NAD+ Decline

NAD+ is a coenzyme required for energy metabolism, DNA repair, and as a substrate for sirtuins and PARP enzymes. Rodent tissue studies consistently show NAD+ falling by roughly 50% or more between young and old age in liver, muscle, and brain, driven by reduced synthesis and increased consumption by the enzyme CD38 [1]. This decline is mechanistically linked to reduced sirtuin activity and mitochondrial dysfunction in mice [1].

NAD+ precursors nicotinamide mononucleotide (NMN) and nicotinamide riboside (NR) raise blood NAD+ metabolite levels in human RCTs; a 2023 trial found NMN supplementation (300-900 mg/day for 60-90 days) safely raised NAD+ levels and improved some fatigue and exercise measures in middle-aged and older adults, though effects on hard clinical endpoints were not assessed [2]. However, a 2025 whole-blood NAD+ measurement study reported no significant association between NAD+ levels and chronological age or major lifestyle factors in a general adult sample, directly challenging the assumption that NAD+ decline is a universal, easily detected human aging signature outside of rodent tissue [3].

What the evidence does not show: no RCT has demonstrated that raising blood NAD+ via NMN or NR changes mortality, and the discrepancy between consistent rodent tissue findings and inconsistent human blood findings suggests blood NAD+ may not track tissue NAD+ well, or that the age-decline pattern seen in rodents does not generalize cleanly to humans [3].

Bryan Johnson's Blueprint protocol has included NAD+ precursor supplementation among its stack, and Blueprint's Biomarkers platform lists NAD+-related metabolic markers among the panels it offers, framed explicitly as one data point among more than 100 biomarkers rather than a validated aging clock on its own [4].

Charles Brenner, a biochemist who helped characterize NR metabolism, has publicly pushed back on marketing claims that link NMN/NR supplementation directly to lifespan extension, noting that human trials to date measure biomarker and metabolite changes, not survival, and that the 2025 blood NAD+ study undercuts the simple "NAD+ declines with age, so raise it" narrative used to sell these supplements [3]. A separate fingerstick-assay project mapping real-world NAD+ variation across a large consumer-testing population also found substantial individual variability not cleanly explained by age alone, consistent with the 2025 cohort finding that simple chronological decline is not the dominant signal in human blood [5]. Commentary responding to that data argued explicitly that a blood NAD+ test cannot currently tell someone their biological age, given how much the readout depends on assay method, time of day, and recent activity rather than a stable underlying trait [6].

The plain takeaway: NAD+ biology is real and rodent decline is well documented, but human blood NAD+ measurement is not yet a reliable aging biomarker, and no product has shown it extends human life.