# NAD+ Research: Mechanism, Precursor Trials and the Honest Gaps > NAD+ research summarized: the salvage pathway, the sirtuin–PARP–CD38 axis, dose-dependent blood NAD+ from NMN and NR trials, and why human efficacy for hard endpoints stays preliminary. Cited. Mechanism first, then the human precursor trials, then the gaps the literature itself flags. Every number is tacked to its source. ## Start here This page covers the NAD+ research in three layers. First, the biochemistry: how cells build and spend NAD+ (a fuel-handling helper molecule every cell uses to make energy). Second, the human trials of its precursors (building blocks the body turns into NAD+, mainly NMN and NR), which reliably raise the NAD+ level in blood. Third, the honest limits — because raising a blood number is not the same as proving a health benefit in people, and the literature says so plainly. Nothing here is medical advice or a dosing instruction; it is a summary of what specific studies measured. ## How cells make and spend NAD+ Cells synthesize NAD+ by three routes: de novo from the amino acid tryptophan, the Preiss-Handler pathway from nicotinic acid (niacin), and — dominant in mammals — the salvage pathway, which recycles nicotinamide back into NAD+ through the rate-limiting enzyme NAMPT [5]. NR (nicotinamide riboside) feeds in through a side door, converted to NMN by NRK kinases and then to NAD+, bypassing NAMPT entirely [5]. The spending side is where the aging story sits. NAD+ is consumed — not just cycled — by sirtuins, PARP1 and the CD38/CD157 ectoenzymes, which compete for a pool that shrinks with age [5]. Exercise pushes back: aerobic training induces NAMPT protein roughly 2-fold higher in athletes than in sedentary subjects, and transgenic NAMPT overexpression gives mice about 3-fold higher exercise endurance than wild-type [7]. The clock matters too — CLOCK:BMAL1 drives circadian expression of NAMPT, so intracellular NAD+ oscillates over 24 hours, with SIRT1 feeding back onto the Nampt promoter [6]. ## Nicotinamide Mononucleotide in Human Trials The strongest human signal for nicotinamide mononucleotide comes from two randomized trials. In a multicenter, double-blind, placebo-controlled, dose-ranging trial, healthy middle-aged adults took NMN at 300, 600 or 900 mg/day for 60 days; blood NAD+ rose significantly at days 30 and 60 across all groups versus placebo (p ≤ 0.001), walking distance improved, a biological-age measure did not increase, and 600 mg/day was identified as optimal with no safety issues at any dose [3]. The second is mechanistically tighter. In prediabetic, postmenopausal women, NMN 250 mg/day for 10 weeks increased muscle insulin sensitivity (measured by hyperinsulinemic-euglycemic clamp) and remodeled insulin signaling, with no change in body composition or HbA1c [1]. Both trials raised the NAD+ pool; neither claimed to treat a disease. ## What Studies Have Measured: Reported Outcomes Reported NAD+ benefits in the literature are best read as discrete measured endpoints, not a blanket claim. Oral NR raised whole-blood NAD+ by 22%, 51% and 142% at 100/300/1000 mg/day over 8 weeks, without elevating LDL cholesterol or disrupting one-carbon metabolism [4]. Oral NMN improved muscle insulin sensitivity at 250 mg/day [1] and improved walking distance at 300–900 mg/day [3]. The animal record is broader and more dramatic, which is exactly why it needs care. In mice, the NAD+ precursor nicotinamide riboside given before noise exposure protected cochlear ribbon synapses and aided hearing recovery [12]. In glaucoma-prone mice, oral nicotinamide was protective, with 93% of eyes at the highest dose not developing glaucoma [14]. Topical nicotinamide, in a review of human and laboratory evidence, reduced progression of skin-aging and pigmentation markers [11]. These are real findings in their own models — and several are mouse-only, which the literature does not let you forget. ## The CD38 axis and age-related decline If one enzyme explains why tissue NAD+ falls with age, the evidence points at CD38. CD38 deletion in mice preserves NAD+ levels and SIRT3 activity and improves mitochondrial function and metabolic health with age [2]. A potent, specific CD38 inhibitor reversed age-related metabolic dysfunction in aged mice by restoring tissue NAD+ decline through an SIRT3-dependent mechanism [8]. Even a dietary flavonoid does it: apigenin inhibits CD38 NADase activity (IC50 about 10.3 µmol/L in vitro) and, at 100 mg/kg IP daily in high-fat-diet mice, raised intracellular NAD+, improved glucose homeostasis and lowered hepatic triglycerides [10]. SIRT3 sits downstream — its loss impairs ATP production and antioxidant defense and tracks with disease pathology in model organisms [9]. ## Where the evidence stops The field's own reviews draw the line clearly. A 2025 narrative review of NAD+ precursor supplementation in human ageing concluded that human trials have shown limited efficacy, that age-related NAD+ decline has been consistently observed in only a limited number of human studies, and that data on tissue-specific NAD+ dynamics remain sparse [13]. The foundational reviews frame restoring NAD+ as a candidate strategy against age-related disease — a candidate, not a settled result [5]. So the honest summary is two-sided. Raising whole-blood NAD+ with oral precursors is reproducible and dose-dependent [4][3]. Translating that into proven longevity or disease prevention in people has not been demonstrated, and much of the strongest anti-aging data comes from rodents [13]. The injectable route, covered on the [NAD injection and IV NAD+](/nad-injection) page, has the weakest controlled evidence of all. Every figure cited above traces to the [full reference list](/references). --- A pinned-up research board on NAD+ — the redox coenzyme and its precursors NMN and NR tacked to their studies, the dose-dependent blood-NAD+ data marked confirmed and the unproven human endpoints flagged in red, with no clinic behind the board and nothing here infused, dispensed, or sold.