NMN clinical trials: a sober reading of the human evidence

Few molecules in the longevity conversation have been studied in humans as quickly as nicotinamide mononucleotide, usually shortened to NMN. The animal work arrived first and was striking enough to draw a wave of human trials. Those trials have now reported, and a small number of reviews and meta-analyses have begun to pool them. This is a good moment to ask a plain question. What have the human studies of NMN actually found, and how far does that evidence really reach? The honest answer is more measured than the enthusiasm around the molecule would suggest.

What NMN is

NMN is a precursor to NAD+, a coenzyme that every cell uses to carry out the chemical reactions of metabolism. NAD+ sits at the centre of how cells turn food into usable energy and participates in a wide range of repair and signalling processes. The body makes NMN from forms of vitamin B3 and from a compound called nicotinamide, and the small amounts found in food are part of normal nutrition. When NMN is taken as a supplement, the working idea is straightforward. Supply more of the precursor, and the body has more raw material from which to make NAD+. Whether that translates into anything a person would notice is a separate question, and it is the question the trials were designed to probe.

Why NMN is studied

The interest in NMN comes from two directions. The first is the observation that NAD+ levels appear to fall in many tissues as animals, and probably humans, grow older. If a decline in NAD+ contributes to features of ageing, then restoring it becomes an obvious thing to test. The second is the body of animal work. In mice, raising NAD+ with precursors such as NMN has been associated with improvements in measures of metabolism, vascular function and physical performance. Those results were promising enough to justify human trials, but it is worth stating clearly that benefits seen in mice do not reliably carry across to people. The history of ageing research is full of interventions that looked compelling in animals and faded under the more demanding conditions of a human trial.

What the human trials have found

The most consistent finding across the human trials is also the most basic one. Taking NMN tends to raise NAD+ in the blood. Several controlled studies have measured a clear, dose-related rise in NAD+ or its related compounds in blood after a period of supplementation, and a 2023 update in Advances in Nutrition summarising the clinical record reports this rise as one of the more dependable results in the literature. A 12-week placebo-controlled trial published in Scientific Reports in 2023 likewise documented an increase in NAD+ metabolism markers in participants taking NMN. So the first link in the proposed chain, that swallowing the precursor raises the measured level of the coenzyme in blood, does appear to hold.

The second consistent finding concerns safety. Across the trials conducted so far, NMN has generally been well tolerated, with adverse events that were mild and broadly similar between the supplement and placebo groups. A 2022 randomised controlled trial in GeroScience reported that NMN was safe and well tolerated over its study period, and the broader reviews echo this, while being careful to note that the trials have mostly been short and the number of people studied remains modest. Good short-term tolerability is reassuring, but it is not the same as established long-term safety, and the reviews are explicit about that distinction.

Beyond raising NAD+ and being tolerated, the picture becomes less clear. A frequently cited 2021 study in Science found that NMN improved muscle insulin sensitivity in a group of prediabetic women, an outcome of genuine interest. But it was a single, relatively small trial in a specific population, and a result in one such study is a starting point for further work rather than a conclusion. Other trials have looked at measures such as physical performance, blood pressure, blood lipids and markers of metabolic health, with results that have been mixed and often modest.

The important caveats

Several features of this evidence base call for caution. The trials have generally been small, enrolling tens rather than hundreds of participants, and short, often running for weeks to a few months. Small, short trials can detect a clear biochemical change such as a rise in blood NAD+, but they are poorly suited to detecting modest effects on health, and they say nothing about what happens over years.

There is also the matter of what is being measured. A rise in blood NAD+ is a surrogate marker. It tells you the supplement is doing something measurable inside the body, but it does not by itself tell you that a person will be healthier, function better or live longer. The outcomes that matter to people are harder to measure and require larger, longer trials. The gap between moving a marker and changing an outcome is where many promising interventions come undone, and NMN has not yet bridged it.

This is where the pooled analyses are most useful, because they look across the individual trials rather than at any one of them. A 2024 meta-analysis in Critical Reviews in Food Science and Nutrition brought the available trials together and found that, while NMN reliably raised NAD+ and was well tolerated, most of the clinical outcomes examined did not reach statistical significance. A separate 2024 meta-analysis in Current Diabetes Reports, focusing on glucose and lipid measures, reached a similarly restrained conclusion, finding little consistent effect on those metabolic markers across the pooled data. In other words, when the studies are combined, the firm signal is the biochemical one. The clinical signal is, so far, weak and inconsistent.

What the evidence does and does not show

It is worth being precise, because precision is what this topic usually lacks. The human evidence does show, with reasonable consistency, that taking NMN raises NAD+ in the blood, and that over the short periods studied it has generally been well tolerated. Those are real findings and they are not trivial. They establish that the basic premise, that an oral precursor can shift the measured level of the coenzyme, is sound.

What the evidence does not yet show is that this biochemical change reliably produces meaningful improvements in human health, function or ageing. The trials are small and short, many of the outcomes are surrogate markers rather than the things people actually care about, and the meta-analyses that pool the data find most clinical outcomes are not significant. None of this proves NMN does nothing. It simply means the human case is unproven and the longer, larger trials that would settle it have not yet been done. The reasonable position is interest tempered by patience. NMN is a serious subject of research rather than a settled one, and the most accurate thing that can be said is that the science is still in progress.

Further reading

Continue reading from the journal: What is NAD+? A plain-English guide to the cell’s busiest coenzyme and The NMN question: when a moving biomarker is not the same as slower ageing.

Sources

• Song Q, Zhou X, Xu K, Liu S, Zhu X, Yang J. The Safety and Antiaging Effects of Nicotinamide Mononucleotide in Human Clinical Trials: an Update. Advances in Nutrition, 2023. doi:10.1016/j.advnut.2023.08.008
• Yi L, Maier AB, Tao R, et al. The efficacy and safety of beta-nicotinamide mononucleotide (NMN) supplementation in healthy middle-aged adults: a randomised, multicentre, double-blind, placebo-controlled, parallel-group, dose-dependent clinical trial. GeroScience, 2022. doi:10.1007/s11357-022-00705-1
• Yoshino M, Yoshino J, Kayser BD, et al. Nicotinamide mononucleotide increases muscle insulin sensitivity in prediabetic women. Science, 2021. doi:10.1126/science.abe9985
• Katayoshi T, Uehata S, Nakashima N, et al. Nicotinamide mononucleotide and NAD+ metabolism: a 12-week trial. Scientific Reports, 2023. doi:10.1038/s41598-023-29787-3
• Zhang R, et al. A systematic review and meta-analysis of NMN supplementation outcomes. Critical Reviews in Food Science and Nutrition, 2024. doi:10.1080/10408398.2024.2387324
• Chen L, et al. Effects of NMN supplementation on glucose and lipid outcomes: a meta-analysis. Current Diabetes Reports, 2024. doi:10.1007/s11892-024-01557-z