# NAD+ Research: What the Coenzyme and Precursor Studies Measured > NAD+ research summarized: the salvage pathway, CD38 and age-related decline, NMN and NR human trials, and the IV NAD+ pilot data, every figure cited to source. Mechanism, the precursor trials that reliably raise blood NAD+, and the thinner IV evidence — each finding tagged by species and route, each number cited. ## The short version This page reads the NAD+ research in plain order. NAD+ (the helper molecule cells use to turn food into energy) drops with age, and the studies test whether feeding the body a **precursor** (a building block it converts into NAD+ — NMN and NR) puts the level back up. The clear result: oral NMN and NR reliably raise the NAD+ measured in blood. The unsettled part: whether that translates into the health outcomes people hope for. Injected and IV NAD+ have the least controlled evidence of all. Every claim below names its study. ## Why Precursors? The Salvage and Preiss-Handler Pathways An NAD+ precursor is a molecule the cell converts into NAD+. Three routes do the building. The **salvage pathway** is dominant in mammals: it recycles nicotinamide back into NAD+ through NMN, with NAMPT (nicotinamide phosphoribosyltransferase) as the rate-limiting enzyme [5]. **Nicotinamide riboside** enters a step downstream — NRK kinases convert NR to NMN, then NMNAT finishes NMN to NAD+ — a route that bypasses the Preiss-Handler steps [5]. The **Preiss-Handler pathway** builds NAD+ from nicotinic acid (niacin), and de novo synthesis starts all the way back at the amino acid tryptophan [5]. This is why "NAD+ supplements" are precursors. NAD+ is large and charged and is not freely imported by cells; a precursor is small enough to be absorbed and then assembled into NAD+ inside the cell. Isotope tracing in mice added a twist: oral nicotinamide riboside appears to reach host NAD+ partly via gut bacteria that convert host nicotinamide to nicotinic acid, a host-microbiome cycling of NAD+ precursors [11]. ## How tissue NAD+ falls with age: CD38, sirtuins, and PARPs NAD+ is not only burned for energy; it is consumed by signaling enzymes that compete for the same pool. Sirtuins (SIRT1-7) are NAD+-dependent deacylases governing metabolism and stress resistance; PARP1 is a DNA-repair enzyme that consumes large amounts of NAD+ when DNA damage activates it; and CD38 is an NAD+-consuming ectoenzyme [5]. CD38 is the key age variable. In mice, CD38 is the principal NAD+-consuming enzyme whose activity rises with age, driving the age-related fall in tissue NAD+; CD38-knockout mice are protected from that decline, preserving NAD+ and SIRT3 activity and improving mitochondrial function with age [2]. A foundational review frames the whole picture: declining NAD+ with age is linked to metabolic dysfunction and disease susceptibility across yeast, worms, mice, and humans, with sirtuins, PARPs, and CD38 as the consuming enzymes that draw the pool down [5]. ## What NAD+ Research Has Measured Set apart from "NAD+ benefits" claims, here is what specific human trials measured. In prediabetic, postmenopausal women, 10 weeks of oral NMN at 250 mg/day significantly increased muscle insulin sensitivity, assessed by hyperinsulinemic-euglycemic clamp, and remodeled insulin signaling — with no change in body composition or HbA1c [1]. In healthy middle-aged adults, NMN at 300-900 mg/day for 60 days raised blood NAD+ dose-dependently and improved six-minute walking distance and quality-of-life scores versus placebo, with 600 mg/day identified as the optimal dose and no safety issues at any dose [3]. In healthy overweight adults, NR at 100-1000 mg/day for 8 weeks raised whole-blood NAD+ by 22%, 51%, and 142% without elevating LDL cholesterol or disrupting one-carbon metabolism [4]. The boundary is just as important. A 2025 *Nature Metabolism* review of the human clinical evidence concluded that trials have shown limited efficacy for hard endpoints, that age-related NAD+ decline has been consistently observed in only a limited number of human studies, and that tissue-specific NAD+ data remain sparse [14]. Raising blood NAD+: established. Curing or reversing anything: not what these studies claim. ## NMN (Nicotinamide Mononucleotide) as an NAD+ Precursor NMN is a direct NAD+ precursor one biochemical step from NAD+, and it carries the most-replicated human dosing among the oral boosters. The 250 mg/day muscle-insulin-sensitivity