# NAD+: A Cited Reading of the Coenzyme and Its Precursor Research > NAD+ is a redox coenzyme every cell uses to make energy and to fuel sirtuins, PARPs and CD38. Oral precursors NMN and NR raised whole-blood NAD+ up to 142% in randomized trials. A cited digest. A precise reading of the nicotinamide adenine dinucleotide literature: the redox chemistry, the precursor pharmacology, the metabolic and muscle endpoints, and the honest gap between raising blood NAD+ and proving a clinical outcome — every figure cited. ## The short version NAD+ (a fuel-handling helper molecule every cell uses to turn food into energy) is short for nicotinamide adenine dinucleotide. It is a coenzyme (a helper molecule an enzyme needs to do its job), not a drug and not a single product you buy. It also powers a set of cellular-maintenance enzymes — sirtuins, PARPs and CD38 — that handle DNA repair, gene control and inflammation. Tissue NAD+ falls as we age. Because the NAD+ molecule itself is large and poorly absorbed by mouth, most oral products supply a precursor (a building block the body converts into NAD+ — NMN and NR are the common ones). This page summarizes what published studies measured. It is not medical advice, and it gives no dosing instructions. ## What Is NAD+? NAD+ is the cell's central redox carrier — "redox" being the chemistry that shuttles electrons to release energy. It moves electrons through glycolysis, the TCA cycle and oxidative phosphorylation to make ATP, the cell's energy currency [5]. Every time a cell breaks down glucose or fat for fuel, NAD+ collects the freed electrons and becomes NADH; the electron transport chain then strips those electrons back off to power ATP synthesis, regenerating NAD+ for another pass. The pool is small and turns over constantly, which is why supply and consumption both matter. The same molecule is a consumed substrate for three families of signaling enzymes: sirtuins (a family of cellular-maintenance enzymes that cannot work without NAD+), PARPs (DNA-repair enzymes), and CD38 (an enzyme that breaks NAD+ down and rises with age) [5][2]. Unlike its redox role, these reactions actually cleave the molecule, so signaling literally spends NAD+ rather than recycling it [5][13]. Structurally, NAD+ is a dinucleotide: two nucleotides — a nicotinamide ring and an adenine ring — joined by a two-phosphate bridge, molecular formula C21H27N7O14P2, molecular weight 663.43 Da (CAS 53-84-9). That size and charge are exactly why it is poorly absorbed by mouth, and why oral products supply smaller precursors instead. NAD+ is synthesized inside every human cell from tryptophan (the de novo route), from nicotinic acid (the Preiss-Handler route), and chiefly through the salvage pathway, which recycles nicotinamide back into NAD+ via the rate-limiting enzyme NAMPT [5]. Tissue NAD+ declines with age across yeast, worms, mice and the human studies surveyed to date [5][12]. ## What the Research Measures: NAD+ and Its Precursors The most reliable human finding is pharmacodynamic, not clinical: oral precursors raise whole-blood NAD+, dose by dose. Nicotinamide riboside (NR) at 100, 300 and 1000 mg/day for 8 weeks raised whole-blood NAD+ by 22%, 51% and 142% respectively in healthy overweight adults, with no flushing and no significant adverse-event difference from placebo at any dose [4]. A multicenter, double-blind trial of NMN (nicotinamide mononucleotide) at 300, 600 and 900 mg/day for 60 days raised blood NAD+ at days 30 and 60 across every dose group versus placebo (p≤0.001) and identified 600 mg/day as the optimal dose [3]. What "NAD+ benefits" means in the literature is therefore specific and measured, not promotional. In prediabetic, postmenopausal women, 10 weeks of oral NMN at 250 mg/day improved muscle insulin sensitivity on a hyperinsulinemic-euglycemic clamp — the reference method for measuring how well muscle responds to insulin — with no change in body composition or HbA1c [1]. In amateur runners, NMN at the same 250 mg/day dose for 12 weeks raised ventilatory thresholds during treadmill testing, read as better skeletal-muscle oxygen use [7]. None of these are oral NAD+ trials — they are precursor trials, because NAD+ itself is poorly taken up intact. The distinction matters: a study that gave NMN or NR is not a study of "taking NAD+." See the [muscle and metabolic findings](/research) and the [NMN vs NR precursors](/nmn-vs-nr) comparison. ## Why Tissue NAD+ Falls With Age Age-related NAD+ decline is partly a consumption problem. CD38, an NAD-consuming ectoenzyme, rises with age and inflammation and is a principal driver of the fall in tissue NAD+ [2]. In mice, deleting CD38 preserved NAD+ levels and SIRT3 activity and improved mitochondrial function and metabolic health with age [2]. Three enzyme families compete for the same NAD+ pool — sirtuins, PARP1 and CD38 — so anything that raises consumption or lowers synthesis lowers the available coenzyme [5][13]. The synthesis side ages too. NAMPT, the rate-limiting salvage enzyme, falls in skeletal muscle with age, which lowers the cell's capacity to recycle nicotinamide back into NAD+ [9]. So the decline is two-sided: more demand from a rising CD38 burden, less supply from a falling NAMPT. Notably, the synthesis side is partly recoverable through exercise, which raises NAMPT in human muscle [9]. This is the rationale behind precursor supplementation: if the salvage pathway can be fed more substrate, blood NAD+ rises measurably [4][3]. Whether that translates into the downstream outcomes people hope for is a separate, and more open, question — covered on the [human clinical trials](/research) page and in the [common questions about NAD+](/faq). ## How This Digest Is Organized The site is built around the distinctions the science keeps and search results blur. The [human clinical trials](/research) page covers mechanism and the precursor trials in depth. The [doses used in the research](/dosage) page reports what was administered, by which route, for how long — never as instruction. The [NMN vs NR precursors](/nmn-vs-nr) page compares the two oral building blocks head to head. The [IV NAD+ therapy research](/iv-nad) page reads the injectable route against its evidence and its documented quality risks. The FAQ answers the twenty-two most-asked questions directly. Each carries the citations behind its claims. ## What This Site Is Medicine NAD is an independent editorial digest of the peer-reviewed NAD+ literature. It does not sell anything, does not provide medical advice, and does not recommend any product or dose. NAD+ is not an FDA-approved drug; it is sold as a dietary supplement, and its oral products are usually precursors. The regulatory picture has live edges: some forms, notably NMN, face an unsettled FDA challenge to their dietary-supplement status [12], and intravenous and injectable NAD+ is an unapproved compounded wellness therapy with documented quality risks — read here strictly as research, not as a treatment offered or endorsed. Every quantitative claim on this site maps to a numbered citation; the full list, with DOIs, is on the [study references and DOIs](/references) page. --- A ruled reading of the NAD+ literature — the coenzyme kept distinct from its precursors, the measured separated from the unproven; not a clinic, not a vendor, not a prescription.