What Are Sirtuins? Longevity Genes Explained
Sirtuins are NAD+-dependent enzymes that adjust protein activity in response to energy state. What they do, and where the popular story outran the evidence.
The Short Answer
Sirtuins are a family of seven enzymes in humans, SIRT1 through SIRT7, that remove acetyl and other acyl groups from proteins and require NAD+ as a co-substrate to do so. Because the reaction consumes NAD+, sirtuin activity is coupled to cellular energy state: when NAD+ is relatively abundant, as during energy scarcity, sirtuins are more active, and they modify histones and transcription factors in ways that favour mitochondrial function, stress resistance and repair. They are frequently called longevity genes because overexpression extends lifespan in yeast, worms and flies. That description overstates the human case considerably, and the sirtuin story is the clearest example in longevity biology of a mechanism that is real while the popular version of it is not.
The Seven Sirtuins and What They Do
They differ in location, substrate and function, and lumping them together loses most of the information.
- SIRT1. Nucleus and cytoplasm. The most studied. Deacetylates histones, PGC-1alpha, FOXO transcription factors, p53 and NF-kB, which places it at the intersection of mitochondrial biogenesis, stress response and inflammatory signalling.
- SIRT2. Mainly cytoplasmic. Acts on tubulin and on metabolic enzymes, with roles in cell cycle and in myelination.
- SIRT3. Mitochondrial, and the principal mitochondrial deacetylase. Acts on enzymes of fatty acid oxidation, the electron transport chain and the antioxidant enzyme SOD2.
- SIRT4 and SIRT5. Mitochondrial, with SIRT5 acting mainly as a desuccinylase and demalonylase rather than a deacetylase, and both regulating amino acid and urea metabolism.
- SIRT6. Nucleus, chromatin-associated. Involved in DNA repair, telomere maintenance and genome stability. Its overexpression extends lifespan in mice, which is a stronger result than most of the family carries.
- SIRT7. Nucleolus. Regulates ribosome biogenesis and has roles in the DNA damage response.
The family shares a mechanism and a co-substrate requirement, and very little else.
Why NAD+ Dependence Is the Whole Point
Every sirtuin reaction consumes one molecule of NAD+, cleaving it and releasing nicotinamide. This is unusual: most enzymes that modify proteins use ATP or a methyl donor, and the choice of NAD+ makes sirtuins into energy sensors rather than merely enzymes.
NAD+ is the oxidised member of a redox pair, and its availability reflects metabolic state. Energy scarcity, fasting, exercise and caloric restriction shift the balance toward NAD+ availability. Nutrient abundance shifts it the other way. Sirtuin activity therefore rises in precisely the conditions that also inhibit mTORC1 and activate AMPK, which is why these three appear together in every account of nutrient-sensing biology.
The released nicotinamide is also a feedback inhibitor of the reaction, and NAD+ pools decline with age in multiple tissues. Both facts are the basis for the interest in NAD+ precursor supplementation, on the logic that raising the co-substrate raises the activity. That logic is sound as far as it goes and it is not the same as demonstrating a clinical outcome.
What Is Solidly Established
The biochemistry. The reaction, the NAD+ requirement, the substrates and the feedback inhibition are all well characterised. This part is not in dispute.
The signalling roles. SIRT1 deacetylation of PGC-1alpha promoting mitochondrial biogenesis, SIRT3 regulation of mitochondrial enzymes, SIRT6 involvement in DNA repair and genome stability: these are reproducible findings across laboratories.
Invertebrate lifespan. Sirtuin overexpression extends lifespan in yeast, worms and flies, though the magnitude of the original results was later revised downward substantially when genetic background was properly controlled.
SIRT6 in mice. Overexpression extends lifespan in mice, which is a mammalian result and therefore carries more weight than the invertebrate work.
Where the Popular Story Outran the Evidence
Four claims deserve direct correction.
"Sirtuins are the longevity genes." They are one node in a network that includes mTOR, AMPK, insulin/IGF-1 signalling and autophagy machinery. Calling any single family the longevity genes misrepresents an interconnected system, and the phrase persists mainly because it is memorable.
"Caloric restriction works through sirtuins." Proposed early and not sustained. Caloric restriction extends lifespan in animals lacking specific sirtuins, so the pathway is at most one contributor among several.
"Resveratrol activates SIRT1." The original assays used a fluorescent substrate, and the apparent activation turned out to be an artefact of that substrate rather than a property of the enzyme. Resveratrol has other actions, and direct SIRT1 activation is not well supported, which is a substantial part of why resveratrol's longevity case weakened over the following decade.
