protection against metabolic disease. The mechanism, however, was indirect. Resveratrol activates AMPK (AMP-activated protein kinase, a central energy sensor), and it is via AMPK that it drives downstream benefits. Critically, those benefits were abolished in animals where SIRT1 had been genetically deleted, demonstrating that SIRT1 is required as a downstream mediator of resveratrol's effects even when it is not the direct target. ¹¹

The controversy resolved, then, not by discrediting the sirtuin pathway but by redefining how it is engaged. Resveratrol activates SIRT1 indirectly. The pathway is real. The direct binding claim was the part that did not survive scrutiny. This distinction matters because it opens the question: is there a stilbene, the class of polyphenols to which both resveratrol and pterostilbene belong, that does directly and efficiently engage SIRT1?

Pterostilbene is a naturally occurring stilbene found in blueberries and grapes. Structurally, it differs from resveratrol in that two of its hydroxyl groups are replaced by methoxy groups. This seemingly minor modification has significant pharmacokinetic consequences.

A 2011 pharmacokinetic study by Kapetanovic and colleagues compared oral bioavailability of pterostilbene and resveratrol in rats. Resveratrol's oral bioavailability was approximately 20%, consistent with the well-documented rapid conjugation and excretion that limits its systemic exposure in animal models. Pterostilbene achieved approximately 80% oral bioavailability under the same conditions (animal study data; human bioavailability has not been separately confirmed in a dedicated PK trial). ¹² The difference is attributable to the methoxy groups: they increase lipophilicity, reduce first-pass conjugation, and prolong plasma half-life. Wang and Sang, in a 2018 review of stilbene pharmacokinetics, confirmed this pattern and noted that pterostilbene's metabolic stability translates to higher tissue concentrations at comparable doses. ¹³ These are animal pharmacokinetic data, and precise human equivalence has not been established, but the structural rationale is sound.

Beyond pharmacokinetics, pterostilbene appears to engage SIRT1 more directly than resveratrol does. A 2021 study by Zhang and colleagues investigated pterostilbene's effects in endothelial cells and confirmed SIRT1 activation as a key mechanism. Molecular docking analysis showed direct binding between pterostilbene and the SIRT1 protein, with downstream protective effects on vascular function that were attenuated when SIRT1 was inhibited. ¹⁴ This is cell and animal data, not a human clinical trial, so the mechanistic picture is still being built. But the combination of better bioavailability and credible direct binding evidence positions pterostilbene as a more interesting sirtuin activator than resveratrol. It is exactly this profile, better absorption paired with credible direct SIRT1 binding, that led us to include pterostilbene at a clinical dose in our triple-action NAD+ booster, The Repair.

The logic of combining an NAD+ precursor with a sirtuin activator follows directly from the biochemistry. Sirtuins require NAD+ as substrate. Pterostilbene may facilitate the conformational activation of SIRT1 at the enzyme level. These are not redundant mechanisms; they address different rate-limiting factors for sirtuin activity. Raising NAD+ removes the substrate bottleneck. A direct activator addresses whether the enzyme is in a catalytically favourable state. The two together cover both constraints.

The most relevant human clinical evidence for this combination comes from a 2017 randomised controlled trial by Dellinger and colleagues, published in npj Aging and Mechanisms of Disease. The combination tested was a commercial NR-plus-pterostilbene supplement (marketed as Basis by Elysium Health), and the trial involved that manufacturer. Adults aged 60 to 80 received varying doses of NR (nicotinamide riboside) combined with pterostilbene. Across doses, whole blood NAD+ levels increased by 40 to 90% above baseline, with the higher-dose group showing the larger effect. The combination was well tolerated with no serious adverse events. ¹⁵

Two honest caveats belong with that headline number. First, this was whole-blood NAD+, a convenient and accessible proxy. What ultimately matters is NAD+ inside cells, particularly in the metabolically demanding tissues that age, and blood levels do not map perfectly onto intracellular concentrations there. A rise in the blood is encouraging, but it is a stand-in for the measurement we actually care about. Second, the NAD+ increase itself is driven by the NR component. Pterostilbene is not an NAD+ precursor and would not be expected to add to that number at all; its proposed contribution sits downstream, at the level of SIRT1 activation, which this measurement does not capture.

That points to the more important limitation of the trial for our specific argument. It measured NAD+ levels and safety. It did not measure sirtuin activity, deacetylation of any target protein, or any functional readout that would demonstrate the raised NAD+, let alone the added pterostilbene, actually translated into more sirtuin work. To our knowledge, no human trial has yet directly measured sirtuin activation after this kind of NAD+-precursor-plus-stilbene combination; the supporting evidence for the activation step remains cellular and preclinical. So the rationale for the combination is mechanistically coherent and reasonable, but at the level of demonstrated sirtuin output in people it is still an inference, not a proven result. That is a distinction we think is worth keeping in plain view rather than glossing over.

