NAD+, NMN and NR: what the longevity evidence actually shows

NAD+ is a coenzyme every cell needs to make energy and run its repair machinery, and its levels fall as we age. NMN and NR are two precursors sold to top it back up, and they do work at the first hurdle: good human trials show both reliably raise blood NAD+, often roughly doubling it. The honest problem is the next step. Raising the number in your blood has so far produced only modest, inconsistent functional benefits in people, nothing like the rejuvenation seen in mice. The molecular biology is real and interesting; the longevity promise is, for now, unproven.

What NAD+ actually does

Nicotinamide adenine dinucleotide, NAD+, is one of the most fundamental molecules in biology. It is a coenzyme, a helper that other enzymes cannot work without, and it sits at the centre of how cells turn food into usable energy. In its two forms, NAD+ and the reduced NADH, it shuttles electrons through the reactions of glycolysis, the citric acid cycle and the mitochondrial chain that produces ATP. Without enough of it, the cell's power supply falters.¹

NAD+ does more than carry electrons, though, and this is where the longevity interest comes from. A separate set of enzymes consumes NAD+ as a substrate, chewing it up to do their jobs. These include the sirtuins, a family of proteins involved in stress resistance and gene regulation; the PARPs, which orchestrate DNA repair; and CD38, an enzyme tied to immune signalling and inflammation. Every time these enzymes act, they break down a molecule of NAD+, so the cell has to keep making more. The pool is in constant flux, made and spent many times a day.¹²

Why it falls with age

Across many tissues, measured NAD+ levels drop as animals and people get older. The decline is not because the body forgets how to make NAD+, but because the balance between production and consumption shifts. Two changes stand out in the research. First, NAMPT, the enzyme that recycles spent nicotinamide back into NAD+ through the salvage pathway, becomes less active with age, so the recycling line slows. Second, levels and activity of CD38 rise with age, and CD38 is a voracious consumer of NAD+. A landmark mouse study showed that CD38 is required for much of the age-related NAD+ decline and the mitochondrial dysfunction that comes with it.²

There is also competition for the pool. PARPs, CD38 and the sirtuins all draw on the same NAD+ supply, so when DNA damage and inflammation rise with age and keep PARPs and CD38 busy, less NAD+ is left for the sirtuins. The logic of supplementation is simple: if the pool is shrinking, refill it. Whether refilling the pool actually reverses the downstream problems is the question the trials are meant to answer.²

How NMN and NR aim to raise it

You cannot usefully swallow NAD+ itself; it is a large, charged molecule that is broken down in the gut before it reaches cells. Instead the supplements supply precursors, smaller building blocks the body converts into NAD+ through the salvage pathway. The two headline precursors are nicotinamide riboside (NR) and nicotinamide mononucleotide (NMN). Both are close chemical relatives of vitamin B3. NR is converted to NMN inside the cell, and NMN is then converted to NAD+, so they feed into the same pipeline a step apart.¹

The route they actually take in humans turns out to be less direct than the textbook diagram suggests. A 2026 head-to-head trial found evidence that gut bacteria convert a portion of both NMN and NR into nicotinic acid, another NAD+ raw material, which is then absorbed and used. In other words the microbiome is part of the delivery system, which helps explain why two molecules that look different on paper end up raising blood NAD+ to a similar degree.³

What the human trials actually show

It is worth being precise here, because this is where marketing and evidence part company. Human trials fall into two buckets: does the supplement raise NAD+ (the biomarker), and does it actually change how people feel or function (the outcome that matters). On the biomarker, the evidence is genuinely solid. On function, it is thin and inconsistent.

Raising NAD+: this part works

Christopher Martens and colleagues, in a 2018 trial in Nature Communications of 24 healthy middle-aged and older adults, showed that 1,000 mg/day of NR raised blood NAD+ by around 60% and was well tolerated.⁴ For NMN, a University of Tokyo trial in 42 healthy men aged 65 and over found that 250 mg/day for 12 weeks significantly raised blood NAD+.⁵ The 2026 head-to-head study sealed the point: 1,000 mg/day of NMN or NR each roughly doubled NAD+ over 14 days, with no significant difference between them, while plain nicotinamide produced only a brief spike.³

Changing function: this part is shaky

The functional results are where enthusiasm should cool. The most cited positive human result is a 2021 Science trial led by Mihoko Yoshino and Samuel Klein: 25 prediabetic, overweight or obese postmenopausal women took 250 mg/day of NMN for 10 weeks, and the NMN group showed improved muscle insulin sensitivity on a gold-standard clamp test. It was a real, well-conducted finding, but it was small, in one narrow group, and other metabolic measures did not move.⁶ The Tokyo NMN trial reported modest gains in walking speed and left-hand grip strength, though right-hand grip did not improve, which is the kind of split result that warrants caution.⁵

NR has fared worse on function despite raising NAD+. Trials in men have generally failed to show improvements in muscle metabolism, and a 2025 pilot trial combining NR with exercise in older adults found that NR did not lower blood pressure any more than exercise plus placebo, even though NAD+ metabolites rose.⁷ The pattern across the literature is consistent: the biomarker goes up, the body does not reliably follow.

