# NAD+: The Coenzyme, the Precursors, and What Human Trials Measured > NAD+ is an endogenous redox coenzyme marketed as a dietary supplement. Oral precursors raised blood NAD+ by up to 142% in trials; IV NAD+ is compounded and unapproved. A cited digest. Oral precursors reliably raise blood NAD+. IV NAD+ is rapidly cleared and unapproved. Every figure here is cited, every route is labeled, and the honest gaps are foregrounded. ## The short version NAD+ (a fuel-handling helper molecule every cell uses to turn food into energy) is not a drug and not one product on a shelf. It is a coenzyme (a helper molecule an enzyme needs to do its job) your body already makes, and its level in tissue falls as you age. Most things sold as a "NAD supplement" are not NAD+ at all — they are precursors (building blocks the body converts into NAD+; NMN and NR are the common ones), because NAD+ itself is too large and charged to absorb well by mouth. In controlled human trials, those precursors raised blood NAD+ by measurable amounts. Whether that translates into the benefits people hope for is still mostly unproven [7]. This page reviews the evidence; it is not advice and recommends no dose. ## What is NAD+? NAD+ (nicotinamide adenine dinucleotide) is the cell's central redox coenzyme — a molecule that shuttles electrons through glycolysis, the TCA cycle, and oxidative phosphorylation to make ATP, the cell's energy currency [5]. "Redox" simply means chemistry that moves electrons to release energy; NAD+ is the oxidized form that accepts an electron to become NADH, and NADH hands it back in the mitochondria to drive ATP synthesis. It is present in every living cell and is synthesized from vitamin-B3-family inputs and from the amino acid tryptophan [6]. Beyond carrying electrons, NAD+ is a *consumed* substrate for three families of signaling enzymes: sirtuins (cellular-maintenance enzymes that cannot work without NAD+), PARPs (DNA-repair enzymes), and CD38 (an enzyme that breaks NAD+ down) [5]. Those enzymes spend NAD+ as they work, so the cell must keep resynthesizing it. Tissue NAD+ declines with age, partly because CD38 activity rises and consumes more of the pool [2]. That decline is the central rationale behind every product in this category — and the reason researchers test whether *raising* NAD+ changes anything measurable. ### What does NAD stand for? NAD stands for nicotinamide adenine dinucleotide. NAD+ is the oxidized form of the coenzyme and NADH is the reduced form; they are the same molecule interconverting as it carries electrons [5]. ### Is NAD a peptide? No. NAD+ is a dinucleotide coenzyme — nicotinamide mononucleotide joined to adenosine monophosphate — not a peptide [5]. It is a single endogenous metabolite, not a chain of amino acids, which is why the chemistry and the regulatory picture differ from research peptides. ## NAD supplement: what the research describes A NAD supplement, in practice, almost never delivers NAD+ itself. Oral NAD+ is poorly taken up by cells intact [13], so the products that have controlled human evidence behind them are *precursors* — nicotinamide riboside (NR), nicotinamide mononucleotide (NMN), and the older niacin/nicotinamide forms — which the body converts into NAD+ through the salvage and Preiss-Handler pathways [6]. The pharmacodynamic readout that trials use is whole-blood NAD+, because sampling tissue NAD+ in living humans is invasive and rare [4]. On that readout, oral precursors perform reliably: an 8-week randomized trial of NR raised whole-blood NAD+ by 22%, 51%, and 142% at 100, 300, and 1000 mg/day respectively, dose-dependently and with no significant adverse-event difference from placebo [4]. A separate multicenter trial of NMN (300-900 mg/day for 60 days) significantly raised blood NAD+ at days 30 and 60 across every dose group versus placebo [3]. ### Why most oral products are precursors NAD+ is a large, charged dinucleotide (molecular weight 663.43 Da), and that size and charge make it poorly orally bioavailable — it is not freely taken up intact by most cells [13]. So oral products use precursors instead: NMN sits one biochemical step from NAD+, and NR is converted to NMN by NRK kinases before reaching NAD+ [6]. This is why the [NMN vs NR precursors](/nmn-vs-nr) distinction matters, and why describing an oral-NMN or oral-NR trial as "taking NAD+" is inaccurate. ## What the literature has measured (so-called 'benefits') The honest framing is *measured findings*, not promised benefits. Two things are well established. First, oral precursors raise blood NAD+ in a dose-dependent way [4][3]. Second, tissue NAD+ declines with age across model organisms and humans, and that decline tracks with metabolic dysfunction [1][5]. Functional endpoints are more mixed. In prediabetic, postmenopausal women, 10 weeks of oral NMN at 250 mg/day significantly improved muscle insulin sensitivity, though it changed neither body composition nor HbA1c [8]. In the NMN dose-ranging trial, walking distance and quality-of-life scores improved versus placebo, and a biological-age measure did not increase [3]. A 2025 *Nature Metabolism* review of the human clinical evidence is the most current synthesis, and its conclusion is sober: human trials have shown limited efficacy for hard clinical endpoints, the age-related NAD+ decline has been consistently observed in only a limited number of human studies, and tissue-specific NAD+ data remain sparse [7]. Raising blood NAD+ is real. Translating that into longevity or disease prevention in humans is, as of now, unproven [7]. For the underlying studies, see [what the research has measured](/research); for the tolerability record and the injectable-route risks, see [NAD+ safety](/nad-safety). ## Common questions ### What is NAD supplement used for? NAD+ is an endogenous redox coenzyme; products marketed as NAD supplements are typically precursors (NMN, NR, niacin) studied for their ability to raise blood NAD+, which declines with age [4][1]. This summarizes research, not a recommendation to use any product. ### What does NAD do for the body? NAD+ carries electrons through glycolysis, the TCA cycle, and oxidative phosphorylation to make ATP, and it is a consumed substrate for sirtuins, PARPs, and CD38 — enzymes that govern DNA repair, gene regulation, and inflammation [5][8]. ### Is NAD just vitamin B3? No. NAD+ is built from vitamin-B3-family precursors (niacin, NR, nicotinamide), but it is itself a dinucleotide coenzyme, not a vitamin [6]. The precursors feed NAD+ synthesis through the salvage and Preiss-Handler pathways. ### What does NAD mean in medical terms? In biochemistry, NAD means nicotinamide adenine dinucleotide — the cell's central redox coenzyme and a substrate for NAD-consuming signaling enzymes [5]. The acronym can denote unrelated clinical phrases elsewhere, but here it refers to the coenzyme. See [common questions about NAD+](/faq) for more. --- A safety-first reel of the NAD+ literature — what the trials cut to, where the evidence drops off, and nothing dispensed, prescribed, or sold.