NAD+ is a cellular coenzyme, and its oral precursors NMN and NR are what the human trials actually measured.

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 and the NMN vs NR precursors 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 page and in the common questions about NAD+.

How This Digest Is Organized

The site is built around the distinctions the science keeps and search results blur. The human clinical trials page covers mechanism and the precursor trials in depth. The doses used in the research page reports what was administered, by which route, for how long — never as instruction. The NMN vs NR precursors page compares the two oral building blocks head to head. The IV NAD+ therapy research 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 page.