The MIB-626 programme

MIB-626 is a pharmaceutical-grade, microcrystalline form of NMN developed by Metro Biotech and studied at Massachusetts General Brigham, with David Sinclair among the named scientists. A 2022 trial in The Journals of Gerontology in overweight middle-aged and older adults showed that 1,000 mg once or twice daily safely raised blood NAD+ and its metabolites in a dose-related way.⁸ Crucially, that was a pharmacokinetic and safety study, not a longevity result. The same compound is being tested for specific medical uses, including a randomised trial in hospitalised COVID-19 patients with acute kidney injury, where it again raised NAD+ safely.⁹ These are drug-development trials with disease endpoints, a different thing from proving a supplement makes healthy people live longer.

The sirtuin connection and the hype

The reason NAD+ became a longevity obsession is the sirtuins. These enzymes need NAD+ to function, and in lower organisms boosting sirtuin activity can extend lifespan. The popular theory, associated most with David Sinclair, runs: ageing lowers NAD+, low NAD+ starves the sirtuins, sluggish sirtuins let ageing accelerate, so restoring NAD+ should reawaken the sirtuins and slow ageing. It is an elegant story and the individual links are real in the lab.¹

The catch is that an elegant mechanism is not the same as a proven effect in people. Resveratrol, the red-wine compound once sold on the same sirtuin logic, raised NAD+ and sirtuin interest for years but never delivered convincing human longevity or healthspan results. NAD+ precursors risk the same fate: a beautiful chain of reasoning that holds up in a dish and in mice but has not, so far, translated into people living longer or healthier lives. Treat the sirtuin narrative as a hypothesis under test, not an established route to longevity. For more on how to weigh this kind of mechanism-led claim, see our insights and the broader health library.

Safety and the cancer question

On short-term safety the news is reassuring. Across trials, NMN and NR have been well tolerated at the doses studied, typically up to 1,000 mg/day and as high as 2,000 mg/day of MIB-626, with no serious adverse events attributed to them. Reported side effects are usually mild: nausea, loose stools, headache or fatigue.³⁸ Long-term safety, over years rather than weeks, simply has not been studied in humans, so confident claims either way are premature.

The FDA wrangle and the UK picture

NMN's legal status in the United States has been a saga. In 2022 the FDA accepted an NMN new-dietary-ingredient notification, then reversed itself months later, ruling that because NMN had already been studied as an investigational drug (the Metro Biotech MIB-626 programme), it could no longer be sold as a supplement under the drug-exclusion clause. Industry groups petitioned and sued. In 2025 the FDA reversed again, accepting that NMN had been marketed as a supplement before the drug investigation and could remain on sale.¹¹ The whiplash tells you more about regulatory technicalities than about whether NMN works.

The UK picture is calmer and clearer. NR (as nicotinamide riboside chloride) is an authorised novel food in Great Britain, with a maximum recommended intake of 300 mg/day for adults and not for use in pregnancy or breastfeeding. These products are regulated as food supplements under food law, overseen by the Food Standards Agency, not as medicines by the MHRA. There are no authorised health claims for NAD+ precursors in the UK or EU, none are prescribed on the NHS, and they appear in no NICE guideline.¹⁰ If you are deciding whether a supplement earns a place in your routine, our stack builder can help you weigh it against better-evidenced options.

An honest verdict

So should you bother? Here is the balanced read. The mechanism is real: NAD+ matters, it declines with age, and NMN and NR genuinely raise it in your blood. That is more than can be said for many longevity products. But raising a biomarker is the easy part. The hard part, showing that doing so makes healthy people stronger, sharper or longer-lived, has not been achieved. The strongest functional signals are small, narrow and not consistently replicated, and the headline lifespan data is entirely from animals.

A reasonable position: these are not snake oil, but they are not the rejuvenation they are marketed as either. They are early-stage interventions with a sound rationale and unproven payoff. If you choose to try one, do so with clear eyes, modest expectations, a sensible dose, and the knowledge that exercise, sleep and not smoking remain far better evidenced for healthy ageing than any NAD+ capsule. If you are new to thinking about this systematically, our start here guide is a better first step than any supplement.

This article is educational and does not constitute medical advice, diagnosis, or a treatment recommendation. Medication uses described as “off-label” are not licensed for that purpose in the UK and should only be considered under qualified clinical supervision. Always speak to your GP, pharmacist, or a registered specialist before starting, stopping, or changing any treatment. If you have severe or alarm symptoms - unintentional weight loss, blood in your stool, difficulty swallowing, persistent vomiting, a fever, or severe pain - seek urgent medical care.