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Supplements such as nicotinamide riboside and NMN that raise cellular NAD+ levels, with strong pharmacokinetic evidence and weak evidence of any functional benefit.
NAD+ precursors are compounds, chiefly nicotinamide riboside (NR) and nicotinamide mononucleotide (NMN), taken to raise cellular levels of nicotinamide adenine dinucleotide, a coenzyme that carries electrons in metabolism and is consumed as a substrate by several classes of enzyme. The rationale is that NAD+ declines with age in at least some tissues and that restoring it should restore the activity of the enzymes that depend on it. The supplements do raise NAD+ in blood reliably; whether that produces any benefit in humans is unresolved, and the field's commercial intensity considerably exceeds its evidence.
NAD+ has two distinct jobs. As a redox cofactor it cycles between NAD+ and NADH without being consumed, shuttling electrons through glycolysis and the mitochondrial electron transport chain. As a substrate it is destroyed: sirtuins cleave it to remove acyl groups from proteins, PARP enzymes consume it while responding to DNA damage, and the ectoenzyme CD38 degrades it outright. The second role is what links NAD+ to aging, because the consuming enzymes are all more active in aged tissue — more DNA damage means more PARP activity, and CD38 rises with the immune activation characteristic of Inflammaging and with the accumulation of senescent cells. Falling NAD+ availability is usually placed downstream of genomic instability and upstream of mitochondrial decline in the standard maps of the Hallmarks of aging.
The salvage pathway recycles nicotinamide back to NAD+ through NMN. NR enters one step earlier, converted to NMN by nicotinamide riboside kinases. Orally administered NMN is largely broken down in the gut and liver before reaching tissues, and how much of an oral dose ever reaches a muscle cell intact remains contested.
The decline is less certain than it sounds"NAD+ falls with age" is repeated as settled. NAD+ degrades within minutes of tissue collection, making measurement difficult, and while declines are well documented in aged mouse tissues, human data are mixed — several studies have failed to find an age-related fall in skeletal muscle. The premise of the whole intervention is less secure than its marketing implies.
Interest in NAD+ derives largely from sirtuins, a family of NAD-dependent enzymes proposed in the early 2000s as conserved longevity regulators and as the mediators of dietary restriction. That programme has aged poorly. The claim that resveratrol directly activates SIRT1 was traced to an artefact of the fluorophore-labelled peptide used in the assay, with activation disappearing when native substrates were used. Reported lifespan extension from sirtuin overexpression in worms and flies failed to replicate when background genotypes were controlled.1
Sirtuins remain real and important enzymes, but the case that they mediate any substantial part of mammalian aging is weaker than it was in 2006. The commercial and popular framing of NAD+ supplementation still rests heavily on the older story, promoted most visibly by David Sinclair, whose claims in this area have drawn sustained criticism from other biogerontologists.
Human studies divide cleanly into pharmacokinetics, where results are consistent, and outcomes, where they are not.
Oral NR raises blood NAD+ in a dose-dependent way, a result established in the first human pharmacokinetic study and replicated repeatedly.2 Beyond that, trials have been small, short, and heterogeneous. Six weeks of NR in healthy middle-aged and older adults raised NAD+ and, in exploratory analysis, was associated with lower systolic blood pressure and arterial stiffness in participants with elevated readings.3 In aged men, NR raised muscle NAD+ metabolites and lowered circulating inflammatory cytokines but produced no change in mitochondrial bioenergetics or physical function.4 Several trials in insulin-resistant and obese participants found no improvement in insulin sensitivity.
For NMN, the most cited result is a small trial in prediabetic postmenopausal women reporting improved muscle insulin sensitivity, with most other endpoints unchanged.5 Trials reporting improved walking distance or reduced fatigue have generally been small and short.
The mouse literature is far more encouraging. NAD+ repletion improves mitochondrial function, counteracts the decline of muscle stem cells, and improves several measures of tissue maintenance in aged mice. The gap between that literature and the human results is the central fact about this intervention. It is a familiar gap, shared with Senolytics and with circulating-factor therapies. Unlike Rapamycin, NAD+ precursors have not extended lifespan in the multi-site protocols that the field treats as the standard for rodent longevity claims.
NR is sold as a dietary supplement in the United States, with regulatory acknowledgment as a new dietary ingredient. NMN's status is disputed: US regulators concluded that because NMN had been authorized for investigation as a new drug, it is excluded from the dietary supplement definition, a determination that led major retailers to delist NMN products and prompted industry petitions that remained unresolved as of 2026. The rule at issue is the one that prevents a company from short-circuiting drug development by selling an investigational compound as a supplement; whether it was correctly applied here is contested, and the situation may have changed since.
The sector has also generated substantial litigation between suppliers over patents and advertising claims. None of that litigation has produced clinical evidence, and the commercial framing has done real damage to the field's credibility: consumers encounter NAD+ as an established anti-aging measure long before they encounter the trials.
NAD+ precursors have been well tolerated in trials at the doses studied, with mild gastrointestinal effects most commonly reported. Three theoretical concerns recur. High-dose nicotinamide consumes methyl groups during clearance, raising questions about methyl-donor availability. NAD+ is required for proliferation, and preclinical work has raised the possibility that supplementation could support the metabolism of existing tumours; this has not been tested in humans and remains unresolved. Finally, intravenous NAD+ infusions, sold by clinics, deliver a molecule that does not readily cross cell membranes and have no controlled evidence behind them.
Three questions determine whether this class amounts to anything. Whether tissue NAD+ actually falls in humans, measured properly and tissue by tissue. Whether raising it changes anything a person would notice, which requires trials with functional endpoints rather than blood levels or shifts in an epigenetic clock reading. And whether the more direct approach, inhibiting the enzymes that consume NAD+ and particularly CD38, works better than supplying more substrate.
The obstacle is the one that affects the whole field. Without accepted biomarkers of aging, a trial powered on healthspan outcomes takes years and costs more than any supplement company will spend, and the incentive to run it disappears once a product can be sold without it. That is the inverse of the problem facing generic drugs like metformin, where the trial is worth running and nobody profits from it.
Until such a trial exists, NAD+ precursors sit in the same category as several other popular geroprotectors: a coherent mechanism, an enormous consumer market, and, measured against exercise, no demonstrated effect on anything that matters. The Geroscience hypothesis does not require every plausible mechanism to work, and this one has had a long run without delivering.
paperBurnett, C. et al. "Absence of effects of Sir2 overexpression on lifespan in C. elegans and Drosophila." Nature, 2011. ↩
paperTrammell, S.A.J. et al. "Nicotinamide riboside is uniquely and orally bioavailable in mice and humans." Nature Communications, 2016.↩A pharmacokinetic study: it establishes that blood NAD+ rises after a dose, not that anything downstream of NAD+ changes.
paperMartens, C.R. et al. "Chronic nicotinamide riboside supplementation is well-tolerated and elevates NAD+ in healthy middle-aged and older adults." Nature Communications, 2018.↩The stated aims were tolerability and NAD+ elevation; the blood-pressure and arterial-stiffness findings are secondary and hypothesis-generating.
paperElhassan, Y.S. et al. "Nicotinamide riboside augments the aged human skeletal muscle NAD+ metabolome and induces transcriptomic and anti-inflammatory signatures." Cell Reports, 2019. ↩
paperYoshino, M. et al. "Nicotinamide mononucleotide increases muscle insulin sensitivity in prediabetic women." Science, 2021.↩Muscle insulin sensitivity was measured by clamp rather than by a surrogate index, but the trial was small and most other endpoints did not move.