NAD+ After 40: What the Evidence Really Says About Boosting It

Walk down the supplement aisle and you will find bottles promising to restore your NAD+. The pitch is everywhere: as you age your NAD+ falls, and topping it back up can slow aging itself. NMN and NR are the two biggest sellers, and urolithin A is the newer mitochondrial angle.

This pillar is the evidence-based version. It explains what NAD+ is, whether the decline is even real, what the human trials show, and where the marketing runs far ahead of the science.

The short answer up front: NAD+ precursors do reliably raise NAD+ in the blood, but after more than a decade of human trials, no supplement has shown it can slow aging or deliver a meaningful health outcome. Most trials have come back null on the outcomes that matter.

The one-sentence summary

NAD+ is a molecule every cell uses for energy and repair, its levels do seem to fall in some tissues with age, and supplements like NMN and NR do raise NAD+ in the blood, but human trials have so far failed to show these supplements produce real health benefits, so the anti-aging marketing greatly exceeds what the data support.

What NAD+ is (and what it does)

NAD+ stands for nicotinamide adenine dinucleotide. It is a small molecule present in every living cell, and it does two broad jobs.

First, it shuttles energy. NAD+ carries electrons between reactions, which is how your cells turn food into usable energy in the mitochondria. Without it, that machinery stops.

Second, it acts as a fuel for enzymes that repair DNA and quiet inflammation. Families of proteins called sirtuins and PARPs consume NAD+ while they work, as does the enzyme CD38. This is the part that interests aging researchers: these are the systems that respond to stress, handle DNA damage, and are thought to support cellular health over time [4].

The core theory is simple. If NAD+ falls with age, and the enzymes that depend on it are needed to keep cells healthy, then restoring NAD+ might restore those systems. The biology is real. Whether it translates into human health is a different question, and that is where the evidence gets thin [5].

Does NAD+ really decline with age? (the honest data)

The idea that NAD+ falls with age comes mostly from animals. A widely cited 2013 study in Cell found that NAD+ declines in multiple tissues of aging mice, including muscle, and linked that drop to mitochondrial problems [1]. That finding seeded most of the modern interest.

Human data are far less clean. One study of human blood found the NAD metabolome shifts with age, with several NAD-related compounds lower in older people [2]. But a 2020 review by researchers who study NAD+ was blunt: the human evidence is limited, the results are not consistent, and much of the “decline” narrative leans on rodent work [3].

Several things muddy the picture. NAD+ can be measured in blood, tissue, or cells, and the numbers do not always move together. Blood levels may not reflect what is happening in muscle or brain. Different labs and methods give different results. And the steep decline you see in the marketing charts does not match what most human studies find, which is a smaller and more variable change [3][5].

This matters, because most people decide whether to buy a supplement after seeing a blood test from a commercial lab. Those tests are not standardized, they do not tell you what is happening in your tissues, and no doctor or guideline uses a single NAD+ number to make treatment decisions [5]. Treat the “check your NAD+” pitch with suspicion.

The precursors: NMN, NR, and what the human trials show

NAD+ is too big to absorb from a pill. So supplements give you a smaller molecule your body can build it from. These are called precursors.

Nicotinamide riboside (NR) and nicotinamide mononucleotide (NMN) are the two most popular, and both are essentially refined forms of vitamin B3. Plain niacin and nicotinamide from your diet are also NAD+ precursors, and far cheaper. The extra marketing is about NMN and NR specifically [6].

Biologically, NR converts into NMN, which converts into NAD+. The two are close cousins rather than rivals with clearly different effects [6].

Here is the honest comparison:

	Nicotinamide riboside (NR)	Nicotinamide mononucleotide (NMN)
Human trial history	Longer and larger	Shorter and smaller
Effect on blood NAD+	Reliably raises it	Reliably raises it
Proven health benefit	Not established	Not established
Regulatory status	Sold freely as a supplement	Complex US history (see below)

The landmark human trial of NR was published in Nature Communications in 2018. It gave healthy middle-aged and older adults NR and found it was well tolerated and clearly raised NAD+ in the blood [7]. A separate study confirmed NR boosted NAD+ inside aged human muscle and shifted some muscle gene activity [11].

NMN arrived in human trials later. A 2022 study in healthy older men given 250 mg per day found blood NAD+ rose, with mixed signals on muscle function [8].

So both work at the level they are designed to work: they raise NAD+. That is the part the marketing leads with.

The uncomfortable part is what happens next. Raising NAD+ in the blood has not translated into meaningful health outcomes in the trials done so far.

