Published in Nature Metabolism, the study found that whole-blood NAD+ levels remained stable across multiple age groups and lifestyle interventions, challenging the idea that a blood test could use the molecule as a reliable marker of aging.
“Our findings provide important clarity in a field marked by conflicting reports, hype and commercial promises,” lead authors Maria Trętowicz and Riekelt Houtkooper of the University of Amsterdam wrote in a research briefing accompanying the study.
“Pragmatically, they indicate that whole-blood NAD+ levels cannot be used to track the aging process in humans and also provide new insight into the regulation of systemic NAD+ pools, which seem to be maintained within a narrow physiological range and are possibly buffered against variation. Consistent with this, whole-blood NAD+ levels do not reflect tissue-specific changes, such as those observed in aging muscle.”
More broadly, they added, the findings underscore the need to avoid oversimplifying or generalizing across distinct compartments of human metabolism—but do not imply that whole-blood NAD+ lacks biological relevance.
Substantial impact of small details
The study used a rigorously validated assay to analyze whole-blood samples from more than 300 people across independent cohorts that included younger and older adults, elite athletes, frail older adults undergoing diet or exercise interventions, and participants receiving supplemental nicotinamide riboside—a form of vitamin B3—to boost NAD+.
Across the cohorts, the researchers observed no meaningful link between whole-blood NAD+ and age, nor did they report any major shifts after lifestyle interventions including exercise and protein-rich diets. They did however note an expected rise in blood NAD+ in the supplementation group.
Importantly, the study highlighted that discrepancies in laboratory handling as much as biology may also influence inconsistencies in the scientific literature.
NAD+ measurements were highly sensitive to how blood samples were stored and handled, where freezing and repeated freeze-thaw cycles could substantially reduce measured levels.
“I applaud the authors for how they have drawn attention to small details in sample prep that may have a substantial impact on downstream measurements, which will be important to the field,” Lindsay Wu, an associate professor at the University of New South Wales Sydney and metabolic biochemist specializing in NAD+, shared in the research briefing.
“Collating data from diverse studies is an important attempt to provide robustness for the field; however, it should be noted that each of these studies is quite small in size … These smaller cohort studies could obscure any real changes.”
The University of Amsterdam researchers acknowledged the moderate cohort sizes used and the need for larger longitudinal studies.
Concentration of circulating NAD+: ‘That convenience has been overinterpreted’
NAD+, short for nicotinamide adenine dinucleotide, has become a major focus in longevity science because of its broad role in energy production and other core cellular processes at the mitochondria level.
Because mitochondrial dysfunction is a key driver of biological aging and contributes to multiple hallmarks of aging, a growing market has emerged around compounds that might preserve mitochondrial function to improve human healthspan. Among these are NAD+ and its precursors nicotinamide riboside (NR) and nicotinamide mononucleotide (NMN), which are proposed to restore cellular NAD+ pools, enhance metabolic resilience and mitigate age-associated physiological decline.
Commenting independently on the study, Mark Tarnopolsky, a professor at McMaster University known for his work on mitochondrial disorders, exercise physiology and aging, said that although findings do not invalidate tissue NAD biology, they do indeed strongly challenge the use of circulating NAD+ as a surrogate biomarker of downstream outcomes.
While he acknowledged ongoing debate about whether NAD+ availability declines in some human tissues with aging and whether this contributes meaningfully to impaired mitochondrial homeostasis, he explained that the biologically relevant factor is not simply the concentration of circulating NAD+ but the redox state within cells and organelles—especially the mitochondrial balance between NAD+ and its reduced form, NADH. It is this chemical ratio that helps determine how efficiently cells produce energy and reflects their overall metabolic and energetic health.
“In practice, these measurements are difficult to obtain in humans, which has led many studies to rely on blood, plasma or serum NAD+ as accessible surrogate markers,” he said. “That convenience has been overinterpreted. Human trials consistently show that NR and NMN increase circulating NAD-related metabolites, but this should be regarded as pharmacodynamic target engagement rather than evidence of improved mitochondrial function, biological age or healthspan.”
Dr. Tarnopolsky added that this distinction matters because the efficacy data for NAD+ precursors have been underwhelming, with the strongest human studies showing “reliable biochemical engagement” but “little convincing evidence of reproducible, clinically meaningful functional benefit.”
Evidence he cited includes several studies led by researchers from the University of Copenhagen, which found no meaningful effects of NR supplementation on endpoints including muscle mitochondrial function and insulin sensitivity in obese men. The studies did not rule out the possibility that NR could have effects in other populations with differences in age, sex or pre-existing diseases.
NAD+ for measurable health benefits
Andrew Shao, PhD, senior vice president of global regulatory & scientific affairs at leading NAD+-boosting ingredient supplier Niagen Bioscience, said that the University of Amsterdam findings are not new to the NAD+ research community but noted that the study adds important rigor to an evolving area of NAD+ science and reinforces the need for a more nuanced understanding of how NAD+ functions throughout the body.
“While the findings suggest that whole-blood NAD+ may not be a standalone biomarker of aging, the study conflicts with several other studies which did observe an inverse association between age and whole blood or blood cell NAD+ levels,” he added. “So, while the study attracted many headlines, it’s not the first to examine the relation between blood NAD+ and age. It did also confirm that NR supplementation raises blood NAD+, thereby validating its pharmacological action.”
One of those headlines,“New Research Upends the Argument for a Popular Longevity Supplement”, featured in The New York Times. In that article, Dr. Shao commented that messages spread by influencers have been oversimplified and that broader clarification is needed to communicate evidence that NAD+ levels decline in tissue and not the blood.
Niagen Biosciences, formerly known as Chromadex, launched its Niagen-branded nicotinamide riboside chloride ingredient in 2013 as the first nicotinamide riboside to market. At the time, the company cited beneficial effects in humans including increased fatty acid oxidation, increased mitochondrial activity, resistance to negative consequences of high-fat diets, protection against oxidative stress, prevention of peripheral neuropathy and blockage of muscle degeneration.
Dr. Shao noted that there is a body of research showing that supplementation with NAD+ precursors such as Niagen may support healthier aging by helping maintain cellular processes tied to energy production, DNA repair, inflammation response and tissue function.
“The clinical research on Niagen, specifically, covers outcomes well beyond NAD+ levels as a biomarker—including improved physical function in patients with peripheral artery disease, reduced airway inflammation in COPD and consistent anti-inflammatory signals across at least eight published human studies in diverse populations,” he said.
“It’s also important to note that research on Niagen continues to mature, with 45+ peer-reviewed human clinical trials and growing evidence across specific populations and health contexts.”
Ultimately, he added, the field should focus not only on changes in NAD+ levels—where blood NAD+ may merely serve as a measure of exposure, but also on whether those changes translate into measurable health benefits.
Source: Nature Metabolism. doi: 10.1038/s42255-026-01537-5. "Human whole-blood NAD+ levels do not vary with age or lifestyle interventions". Authors: Maria Tretowicz et al.
