NAD⁺ Biology in Ageing and Chronic Disease: Mechanisms and Evidence across Skin, Fertility, Osteoarthritis, Hearing and Vision Loss, Gut Health, Cardiovascular–Hepatic Metabolism, Neurological Disorders, and Muscle

Background: Nicotinamide adenine dinucleotide (NAD⁺) is a pivotal coenzyme and signaling substrate that integrates redox balance with mitochondrial energy production, DNA repair, epigenetic control, and cellular stress resilience. Declines in NAD⁺ availability—frequently observed with ageing, chronic inflammation, and metabolic stress—have intensified interest in NAD⁺ restoration as a potential strategy to influence disease biology across multiple organ systems. Objective: This narrative review summarizes contemporary mechanistic and translational evidence on NAD⁺ biosynthesis and turnover, highlighting the de novo kynurenine pathway and vitamin B3–dependent salvage routes (nicotinic acid, nicotinamide, nicotinamide riboside, and nicotinamide mononucleotide). We also examine how major NAD⁺ consumers and sensors, sirtuins, poly(ADP-ribose) polymerases (PARPs), and CD38 link NAD⁺ status to inflammation, oxidative stress, and tissue dysfunction in diverse clinical contexts. Methods: Peer-reviewed literature on NAD⁺ metabolism, NAD⁺-dependent signaling, and preclinical/clinical studies of NAD⁺ precursors was evaluated and organized into: (i) core biochemical functions in cellular energetics, (ii) NAD⁺ consumption in genome maintenance and immune signaling, and (iii) organ-focused evidence relevant to skin disorders, infertility and reproductive health, osteoarthritis, hearing loss, vision decline, gut barrier dysfunction, cardiovascular and renal metabolism, hepatic steatosis, neurological diseases, and skeletal muscle health. Results: NAD⁺ supports glycolysis, the tricarboxylic acid cycle, and oxidative phosphorylation, while acting as an essential substrate for PARP-driven DNA repair and sirtuin-mediated deacylation programs that shape mitochondrial fitness, inflammatory tone, and metabolic flexibility. Across experimental models, impaired NAD⁺ homeostasis repeatedly associates with mitochondrial dysfunction, heightened oxidative injury, and dysregulated immune–barrier responses, features shared by intestinal inflammation, neurodegeneration and ischemic injury, cardiometabolic disease, kidney injury, and fatty liver disease. Supplementation with NAD⁺ precursors (notably NR and NMN) reliably elevates NAD⁺ in preclinical systems and increases circulating NAD⁺ metabolites in humans, with early signals of pathway engagement; however, clinical outcomes remain heterogeneous across populations, dosing regimens, and endpoints. Evidence for intravenous NAD⁺ “drip” therapy is comparatively limited and insufficiently standardized, with constraints related to tolerability, dose consistency, and cost, underscoring the need for controlled trials. Conclusion: NAD⁺ occupies a central position at the interface of energy metabolism, genome integrity, and immunometabolic signaling, providing a coherent framework for understanding how cellular stress can propagate multisystem dysfunction. Although NAD⁺-boosting strategies are biologically plausible and mechanistically supported, definitive clinical benefit across skin, fertility, osteoarthritis, sensory decline, gut disorders, cardiovascular and hepatic disease, neurological conditions, and muscle health will require well-designed human studies with standardized biomarkers, safety surveillance, and clinically meaningful endpoints.