# NAD+ Research: Mechanism, Precursor Trials, and the Consumer Enzymes > NAD+ research summarized: the redox role, the salvage pathway, NMN and NR precursor trials, and the sirtuin/PARP/CD38 enzymes that consume NAD+. Every claim cited. The redox role, the biosynthetic routes, the enzymes that consume it, and the human precursor data — organized by evidence tier and cited to source. ## Start here NAD+ does two jobs in a cell. First, it is a redox (chemistry that shuttles electrons to release energy) carrier: it picks up electrons from food breakdown and hands them to the machinery that makes ATP, the cell's energy currency. Second, it is a consumable fuel for repair-and-maintenance enzymes that fix DNA and tune inflammation. Because those enzymes use NAD+ up, and because the cell's NAD+ supply drops with age, scientists have tested whether feeding the body NAD+ precursors (NMN, NR) restores the supply. This page walks through that mechanism and then the human precursor trials, lightest claims flagged as such. ## NAD+ as the cell's redox coenzyme NAD+ cycles between an oxidized form (NAD+) and a reduced form (NADH). In catabolism it accepts electrons to become NADH; in the mitochondrial electron transport chain NADH donates them to drive ATP synthesis. This redox cycling runs through glycolysis, the TCA cycle, and oxidative phosphorylation, which makes NAD+ a coenzyme for hundreds of oxidoreductase reactions [5]. Beyond redox, NAD+ is a consumed substrate. A foundational review identifies the major NAD-consuming enzymes — sirtuins (SIRT1-7), PARPs (chiefly PARP1), and CD38/CD157 — that compete for a shared, finite NAD+ pool, and frames restoring NAD+ as a candidate strategy against age-related disease [5]. When NAD+ is consumed by signaling, it must be resynthesized, which is why the supply routes matter as much as the demand. ## How the body makes NAD+: salvage, NRK, and de novo routes Mammals make NAD+ by three main routes. The dominant one is the salvage pathway, which recycles nicotinamide back into NAD+ via NAMPT (nicotinamide phosphoribosyltransferase), the rate-limiting enzyme, producing NMN, which NMNAT then converts to NAD+ [5]. A second route, used by NR, runs nicotinamide riboside through the NRK1/NRK2 kinases to NMN and then to NAD+, bypassing NAMPT. The third is de novo synthesis from tryptophan, with the Preiss-Handler pathway feeding in from nicotinic acid [5]. These routes are the reason precursor supplements work at all: an oral precursor enters one of these pathways and is converted to NAD+ downstream, raising the measurable pool [4]. ## NMN (Nicotinamide Mononucleotide): The Most-Studied Oral NAD+ Precursor NMN is a direct NAD+ precursor one biochemical step from NAD+, and it has the strongest body of human trial data among the precursors. In a randomized trial in prediabetic, postmenopausal women, 250 mg/day of oral NMN for 10 weeks significantly increased muscle insulin sensitivity (measured by hyperinsulinemic-euglycemic clamp) and remodeled insulin signaling, with no change in body composition or HbA1c [1]. In a multicenter, double-blind, placebo-controlled trial in healthy middle-aged adults, NMN at 300, 600, or 900 mg/day for 60 days dose-dependently raised blood NAD+ at days 30 and 60 (p ≤ 0.001 across all NMN groups), improved walking distance, and identified 600 mg/day as the optimal dose among those tested, with no safety issues at any dose [3]. NMN's regulatory status is contested: the FDA has taken the position that NMN is excluded from the dietary-supplement definition because it was investigated as a drug — a marketplace dispute, not a determination that NMN is illegal to possess [10]. The trials above measured blood NAD+, insulin sensitivity, and physical performance; none demonstrated treatment of any disease. ## Nicotinamide Riboside (NR): A Well-Tolerated, Dose-Scalable NAD+ Booster Nicotinamide riboside is a vitamin-B3-family precursor converted to NMN by the NRK kinases, then to NAD+. In a randomized, double-blind, placebo-controlled trial in healthy overweight adults, NR at 100, 300, and 1000 mg/day for 8 weeks raised whole-blood NAD+ by 22%, 51%, and 142% respectively — a dose-dependent