A 2026 systematic review in Ageing Research Reviews pulled together 33 human intervention studies and 80 rodent studies. In rodents, NAD+ boosting often improved metabolic, mitochondrial, and inflammatory outcomes. In humans, the reviewers found effects on function, metabolism, and healthspan-related measures were “heterogeneous and often null or endpoint-specific,” and that clinical effectiveness for anti-aging or wellness outcomes “remains inconclusive” [10].

A 2026 meta-analysis of NMN randomised trials reached a similar place. Fifteen trials, doses from 250 mg to 2,000 mg a day, durations up to 24 weeks. NMN was well tolerated, but it had no significant effect on body weight, blood sugar, cholesterol, or systolic blood pressure. The authors concluded that broad metabolic benefits were “not evident” [9].

Put plainly: the supplements reliably change a blood biomarker, and the biomarker does not yet translate into a health benefit anyone can measure. That gap is the whole story of this market.

Urolithin A: the different (mitochondrial) angle

Urolithin A is often grouped with NMN and NR, but it works differently. It is not an NAD+ precursor at all.

Where NAD+ supplements try to refill a depleted fuel tank, urolithin A targets a process called mitophagy, the way cells recycle worn-out mitochondria. As you age that recycling slows, and the theory is that urolithin A switches it back on. The goal is healthier mitochondria rather than more NAD+ [12].

The human evidence is early and thin. A 2026 systematic review and meta-analysis could find only five small randomised trials, and the only outcome it could combine across studies was the six-minute walk test, which did not reach statistical significance [12]. Several of the positive trials were funded by the company that makes the main product.

We cover urolithin A in depth in its own article in this series. For the NAD+ story, the point is simple: it is a separate mechanism with separate, still-unproven claims.

What has evidence vs the marketing

Let us be direct about what has evidence and what does not.

The marketing says	The evidence supports
NAD+ falls steeply with age in everyone	A variable decline, mostly shown in animals; human data are limited and inconsistent [3]
Blood tests tell you your NAD+ and guide treatment	Tests are unstandardised and do not reflect tissue levels [5]
NMN and NR slow or reverse aging	They raise NAD+ in blood; no trial shows they slow aging [10]
NAD+ boosters improve energy, muscle, metabolism	Results are largely null or endpoint-specific in humans [9][10]
NAD+ IV drips are powerful anti-aging therapy	No randomised outcomes trial exists for IV NAD+ [10]

Where does that leave you? The honest answer is that lifestyle has far stronger evidence for protecting metabolic and physical health as you age than any NAD+ product.

Regular exercise, a diet without excess calories, good sleep, and not smoking support the very systems that NAD+ researchers study, with decades of human evidence behind them. If you want to protect cellular and metabolic health, these remain the tools with the best track record.

That is not to say NAD+ precursors are useless or dangerous. They are generally well tolerated in the short term [7][9]. But “well tolerated” is not the same as “works,” and no dose, brand, or “NMN complex” has been shown to beat placebo on a health outcome you would notice.

Safety & the NMN regulatory reality

Short-term safety is the one thing the data support. Across trials, NMN and NR have not increased serious side effects or liver enzymes at the doses tested, over periods of weeks to a few months [9]. That is real but narrow: there is no long-term safety data over years of daily use, and these are not FDA-approved drugs [10].

The regulatory story around NMN is worth knowing because it affects what you can even buy.

In 2022 the US Food and Drug Administration took the position that NMN could not be sold as a dietary supplement, on the grounds that it had been the subject of substantial investigation as a drug. That effectively removed NMN from the US supplement market, though it remained available elsewhere.

After petitions and a federal lawsuit, the FDA reversed course. In September 2025 the agency confirmed that NMN may lawfully be marketed as a dietary supplement in the US, accepting evidence that it had been sold that way since at least 2017. NR never faced this restriction, because it is treated as a form of vitamin B3.

Two things to keep straight. First, “lawful to sell as a supplement” does not mean the FDA evaluated NMN for safety or effectiveness. Supplements are not reviewed the way drugs are. Second, no company is allowed to claim that NMN, NR, or any product prevents or treats a disease, and an “anti-aging” claim is marketing, not an FDA-approved statement of benefit.

The honest bottom line

NAD+ is a real and important molecule, and the science behind why it matters is sound. But there is a wide gap between the biology and the products built on it.

What we know: NAD+ supports cellular energy and repair, its levels appear to fall in some tissues with age, and NMN and NR reliably raise it in the blood.

What we do not know: whether raising it produces a health benefit in humans. After more than a decade of trials, the answer is still no clear, consistent clinical effect, and several recent reviews say so in plain language [9][10].

If you are over 40 and wondering whether to spend on an NAD+ booster, the reasonable position is this. There is no proven downside to short-term use for most healthy adults, and there is also no proven upside beyond a blood test. If you buy one, buy it knowing it is an experiment on yourself, not a treatment with evidence.