One further precision point: this trial used NR as the NAD+ precursor, not NMN. NR and NMN are both precursors to NAD+ and both raise intracellular NAD+, but they enter the biosynthesis pathway at different steps and have different cellular uptake routes. The mechanistic rationale for combining an NAD+ precursor with pterostilbene applies equally to NMN, but a direct human RCT with NMN plus pterostilbene specifically has not yet been published. Any formulation that substitutes NMN for NR in this combination is extrapolating from the available evidence, reasonably but not identically.

Most relevant for:

• Adults over 40 with metabolic concerns - SIRT1 and SIRT3 regulate glucose metabolism and mitochondrial function; the evidence base in metabolically compromised adults is the strongest.

• Sedentary adults - exercise is a natural sirtuin activator via NAMPT; those who cannot exercise regularly have the most to gain from supplemental NAD+ support.

• Adults with family history of metabolic or neurodegenerative conditions - sirtuins regulate several protective pathways relevant to these risk categories.

• Anyone already taking NMN who wants to optimise its effect - adding apigenin (CD38 inhibitor) and pterostilbene (independent SIRT1 activator) may amplify the NAD+ signal NMN provides.

Lower priority if:

• Under 25, highly active, no metabolic concerns - sirtuin activity and NAD+ levels are typically higher in this group; incremental supplemental benefit is less predictable.

• Budget-constrained and new to NMN - start with The Base (250mg NMN alone) before adding the full apigenin + pterostilbene stack.

  
  

SIRT1, SIRT3, and SIRT6 carry the clearest ageing stories, but the family has four more members. They are worth a brief tour, partly because they get progressively less relevant to the NAD+ supplementation thesis, and that is itself a useful point: not every sirtuin is a longevity lever, and "activate all sirtuins" is too crude a goal.

SIRT2 is mainly cytoplasmic (it slips into the nucleus during cell division) and is the most abundant sirtuin in the brain. Its signature target is alpha-tubulin, a building block of the cell's internal scaffolding, so it is involved in cell structure and in orchestrating cell division. Its relevance to ageing is real but ambiguous: in neurodegeneration it can look protective in some settings and harmful in others, so it does not fit neatly into a "more is better" narrative.

SIRT4 is mitochondrial and the contrarian of the family. Rather than acting as a classic deacetylase, its main effect is to restrain metabolism, dampening amino-acid-driven insulin secretion and suppressing fat burning. The takeaway is that SIRT4 is a brake, not an accelerator, another reminder that switching every sirtuin up is not the goal.

SIRT5 is mitochondrial and rewrote the textbook definition of what sirtuins do. It is a weak deacetylase but a potent remover of other chemical marks (succinyl, malonyl, and glutaryl groups), and it governs the urea cycle, ammonia detoxification, and reactive-oxygen handling. It is worth knowing about mainly because it shows that NAD+-dependent sirtuin chemistry extends well beyond the acetylation story above.

SIRT6 is covered in the main section above. It is the member that genuinely belongs alongside SIRT1 and SIRT3 in any ageing discussion, which is why we promoted it out of this appendix.

SIRT7 is the least studied. It lives in the nucleolus and helps regulate ribosome production, essentially controlling how much protein-building capacity a cell maintains. It contributes to stress resistance and genome stability, but human-relevant data is still thin.

The sirtuin field has matured considerably from the initial excitement of 2003 and the apparent setback of 2009. What has emerged is a cleaner and more actionable picture. SIRT1, SIRT3 and SIRT6 are genuine longevity-relevant enzymes: they repair DNA, suppress chronic inflammation via NF-kB, drive mitochondrial biogenesis through PGC-1alpha, and optimise mitochondrial efficiency. Their activity falls with age in human tissue, and the primary reason it falls is the parallel decline in NAD+.

Restoring NAD+ via precursors such as NMN is therefore mechanistically sound, but it addresses only the substrate side of the equation. The resveratrol controversy clarified that sirtuin activation also depends on how the enzyme itself is regulated, and it pointed toward more bioavailable stilbenes, particularly pterostilbene, as more appropriate tools for that purpose. The human clinical trial that combined an NAD+ precursor with pterostilbene demonstrated meaningful NAD+ increases in older adults, though it used NR not NMN as the precursor, measured NAD+ rather than sirtuin activity itself, and has no direct NMN equivalent published to date.

The broader scientific direction is clear. Raising NAD+ alone is only half the equation. The most coherent approach addresses both substrate availability and enzyme activation together. That is the reasoning behind the dual-mechanism design in The Repair, and it is grounded in the best available evidence for how sirtuins actually work, evidence that is genuinely strong on the biology and still maturing on the clinical endpoints. We follow the evidence. When it changes, so will we.

For Youth is a science-led longevity brand focused on developing clinically relevant supplements that support healthy ageing and performance at the cellular level. Formulated in collaboration with leading academic researchers, the brand prioritises evidence-based ingredients, advanced delivery technologies, and transparent quality standards.