# NAD+ and Aging in the Research Literature | NAD+ Digest > NAD+ and aging: tissue NAD+ falls 10-50% over adult life as CD38 and PARP rise and salvage drops. Human skin biopsy and mouse evidence, summarized and cited. Why tissue NAD+ falls with age, which enzymes drive the decline, and what raising it has — and has not — been shown to do in humans. ## The short version NAD+ and aging is the headline reason anyone cares about this molecule. The fact behind it is real: NAD+ (the cell's fuel-handling coenzyme) drops as we get older — by an estimated 10% to 50% across tissues — partly because an enzyme called CD38 (which destroys NAD+) climbs with age, and partly because DNA-repair enzymes spend more of it [10]. That decline has been measured directly in human skin [7]. What has *not* been shown is that topping NAD+ back up reverses aging in people. The decline is documented; the cure is not. ## Tissue NAD+ declines with age — measured directly in humans The clearest human evidence comes from skin. In biopsies spanning a range of ages, NAD+ and NADH levels were lower in older skin than younger, and PARP activity was higher and correlated inversely with NAD+ — the first quantitative human-tissue evidence tying oxidative stress and PARP activation to age-related NAD+ depletion [7]. Across the broader literature, a review documented NAD+ decline in skin, brain, liver, skeletal muscle, plasma and macrophages, estimating drops of roughly `10-50%` over adult aging while noting that human data are limited relative to rodent data and that measurement methods vary substantially [10]. The decline is real and reproducible; its exact magnitude is method-dependent. ## Three mechanisms: CD38 up, PARP up, NAMPT salvage down Reviews converge on three causes of the age-related fall [10]. First, **CD38** — an NAD-consuming ectoenzyme — rises with age and inflammation; CD38-knockout mice are protected against age-related NAD+ decline and retain better mitochondrial function and SIRT3 activity [2]. Second, **PARP** activation by accumulating DNA damage consumes more NAD+, as the human-skin data show [7][13]. Third, **NAMPT** — the rate-limiting enzyme of the salvage pathway that recycles nicotinamide back into NAD+ — declines, reducing synthesis [10]. Senescent cells add to the load: their inflammatory secretions activate CD38+ macrophages, accelerating tissue NAD+ loss. Falling NAD+ has been linked to **pseudohypoxia** — disrupted communication between the nucleus and mitochondria — proposed as a route from low NAD+ to mitochondrial dysfunction [5]. ## Sirtuins are the aging link — and they are NAD+-limited The reason NAD+ decline matters for aging biology is the **sirtuins**. SIRT1-7 are NAD+-dependent enzymes that regulate metabolism, DNA repair and lifespan in model organisms, and their activity is rate-limited by intracellular NAD+ [6]. When NAD+ falls, sirtuin output falls with it. In mice, SIRT1 activation — through caloric restriction or NAD+-precursor regimens — extends lifespan and improves metabolic health [6]. This is the mechanistic spine of the "restore NAD+" hypothesis: if low NAD+ throttles the maintenance enzymes, replacing it should release the brake. The hypothesis is well supported in animals [5][6]. ## Does NAD make you look younger? No trial has shown that NAD+ or its precursors reverse visible aging. Human skin biopsies show NAD+ falls and PARP activity rises with age [7], but raising blood NAD+ has not been demonstrated to change appearance. The strongest anti-aging data — extended lifespan, restored tissue function — remain in rodents [6][9]. A 2025 review concluded human efficacy data for precursors in aging are still limited [14]. The decline is documented in people; the cosmetic or longevity payoff is not. ## The honest gap: biomarker proven, outcomes not Raising blood NAD+ with oral precursors is well demonstrated [3][4]. Translating that into slower aging, disease prevention or longer life in humans is not [14]. Much of the strongest anti-aging evidence comes from rodents and may not extrapolate. A 2025 *Nature Metabolism* review 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+ dynamics remain sparsely characterized — a call for more clinical work, not a verdict that precursors work [14]. This page reports that gap plainly rather than papering over it. --- An orbital reading of the NAD+ literature — the coenzyme at the core, its precursors NMN and NR held apart from the molecule itself, and the human, rodent and gap evidence each logged to source; no clinic behind the console and nothing here infused, dispensed or sold.