Protect your health the way the evidence supports: move your body, eat well, sleep, and keep expectations honest. The supplement can sit at the margins. It does not replace the basics, and no amount of NAD+ will make up for neglecting them.

This article is for general information and is not medical advice. It is not a substitute for a doctor’s opinion, and it does not endorse any product. Talk to a clinician before starting any supplement, especially if you take medication or have a health condition.

FAQ

What is a normal NAD+ level, and should I test mine?

There is no established “normal” NAD+ range that doctors use to guide treatment. Commercial blood tests report a number, but the tests are not standardised and blood levels do not reliably reflect what is happening in your tissues. Most researchers regard a single NAD+ blood test as not clinically meaningful [3][5].

At what age do NAD+ levels decline?

The decline has been reported mainly in animal tissues, and human data are limited and inconsistent. Some human studies find NAD-related compounds are lower in older people, but the timing, size, and even the reality of a human decline are not settled [2][3]. The steep “declines by decade” charts in the marketing oversimplify a messy picture.

Is NMN or NR better?

Neither has been shown to beat the other, and there is no trial showing either slows aging. NR has a longer and larger human history; NMN has fewer, shorter trials. Both raise blood NAD+ reliably, and both lack evidence of a meaningful health benefit [6][9][10].

Do NMN or NR have side effects?

In short-term trials they appear well tolerated, with no clear increase in serious side effects or liver problems at the doses studied [9]. There is little long-term data, so no one can promise safety over years of daily use, and neither is an FDA-approved treatment [10].

Why was NMN banned and then unbanned in the US?

In 2022 the FDA said NMN could not be sold as a supplement because it had been investigated as a drug. After industry petitions and a lawsuit, the FDA reversed that decision in September 2025 and confirmed NMN is lawful in dietary supplements. Being lawful to sell is not the same as being proven effective or reviewed for safety.

Can I raise my NAD+ without supplements?

Your body makes NAD+ from vitamin B3, which a normal diet provides, and niacin and nicotinamide are cheap NAD+ precursors [6]. The bigger point is that the lifestyle habits with the strongest evidence for metabolic and physical health as you age, exercise, diet, and sleep, are the same ones worth prioritising regardless of your NAD+ level.

References & sources (12)

References

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2. Clement J, Wong M, Poljak A, Sachdev P, Braidy N. The Plasma NAD(+) Metabolome Is Dysregulated in “Normal” Aging. Rejuvenation Res. 2019;22(2):121-30. PMID: 30124109.
3. McReynolds MR, Chellappa K, Baur JA. Age-related NAD(+) decline. Exp Gerontol. 2020;134:110888. PMID: 32097708.
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5. Bhasin S, Seals DR, Migaud ME, Musi N, Baur JA. Nicotinamide Adenine Dinucleotide in Aging Biology: Potential Applications and Many Unknowns. Endocr Rev. 2023;44(6):1047-73. PMID: 37364580.
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7. Martens CR, Denman BA, Mazzo MR, et al. Chronic nicotinamide riboside supplementation is well-tolerated and elevates NAD(+) in healthy middle-aged and older adults. Nat Commun. 2018;9(1):1286. PMID: 29599478.
8. Igarashi M, Nakagawa-Nagahama Y, Miura M, et al. Chronic nicotinamide mononucleotide supplementation elevates blood nicotinamide adenine dinucleotide levels and alters muscle function in healthy older men. NPJ Aging. 2022;8(1):5. PMID: 35927255.
9. Yang W, Huang J, Tang Z, Chen C, Sun Y. Safety and Metabolism-Related Outcomes of Oral Nicotinamide Mononucleotide Supplementation in Adults: A Systematic Review and Meta-Analysis. Nutrients. 2026;18(14):2251. PMID: 42514320.
10. Gallagher C, Emmanuel OO. NAD(+) supplementation for anti-aging and wellness: A PRISMA-guided systematic review of preclinical and clinical evidence. Ageing Res Rev. 2026;116:103057. PMID: 41655607.
11. Elhassan YS, Kluckova K, Fletcher RS, et al. Nicotinamide Riboside Augments the Aged Human Skeletal Muscle NAD(+) Metabolome and Induces Transcriptomic and Anti-inflammatory Signatures. Cell Rep. 2019;28(7):1717-28. PMID: 31412242.
12. Dao T, Nguyen TT, Gariani K, Kim HJ, Ryu D. Effects of Urolithin A supplementation on muscle health outcomes in humans from randomized controlled trials. Front Nutr. 2026;13:1834344. PMID: 42559151.