Nicotinamide Adenine Dinucleotide (NAD⁺) and the Biology of Aging Mechanisms of Decline and Therapeutic Restoration

The inexorable process of aging is underpinned by a cascade of molecular deficits that erode cellular homeostasis and organismal function. A pivotal molecule within this cascade is nicotinamide adenine dinucleotide (NAD⁺), a fundamental redox cofactor in energy metabolism and an essential substrate for signaling enzymes such as the sirtuin family of deacylases and poly (ADP-ribose) polymerases (PARPs). This review synthesizes current evidence establishing the age-related decline in NAD⁺ bioavailability as a key driver of aging phenotypes and evaluates strategies for its therapeutic restoration. We detail the mechanisms of depletion, emphasizing heightened consumption by CD38 and PARP1 amid chronic inflammation and genomic stress. The review critically analyzes three principal intervention paradigms: supplementation with NAD⁺ precursors like nicotinamide riboside (NR) and nicotinamide mononucleotide (NMN); lifestyle modalities including caloric restriction and exercise; and pharmacological inhibition of NAD⁺-catabolizing enzymes. Compelling preclinical data demonstrate that these approaches can rejuvenate mitochondrial function, improve metabolic health, and extend health span. Early human trials confirm precursor safety and bioavailability, indicating benefits for vascular and metabolic parameters. However, significant translational gaps remain, including questions of long-term safety, optimal dosing, tissue-specific delivery, and potential pro-tumorigenic effects in certain contexts. We conclude that NAD⁺ repletion constitutes a potent, mechanism-based axis for aging intervention, yet its definitive clinical integration mandates rigorous long-term studies to transform robust biological insight into effective human therapeutics.