NAD+ declines roughly fifty percent by age sixty — here is what the clinical literature actually measures.
A clinician-facing digest of NAD+ coenzyme biology, IV infusion protocols, subcutaneous injection data, and the precursor pharmacology trial record — every quantitative claim cited.
What the NAD+ literature has established
NAD+ — nicotinamide adenine dinucleotide — is present in every living cell. It is the primary electron carrier in glycolysis and the TCA cycle, the required substrate for sirtuins (SIRT1–7) that regulate gene expression and mitochondrial biogenesis, and the fuel consumed by PARP1 at every DNA strand break.[13] These three functions are not separate: each one depletes the same cellular NAD+ pool.
Across human biopsy studies, blood NAD+ and NAD+ in skin tissue fall measurably with age, and PARP activity rises in inverse proportion.[22] In mouse genetic models, CD38 NADase expression — one of the three primary drivers of age-related NAD+ depletion — increases 2–3-fold across multiple tissues during chronological aging; knocking out CD38 preserved mitochondrial function and SIRT3 activity in aged mice.[1] A subsequent study identified senescent cells as an upstream cause: their inflammatory secretions activate CD38-expressing macrophages in liver and visceral fat, accelerating the drain.[2]
These mechanisms give researchers a precise pharmacological target. The clinical trial literature has now tested multiple routes of NAD+ repletion in humans: oral precursors NMN (nicotinamide mononucleotide) and NR (nicotinamide riboside), intravenous NAD+ infusion, and — in the most recent work — subcutaneous and intramuscular injection.
Blood NAD+ elevation by oral NMN and NR is now well-documented across at least eighteen completed human trials. The clinical-endpoint picture is more selective: skeletal muscle insulin sensitivity improved significantly in a Washington University RCT of NMN in postmenopausal women with prediabetes;[5] walking distance and VO₂max improved with NMN in middle-aged adults;[6][17] grip strength and gait speed improved in older men at 250 mg/day for 12 weeks.[7] Cognitive outcomes in mild cognitive impairment did not improve despite confirmed NAD+ elevation.[20]
IV NAD+ administration achieves plasma elevations orders of magnitude above oral dosing. A pharmacokinetic pilot study characterized the plasma and urine NAD+ metabolome during a 750 mg IV infusion in healthy males, establishing the first human IV NAD+ PK profile.[9] Addiction-treatment data from a 50-case series showed statistically significant reductions in craving, anxiety, and depression in substance use disorder patients receiving IV NAD+ alongside enkephalinase inhibition.[11]
The full injectable and IV evidence base is summarized in NAD+ IV therapy and NAD+ injection protocols. The mechanism, precursor pharmacology, and NMN/NR head-to-head data are in Research. The FAQ addresses NAD+ side effects and regulatory status.
NAD+ Benefits Documented in Clinical and Preclinical Research
The benefits documented in peer-reviewed research fall into four clusters, each tied to a specific NAD+-dependent pathway.
Energy metabolism and aerobic capacity. NMN at 300–1200 mg/day combined with exercise improved VO₂max and ventilatory thresholds dose-dependently in middle-aged amateur runners in a double-blind RCT (Liao et al. 2021).[17] The mechanism is direct: NAD+ is required for every NADH-generating step in the TCA cycle; depleting it slows mitochondrial ATP synthesis at the electron transport chain.
Skeletal muscle insulin sensitivity. NMN at 250 mg/day for 10 weeks significantly increased skeletal muscle insulin-stimulated glucose disposal in postmenopausal women with prediabetes vs placebo (Yoshino et al. 2021, Science).[5] Liver insulin sensitivity was unchanged — an important methodological note showing tissue-selective rather than systemic translation.
Physical function in older adults. A 12-week RCT of 250 mg/day oral NMN in healthy older men showed significant improvements in gait speed (p=0.033) and left-hand grip strength (p=0.019).[7] A multicenter 60-day RCT identified 600 mg/day as the optimal dose and reported improved walking distance vs placebo.[6]
DNA repair and rare aging diseases. NR supplementation in Werner syndrome patients (52-week RCT, n=9) produced a ~140% plasma NAD+ increase with improved arterial stiffness, HDL counts, and kidney function. In ataxia-telangiectasia trials, NR slowed neuromotor decline (Bohr 2025).[21] The mechanism involves PARP hyperactivation in DNA-repair-deficient cells consuming the NAD+ pool; repletion restores repair capacity.[13]
What the literature has not established: human lifespan extension, cognitive improvement in MCI,[20] or liver metabolic benefit separate from skeletal muscle.[5] The 2025 Nature Metabolism systematic review concluded that blood NAD+ elevation is consistent across trials, but translation to clinical aging endpoints is selective and incomplete — larger, longer RCTs with tissue-level measurements are needed.[19]
How Does NAD+ Decline With Age?
Three mechanisms drive the age-related fall in tissue NAD+.
CD38 upregulation. CD38 is an ectoenzyme that cleaves NAD+ to produce calcium-signaling metabolites. Its expression rises 2–3-fold across liver, skeletal muscle, and adipose tissue with chronological aging in mice (Camacho-Pereira et al. 2016).[1] The upstream driver: inflammatory SASP cytokines secreted by senescent cells accumulating with age activate CD38-expressing macrophages, accelerating the drain.[2]
NAMPT decline. NAMPT is the rate-limiting enzyme in the NAD+ salvage pathway, converting nicotinamide back to NMN. Its expression falls with age in skeletal muscle but is reversible — aerobic and resistance exercise training increased NAMPT protein 12–30% in human muscle biopsies, with proportionally larger gains in older adults (de Guia et al. 2019).[23]
PARP competition. PARP1 consumes NAD+ stoichiometrically at every DNA strand break; severe DNA damage can deplete up to 90% of the cellular NAD+ pool. With age, accumulated DNA damage load increases baseline PARP demand. In aged human skin biopsies, PARP activity correlated inversely with NAD+ level — the first direct human tissue evidence for PARP-driven depletion.[22]
Do supplements reverse the decline? Oral NMN and NR consistently restore blood NAD+ in human RCTs.[3][4][6][8][16] The 2025 Nature Metabolism review by Vinten et al. confirmed this consistency while noting that the functional reversal question remains under active investigation.[19]
NAD+ elevation is consistently achievable. Translation to clinical endpoints requires larger, longer trials.
Vinten et al., Nature Metabolism 2025