result in prediabetic women is the most-cited single-dose finding [1]. The 300/600/900 mg/day multicenter trial added dose-response: blood NAD+ rose at days 30 and 60 across all groups versus placebo (p ≤ 0.001), and a biological-age measure did not increase over the 60 days [3]. NMN's preclinical reach is broad — in mice, NMN rescued photoreceptor death caused by genetic deletion of the NAD+ enzyme NAMPT, identifying NAD+ biosynthesis as essential for vision [10]. NMN's regulatory status is contested in the United States: the FDA has taken the position that NMN is excluded from the dietary-supplement definition because it was authorized for investigation as a drug. Read that as a marketplace category dispute, not a safety ban or a finding of harm [14]. ## Nicotinamide Riboside (NR): The Most Clinically Studied Precursor Nicotinamide riboside is a vitamin-B3-family NAD+ precursor that NRK kinases convert to NMN and then to NAD+, and it is the most clinically studied oral NAD+ booster. The defining human result is dose-scalable: NR at 100, 300, and 1000 mg/day for 8 weeks raised whole-blood NAD+ by 22%, 51%, and 142% respectively, with elevation maintained throughout the study and no significant adverse-event difference from placebo — no flushing, no LDL elevation, no disruption of one-carbon metabolism [4]. NR's preclinical record spans more than metabolism. In a mouse noise-exposure model, NR given before exposure preserved cochlear hair-cell ribbon synapses and aided recovery from noise-induced hearing loss via enhanced oxidation resistance [13]. As covered on [NAD IV therapy](/nad-iv-therapy), a real-world retrospective comparison also found IV NR was far better tolerated than IV NAD+ [15]. ## Nicotinamide and topical NAD+ biology Nicotinamide (niacinamide) replenishes the cellular NAD+ pool and has its own clinical literature, largely topical. A review of human and laboratory evidence reports that topically applied nicotinamide reduced the progression of skin aging and hyperpigmentation in clinical trials while being well tolerated, acting as an NAD+-restoring, antioxidant, barrier-enhancing, and depigmenting cosmeceutical [12]. This is a skin effect of a topical precursor — not evidence that oral or IV NAD+ produces systemic anti-aging. ## NAD+ vs NMN: The Molecule vs Its Precursor NAD+ vs NMN is not a contest between two interchangeable products; it is the coenzyme versus its building block. NAD+ is the finished dinucleotide the cell uses; NMN is one biochemical step upstream [5]. Because NAD+ itself is large, charged, and poorly absorbed orally, NMN is taken instead — small enough to absorb, then converted to NAD+ inside the cell. So when a study reports an oral-NMN result, it is measuring a precursor's effect on blood NAD+ [1][3], not the effect of swallowing NAD+. Keeping that distinction exact is the difference between reading the literature and reading the marketing. ## The IV NAD+ studies: small, preliminary, and metabolism-heavy Intravenous NAD+ has the weakest controlled evidence of any route, and the studies that exist are mostly pilots and case series. A pharmacokinetic pilot infusing 750 mg over 6 hours in healthy men found plasma NAD+ undetectable for roughly the first 2 hours before rising late, with urinary methylated nicotinamide metabolites climbing — evidence of extensive extracellular metabolism, and mild infusion-related symptoms (chest tightness, abdominal discomfort) at higher infusion rates [6]. A pilot of five daily 750 mg IV infusions reported improvement on 6 of 8 neuropsychological tests versus 2 of 8 for saline [9]. In 50 treatment-resistant substance-use-disorder patients, IV NAD+ with enkephalinase inhibition was associated with statistically significant reductions in craving (p = 1.06e-9), anxiety (p = 5.49e-7), and depression (p = 1.76e-4) [7], and a narrative review traces IV NAD+ use in addiction back to a 1961 case series of 104+ patients while noting IV NAD+ remains unapproved by the FDA and calling for rigorous randomized trials [8]. Read these as preliminary signals, not settled outcomes. --- A chrome console reading of the NAD+ record — the coenzyme, its oral precursors, and the injectable studies logged route by route and cited to source, with oral NAD+'s poor absorption and IV NAD+'s rapid clearance flagged in plain sight; no clinic behind the readout and nothing here prescribed, compounded, or sold.