High-Dose NMN Raised NAD⁺ Levels in Older Adults, But Didn't Boost Hearing or Fitness

Nicotinamide mononucleotide, better known as NMN, has built a loyal following among people chasing healthier aging. The idea behind it is simple enough: NMN is a building block the body uses to make NAD⁺, a molecule involved in energy production and cellular repair that naturally declines with age. Animal studies have linked restoring NAD⁺ to improvements in various age-related conditions, which has fueled interest in whether the same might hold true for people. A new 24-week trial out of Japan, published in npj Aging, offers one of the more rigorous looks yet at what high-dose NMN actually does in older adults.

What the researchers tested

The study enrolled adults aged 65 and older and randomly assigned them to take either 1250 mg of NMN daily or a placebo, with neither participants nor researchers knowing who received which, for 24 weeks. That dose is notably higher than what's used in many prior human trials. The research team wanted to answer two main questions: does this level of supplementation meaningfully and durably raise NAD⁺ in the blood, and does it translate into measurable improvements in physical function?

To check the first question, they ran detailed metabolomic analysis on blood samples, tracking NAD⁺ and its related compounds throughout the trial. For the second, their main outcome of interest was auditory function, chosen because hearing decline is a common and measurable marker of aging, alongside a battery of physical performance tests.

NAD⁺ went up, reliably and substantially

On the biochemical side, the results were clear. Participants taking NMN showed a sustained, significant rise in blood NAD⁺ and related metabolites over the full 24 weeks, confirming that oral NMN at this dose is absorbed and converted into NAD⁺ in the body over the long term. This builds on earlier, shorter trials by showing the effect holds up with extended daily use.

The supplement was also generally well tolerated. The researchers didn't observe significant adverse effects tied to taking NMN, though they note that careful safety monitoring remains warranted given the high dose used and the relatively limited pool of long-term human data on NMN overall.

Where the benefits didn't show up

This is where the findings get more sobering for NMN enthusiasts. Despite the robust increase in NAD⁺, there was no significant improvement in auditory function, the trial's primary measure, and no meaningful change in physical performance tests either. In other words, raising NAD⁺ levels in the blood didn't translate into the functional improvements the researchers were specifically looking for.

Some secondary measurements, including aspects of body composition, blood lipids, and dietary habits, did show nominal differences between groups. But once the researchers accounted for the number of comparisons being made, a standard statistical correction to avoid false positives, none of these differences held up as statistically meaningful. The authors are careful to frame these as exploratory signals rather than real effects, suggesting they'd need to be tested directly in future studies before anyone draws conclusions from them.

Separating the biochemistry from the benefit

The gap between these two sets of results is the real story here. It's tempting to assume that if a supplement changes a biological marker, it must be doing something clinically meaningful. This trial pushes back on that assumption. NMN clearly did what it was designed to do at the molecular level, elevating NAD⁺ substantially and consistently. But that molecular shift didn't show up as a detectable benefit in hearing or physical performance over six months in this group of healthy older adults.

That doesn't necessarily close the door on NMN's potential. Hearing and physical performance are just two outcomes among many that aging research is interested in, and this trial doesn't address others, such as cognitive function or metabolic disease risk, that other studies have explored. It's also possible that benefits take longer than 24 weeks to appear, or that certain subgroups might respond differently. But based on what this trial actually measured, the evidence for broader physiological payoff from high-dose NMN remains thin.

What this means if you're considering NMN

If you've been taking or thinking about taking NMN supplements, this study is a useful reality check rather than a verdict. Raising a biomarker isn't the same as improving how you feel or function, and supplement marketing doesn't always make that distinction clear. Anyone considering NMN or other NAD⁺-boosting supplements should talk with their own doctor, particularly given the high doses used in research settings and the still-limited long-term safety data in humans.

Source: npj Aging, DOI: 10.1038/s41514-026-00482-7, based on a randomized controlled trial by Igarashi, Naruse, Takaoka, and colleagues.