# NAD+ Research: Biosynthetic Pathways, Consuming Enzymes, and the Human Trial Record | NAD+ > NAD+ research summary: the salvage, de novo, and Preiss-Handler pathways, the sirtuin/PARP/CD38 consuming enzymes, and the controlled human NMN and NR precursor trials. Cited and route-tagged. The biosynthetic map, the consuming-enzyme set, and the controlled human trials — documented with the species and route on every figure. ## Start here NAD+ does two jobs at once. As a redox coenzyme (helper molecule for electron-shuttling chemistry), it moves electrons through the reactions that make cellular energy. As a signaling substrate, it is the raw material consumed by maintenance enzymes that repair DNA and regulate genes. This page maps how cells make NAD+ (the pathways), which enzymes burn through it (the consumers), and what the human trials of its precursors NMN and NR actually measured. Read the pathway sections for mechanism; read the trial sections for evidence; every number is cited. ## NAD+ precursors and biosynthetic pathways Cells build NAD+ by three converging routes [13]. The dominant mammalian route is the **salvage pathway**: nicotinamide is recycled to NMN by NAMPT — the rate-limiting enzyme — then to NAD+ by NMNAT [10][5]. A parallel **NR route** lets nicotinamide riboside enter as NMN via the NRK1/NRK2 kinases, bypassing NAMPT; the crystal structure of human Nrk1 defined exactly how NR is phosphorylated on its way to NAD+ [12]. Two further routes feed the pool: **de novo** synthesis from the amino acid tryptophan, and the **Preiss-Handler pathway** from nicotinic acid (niacin) [13]. The adenylyltransferase step itself — NMN + ATP → NAD+ — was solved by X-ray crystallography of human NMNAT, revealing a hexamer with conserved ATP-recognition motifs [16]. A comparative analysis across 45 species found the Preiss-Handler route and NAD+ kinase present in every organism examined, suggesting they are ancestral [13]. That is the NAD+ precursor landscape: several inputs, one shared coenzyme output. ## The consuming enzymes: sirtuins, PARPs, and CD38 NAD+ is not only carried and recycled — it is consumed. Three enzyme families spend it: **sirtuins** (SIRT1–7, NAD+-dependent deacylases that regulate metabolism, stress resistance, and DNA repair), **PARP1** (a DNA-repair enzyme that burns large amounts of NAD+ when DNA is damaged), and **CD38/CD157** (NAD-consuming ectoenzymes that rise with age and inflammation) [5]. The CD38 story is central to the age-decline rationale: in mice, CD38 is the principal NAD+-consuming enzyme whose activity climbs with age, and CD38-knockout animals are protected from the age-related NAD+ fall, preserving SIRT3 activity and mitochondrial function [2]. NAMPT, the salvage gatekeeper, is also a candidate target in neurodegeneration biology, with high hippocampal expression linked to neural stem-cell NAD+ synthesis [12]. ## Why declining NAD+ is thought to matter The mechanistic case for caring about the age-related NAD+ drop runs through the mitochondria (the cell's energy organelles). As NAD+ falls, the foundational review describes a state sometimes called pseudohypoxia — a disruption of communication between the nucleus and the mitochondria that mimics low oxygen and degrades mitochondrial function, even when oxygen is plentiful [5]. The decline also feeds a loop with inflammation: senescent (aged, no-longer-dividing) cells secrete inflammatory factors that recruit CD38-bearing immune cells, and those CD38+ cells consume still more NAD+, accelerating the local fall [5][2]. This is the chain the precursor strategy aims at — top up the substrate (NAD+) so the sirtuins and repair enzymes that depend on it keep working [5]. It is a coherent mechanism with strong rodent support; whether topping up blood NAD+ in humans translates down this chain to tissue-level and clinical benefit is exactly the question the 2025 review flags as still open [14]. ## NAD+ as a dietary supplement: oral precursors NMN and NR As a NAD supplement, NAD+ is sold as a dietary supplement, not an approved drug — and most oral products are precursors, because NAD+ itself is large and charged and poorly absorbed intact [8]. The two leading oral precursors are NMN (nicotinamide mononucleotide) and NR (nicotinamide riboside); niacin and nicotinamide are the older vitamin-B3 forms. The controlled