elevation maintained throughout the study, with no flushing and no significant adverse-event differences from placebo at any dose [4]. NR did not elevate LDL cholesterol or disrupt one-carbon metabolism, which supports its profile as a dose-scalable oral NAD+ booster [4]. A 14-day randomized trial comparing NR, NMN, nicotinamide, and placebo found both NR and NMN increased whole-blood NAD+ roughly 2-fold versus placebo (differences of ~49 µM and ~43 µM; p < 0.001), while NAM had no significant effect; the trial also reported that the gut microbiome converts NR and NMN to nicotinic acid, which may mediate part of the NAD+ boost [13]. ## The consumer enzymes: sirtuins, PARP1, and CD38 Three enzyme families consume NAD+ and explain why its supply matters. Sirtuins (SIRT1-7) are NAD+-dependent deacylases that regulate metabolism, stress resistance, and DNA repair [5]. PARP1 is a DNA-repair enzyme that consumes large amounts of NAD+ when activated by DNA damage; in reconstitution biochemistry, PARP1 inhibited mitochondrial DNA polymerase Pol γ when NAD+ was absent and allowed full repair activity as NAD+ rose to physiological levels, coupling mitochondrial DNA repair to the cellular NAD+ state [8]. A review of PARP1/ARTD1 activation describes NAD+ as the limiting substrate that links DNA repair to energy depletion and, when PARP1 over-activates, to cell death relevant to cancer, inflammation, and ischaemia/reperfusion [7]. CD38 is the principal NAD-consuming ectoenzyme whose activity rises with age. In mice, CD38 deletion preserves NAD+ and SIRT3 activity and improves mitochondrial and metabolic health with age, identifying CD38 as a key driver of the age-related NAD+ decline [2]. Senescent cells amplify this: they increase CD38 expression via their secretory phenotype, creating a feedback loop that depletes tissue NAD+ during aging, and NMN supplementation in aged mice reduced markers of cellular senescence [14]. In inflammatory contexts, activated macrophages become dependent on NAMPT-driven NAD+ salvage after ROS-mediated DNA damage consumes NAD+ through PARP [6], and NAD+ metabolism more broadly regulates immune function and inflammageing [9]. ## Does NAD make you look younger? No study shows NAD+ or its precursors make people look younger. Much of the strongest anti-aging data come from rodents, and a 2025 Nature Metabolism review concluded human efficacy for hard clinical endpoints remains preliminary [10]. NAD+ supports cellular metabolism; it is not a cosmetic treatment, and no cited trial measured appearance. ## Does NAD help with weight loss? No cited trial demonstrates weight loss from NAD+ or its precursors. In the NMN insulin-sensitivity trial, 250 mg/day for 10 weeks improved muscle insulin sensitivity but produced no change in body composition [1]. The human evidence centers on blood NAD+, insulin signaling, and physical performance, not fat loss. ## Does NAD cause weight gain? No cited trial reported weight gain from NAD+ precursors. The 10-week NMN insulin-sensitivity trial found no change in body composition or HbA1c [1]. Effects in the controlled studies centered on insulin sensitivity and physical performance rather than body weight. ## Does NAD help with fertility? The studies summarized in this digest do not test fertility endpoints. The human evidence here centers on blood NAD+ elevation, muscle insulin sensitivity, and physical performance [1][3][4]; no fertility claim can be supported from these citations. ## Is taking NAD orally effective? Oral NAD+ itself is poorly taken up by cells intact, so most researchers consider the precursors NMN and NR the rational oral approach [10]. In randomized trials, oral NR raised whole-blood NAD+ by 22/51/142% at 100/300/1000 mg/day [4], and oral NMN dose-dependently raised blood NAD+ at 300-900 mg/day [3]. Blood-level elevation is well established; clinical-outcome translation is not. --- A flat, color-blocked index of the NAD+ literature — the coenzyme set apart from the precursors NMN and NR that rebuild it, what the trials measured stamped to each study and what they did not left in plain view; no clinic behind the index and nothing here infused, dispensed, or sold.