# NAD+ and Aging in the Research Literature — What the Studies Measured | NAD+ > NAD+ and aging: tissue NAD+ declines with age as CD38 rises. A cited digest of the human-tissue, rodent, and precursor evidence on age-related NAD+ decline. Tissue NAD+ falls with age, CD38 rises, and the salvage pathway loses ground — the energy meter draining, read study by study. ## The short version Here is the core of **NAD+ and aging** in plain terms. NAD+ (the energy-handling helper molecule every cell uses) sits at a lower level in older tissue than in younger tissue — picture a power meter that drains as the decades pass. Part of why: an enzyme called CD38 (a molecule on cell surfaces that *eats* NAD+) becomes more active with age and inflammation, while the cell's main recycling route loses capacity. In mice, blocking CD38 or supplying NAD+ building blocks pushes the meter back up. In people, NAD+ does measurably fall, but proof that refilling it slows aging is still thin. This page reads that evidence honestly — the strong rodent findings and the limited human ones, kept apart. ## Tissue NAD+ declines with age — the human evidence The age-related fall in NAD+ is not just a mouse phenomenon. In one of the earliest direct human-tissue studies, skin biopsies showed lower NAD+ and NADH in older donors than younger ones, and PARP activity — a major NAD-consuming, DNA-damage-triggered process — rose with age and correlated inversely with NAD+ [6]. That made [age-related NAD+ decline](/nad-and-aging) measurable in human tissue, not just inferred. The pattern recurs in muscle. In biopsies from 119 older men across three populations, sarcopenia (age-related muscle loss) tracked with a signature of mitochondrial dysfunction, fewer mitochondria, and low NAD+ driven by perturbed NAD+ biosynthesis and salvage [12]. A foundational review tied these threads together: declining NAD+ with age, observed from yeast to humans, is linked to metabolic dysfunction and disease susceptibility, with sirtuins, PARP1, and CD38 the enzymes drawing down the pool [5]. ## Why the meter drains: CD38 rises with age The clearest mechanistic culprit is CD38, an NAD-consuming ectoenzyme that climbs with age and inflammation. In mice, CD38 was identified as the principal enzyme dictating age-related NAD+ decline through a SIRT3-dependent mechanism, and CD38-knockout mice were protected — they preserved NAD+ levels, SIRT3 activity, mitochondrial function, and metabolic health into old age [2]. A follow-up showed a specific CD38 inhibitor reversed age-related metabolic dysfunction in aged mice by restoring tissue NAD+ [9]. Natural CD38 inhibitors point the same way. Apigenin, a dietary flavonoid, blocked CD38's NADase activity with an IC50 of 10.3 micromol/L in vitro and raised intracellular NAD+ in a CD38-dependent manner; in high-fat-diet mice at 100 mg/kg, it improved glucose homeostasis and lowered hepatic triglycerides [10]. These are mechanism and rodent data — informative about *why* NAD+ falls, not proof of a human anti-aging treatment. ## Refilling the pool: what raised NAD+ in older adults If CD38 and salvage decline drain the meter, the obvious question is whether precursors refill it. They do, at least in blood. In a multicenter double-blind RCT, oral NMN at 300, 600, and 900 mg/day for 60 days dose-dependently raised blood NAD+ in healthy middle-aged adults versus placebo, alongside improved walking distance and quality-of-life scores; 600 mg/day was identified as the optimal dose and no safety issues arose [3]. Nicotinamide riboside raised whole-blood NAD+ by up to 142% at 1000 mg/day over eight weeks [4]. Exercise restores salvage capacity without a supplement: aerobic and resistance training raised NAMPT — the rate-limiting salvage enzyme — by 12-30% in human skeletal muscle, with proportionally larger gains in older adults [7]. The honest ceiling on all of this is human *outcomes*: a 2025 review of NAD+ precursor supplementation in aging concluded that human trials have shown limited efficacy, that age-related NAD+ decline has been confirmed in only a limited number of human studies, and that tissue-specific NAD+ data remain sparse [15]. ## Does NAD make you look younger? Tissue NAD+ declines with age, including in human skin [6], and rodent studies link restoring NAD+ to anti-aging effects [2][9]. But a human cosmetic or longevity benefit is unproven: a 2025 review found human efficacy data still limited [15]. This digest reports the measured biology, not a beauty or anti-aging claim. --- An arcade-console reading of the NAD+ literature — the coenzyme logged apart from its precursors NMN and NR, the blood-NAD+ the trials actually moved scored against the gaps the rodent and IV data leave open; no clinic behind the console, nothing here infused, ordered, or sold.