human evidence is precursor evidence: NR raised whole-blood NAD+ by 22%, 51%, and 142% at 100/300/1000 mg/day over eight weeks without elevating LDL cholesterol or disrupting one-carbon metabolism [4], and NMN raised blood NAD+ dose-dependently across 300/600/900 mg/day [3]. One regulatory wrinkle belongs here: the FDA has taken the position that NMN is excluded from the dietary-supplement definition because it was authorized for investigation as a drug — an unsettled marketplace dispute, not a finding that NMN is banned or illegal. ## Nicotinamide riboside (NR): the most clinically studied oral NAD+ precursor Nicotinamide riboside is the precursor with the strongest controlled human dose-response. In healthy overweight adults, NR raised whole-blood NAD+ by 22%, 51%, and 142% at 100, 300, and 1000 mg/day across eight weeks, with the elevation maintained throughout and no significant adverse-event difference from placebo at any dose [4]. In aged men, NR at 1 g/day for 21 days augmented the skeletal-muscle NAD+ metabolome and produced an anti-inflammatory cytokine signature [6]. The enzymology is well defined: NR enters via the NRK kinases, whose structural basis was resolved in human Nrk1 [12]. NR's combination of a clean dose-response, good tolerability, and a defined uptake route is why it is the most clinically studied oral NAD+ precursor. ## NAD+ vs NMN: why oral products are usually precursors NAD+ vs NMN is the distinction that keeps this whole literature honest. NAD+ is the large, charged, finished coenzyme; NMN is a smaller precursor one enzymatic step away from it. Because NAD+ itself is poorly taken up by cells intact, oral "NAD+" is largely a precursor strategy in practice — the molecule that gets absorbed and converted is NMN or NR, not NAD+ [8]. This is why a study of oral NMN or oral NR is never reported here as "taking NAD+," and why plain oral NAD+ capsules are widely considered largely ineffective by comparison [8]. Throughout this digest, route and form tags keep oral-precursor findings separate from infused-NAD+ claims. ## Recent data: the 2024–2025 record The most recent literature sharpens both the promise and the limits. A 2024 systematic review pooled ten randomized NMN trials (437 participants, mean age 58, doses 150–1200 mg/day, mean follow-up 9.6 weeks) and reported statistically significant improvements in gait speed (p=0.002), five-times sit-to-stand performance (p=0.04), six-minute walking distance, and SF-36 quality-of-life scores, with only minor adverse events (8.2%, independent of NMN) and no serious events [15]. That is the strongest synthesis to date that oral NMN moves physical-performance measures, not just blood chemistry. Against it, a 2025 *Nature Metabolism* review of NAD+ precursor supplementation in human ageing concluded that trials have shown limited efficacy for hard endpoints, that consistent age-related NAD+ decline has been demonstrated in only a limited number of human studies, and that tissue-specific NAD+ dynamics remain sparse [14]. The honest reading of the 2024–2025 data: the pharmacodynamic effect (blood NAD+ up, some functional gains) is real and replicated, while the disease-endpoint case is still open. ## Where the evidence is strong, and where it stops The pattern across the record is consistent. Raising blood NAD+ with oral precursors is well demonstrated and dose-scalable [4][3]. Specific metabolic and functional signals have appeared in controlled trials — muscle insulin sensitivity with NMN [1], physical-performance gains in a pooled review [15], an anti-inflammatory muscle signature with NR [6]. But the 2025 *Nature Metabolism* review concluded that human efficacy for hard clinical endpoints remains limited, that consistent age-related NAD+ decline has been shown in only a limited number of human studies, and that tissue-specific NAD+ data are sparse — a call for more [human clinical trials](/research), not a verdict from rodent extrapolation [14]. A separate context-dependent caution: because NAD+ supports proliferating cells, its role in oncology is dual, so caution is advised in cancer populations [8]. --- Documentation of the NAD+ record: the precursor trials logged green, the IV and injection gaps flagged amber, the species and route on every figure — no clinic behind the docs and nothing here dispensed, dosed, or sold.