"Raising NAD+ activates sirtuins and therefore extends lifespan." Each step is plausible and the chain has not been demonstrated end to end in humans. NAD+ precursors do raise blood NAD+ measurably. Whether that translates into altered sirtuin activity in relevant tissues, and whether that alters aging trajectory, are both open.
What Follows Practically
Sirtuin biology mostly justifies things that are already justified for other reasons, which is a common and underappreciated outcome in this field.
Exercise raises NAD+ availability and activates the same nutrient-scarcity signalling. Energy deficit and fasting windows do likewise. Sleep matters because NAD+ metabolism is circadian, with the salvage pathway enzyme NAMPT under clock control. Excess alcohol consumes NAD+ through alcohol dehydrogenase, which is a small mechanistic argument for moderation on top of the larger ones.
Among supplements, NAD+ precursors, nicotinamide riboside and nicotinamide mononucleotide, raise the co-substrate and are the most direct route available. Their evidence supports the biochemical step and not yet a clinical outcome. Compounds sold as direct sirtuin activators, resveratrol foremost, rest on assay work that did not survive scrutiny.
The honest summary: sirtuins are a genuine and important part of how cells respond to energy state, and there is currently no intervention that reliably improves human outcomes by targeting them specifically.
The AEONNN Perspective
Sirtuins map to Cellular Energy and Repair, Pillar 1, and they illustrate why AEONNN separates the Mechanistic layer from the Evidence layer rather than allowing a mechanism to stand as evidence for a recommendation.
The mechanistic case for sirtuins is strong. The evidence case for any specific sirtuin-directed intervention in humans is weak. Those two facts belong in different layers of the Database Matrix precisely so that a compelling mechanism cannot be silently promoted into a recommendation. Where the two disagree, the Evidence layer governs what Insight Protocol will suggest, and the Mechanistic layer governs how it is explained.
This is also where Evidence Levels do work. A sirtuin-targeting compound sits at Level C, experimental and educational, regardless of how good the pathway diagram looks. The Innovation layer tracks it, at two percent of Matrix weight, which is the right weight for a signal that is interesting and not yet actionable.
Pillar Matrix mapping
Database Matrix layers
- Mechanistic Layer (KEGG, Reactome, UniProt)
- Evidence Layer (PubMed, Cochrane, ClinicalTrials.gov)
- Meta / Consensus Layer (JAMA, BMJ, specialty society positions)
- Innovation Layer (bioRxiv preprints, patent filings)
Frequently Asked
What are sirtuins in simple terms?
A family of seven enzymes that remove chemical tags from proteins and need NAD+ to do it. Because the reaction uses NAD+, their activity tracks cellular energy state, rising when energy is scarce and favouring repair and mitochondrial function.
Why are sirtuins called longevity genes?
Because overexpressing them extends lifespan in yeast, worms and flies, and SIRT6 overexpression extends lifespan in mice. The label overstates the case: sirtuins are one node in a network that includes mTOR, AMPK and autophagy machinery.
What is the difference between the seven sirtuins?
They differ in location and substrate. SIRT1 is nuclear and cytoplasmic and the most studied; SIRT3, 4 and 5 are mitochondrial; SIRT6 is chromatin-associated and involved in DNA repair; SIRT7 is nucleolar. They share a mechanism and little else.
Does resveratrol activate sirtuins?
Not directly, on current evidence. The original activation finding was an artefact of the fluorescent substrate used in the assay. Resveratrol has other actions, and direct SIRT1 activation is not well supported.
How is NAD+ related to sirtuins?
NAD+ is consumed as a co-substrate in every sirtuin reaction, which is what couples their activity to energy state. NAD+ pools decline with age, which is the basis for interest in precursor supplementation.
Can you increase sirtuin activity?
Indirectly, through the conditions that raise NAD+ availability: exercise, energy deficit, fasting windows and adequate sleep, since NAD+ salvage is circadian. No compound reliably raises sirtuin activity in human tissue with a demonstrated clinical outcome.
Does caloric restriction work through sirtuins?
Not primarily. Caloric restriction still extends lifespan in animals lacking specific sirtuins, so the pathway is at most one contributor among several.
Evidence and review
Any dosage ranges cited here reflect the ranges used in published human trials, not personal recommendations. Evidence in this field moves, so this article is reviewed quarterly and carries its last-updated date above. Nothing here is intended as medical advice, and supplementation should be discussed with a qualified clinician, particularly alongside prescribed medication or an existing condition.