NAD+

NAD+ (nicotinamide adenine dinucleotide) is a dinucleotide coenzyme consisting of two nucleotides — one containing an adenine base and one containing a nicotinamide base — joined by a phosphate-to-phosphate bridge. It is found in every living cell and is absolutely required for life: without NAD+, cellular respiration cannot proceed, and the cell dies. NAD+ functions as the critical electron carrier in the central metabolic pathways of cellular energy production — it accepts electrons (becoming NADH) during glycolysis, the pyruvate dehydrogenase reaction, and the citric acid (Krebs) cycle, then donates those electrons to the electron transport chain in the inner mitochondrial membrane, where the energy is captured as ATP through oxidative phosphorylation. The continuous regeneration of NAD+ from NADH by the electron transport chain is essential for the ongoing operation of these pathways — without it, NADH accumulates and all upstream oxidative reactions halt.

This central metabolic role, while essential, was understood for decades before the modern explosion of NAD+ research. The transformative realisation of the 1990s–2000s was that NAD+ is not merely a passive electron carrier but an active substrate — consumed and depleted — by a growing family of NAD+-dependent signalling enzymes whose activities directly regulate gene expression, DNA repair, circadian rhythms, stress responses, and cellular lifespan. This recognition repositioned NAD+ from a boring metabolic cofactor to one of the most strategically important molecules in biology and ageing science.

The most studied NAD+-consuming signalling enzymes are the sirtuins — a family of seven NAD+-dependent deacylase enzymes (SIRT1 through SIRT7) that regulate histones, transcription factors, and metabolic enzymes through protein deacylation reactions that consume NAD+ as a co-substrate (not merely a cofactor). The distinction matters: because sirtuins consume NAD+ rather than just using it as a cofactor, their activity is directly limited by intracellular NAD+ availability. When NAD+ levels fall — as they do with ageing, inflammation, genotoxic stress, and sedentary lifestyle — sirtuin activity falls proportionally. When NAD+ levels are restored by supplementation with precursors, sirtuin activity is restored. This causal chain from NAD+ availability to sirtuin activity to downstream gene regulation is mechanistically well-established and forms the scientific foundation of the NAD+ longevity hypothesis.

The downstream effects of sirtuin activation are numerous and profoundly relevant to ageing biology. SIRT1 (the most studied sirtuin) deacetylates FOXO transcription factors (promoting stress resistance and longevity gene expression), PGC-1α (promoting mitochondrial biogenesis and oxidative metabolism), p53 (regulating DNA damage response and apoptosis), NF-κB (reducing inflammatory gene transcription), and many other targets. SIRT3, located in the mitochondrial matrix, regulates mitochondrial enzyme activity and reduces reactive oxygen species (ROS) production by deacetylating and activating key antioxidant enzymes including SOD2. SIRT6 regulates DNA double-strand break repair, genome stability, and metabolic gene expression. SIRT1 additionally deacetylates BMAL1, a core component of the molecular circadian clock, linking NAD+ availability directly to circadian rhythm regulation — an important mechanistic connection to the circadian dysregulation observed in ageing.

The PARP (Poly ADP-ribose polymerase) family of DNA repair enzymes are the other major NAD+-consuming signalling system. PARP1, the most important for DNA repair, is activated by DNA strand breaks — which are generated by oxidative damage, ionising radiation, replication errors, and other genotoxic stresses. When activated, PARP1 consumes massive quantities of NAD+ to generate poly-ADP-ribose chains that recruit and coordinate the DNA repair machinery. In conditions of high DNA damage burden — which is precisely the situation in ageing, where oxidative damage accumulates and repair capacity declines — PARP1 hyperactivation can deplete cellular NAD+ reserves precipitously, leaving insufficient NAD+ for sirtuin activity and normal bioenergetics. This competition between PARP-mediated DNA repair (which consumes NAD+) and sirtuin-mediated longevity gene regulation (which requires NAD+) for a limited NAD+ pool creates a fundamental tension in the ageing cell, and NAD+ supplementation expands the pool available for both systems simultaneously.

CD38, an ectoenzyme expressed on immune cells and in other tissues, is the third major NAD+-consuming pathway and is now understood to be the primary driver of the age-related decline in systemic NAD+ levels. CD38 expression increases dramatically with ageing and with chronic inflammation (a state called "inflammaging"), consuming NAD+ to generate cyclic ADP-ribose (cADPR) and ADPR for calcium signalling purposes. The age-related increase in CD38 activity, combined with chronic inflammatory activation of CD38 in immune cells, produces a progressive, accelerating decline in cellular and circulating NAD+ that begins in the third or fourth decade of life and worsens with each subsequent decade. This decline has been measured in human skeletal muscle biopsies, blood, and other tissues, consistently showing 40–50% reductions in NAD+ levels between young adulthood and late middle age.

The interest in IV NAD+ infusion arises from this NAD+ decline and from the rapid clinical effects that IV administration produces. When NAD+ is infused intravenously, it enters cells through connexin channels and other transport mechanisms and is immediately available for metabolic and signalling functions, bypassing the multi-step biosynthetic pathways required for NAD+ production from oral precursors. Many subjects receiving IV NAD+ infusions report pronounced, often dramatic improvements in energy, mental clarity, mood, and sense of wellbeing within the infusion itself or immediately following — experiences sufficiently consistent and vivid to have driven the growth of a substantial IV NAD+ clinic industry. The mechanistic basis of these acute effects likely involves rapid mitochondrial respiratory chain optimisation and potentially rapid sirtuin and PARP activation in energy-intensive tissues including the brain, cardiac muscle, and skeletal muscle.

Oral NAD+ precursor supplementation — particularly NMN (nicotinamide mononucleotide) and NR (nicotinamide riboside) — provides a more accessible and practical approach to raising NAD+ than IV administration. Multiple human randomised controlled trials have confirmed that NMN (300–900 mg/day) and NR (250–1000 mg/day) significantly elevate blood and tissue NAD+ levels in older adults, with mechanistic biomarker changes (SIRT1 activity, mitochondrial gene expression) consistent with sirtuin activation. The clinical translation of these NAD+ elevations to meaningful health outcomes is under active investigation in multiple ongoing trials.