Nicotinamide adenine dinucleotide, better known as NAD, is a molecule found in every living cell, where it shuttles electrons during the chemical reactions that turn food into energy. It also acts as fuel for enzymes involved in DNA repair, gene regulation, immune defense, and the body’s internal clock. Your cells both produce and consume NAD continuously, and the balance between those two processes shapes everything from how efficiently your mitochondria work to how quickly certain tissues age. The science around NAD has expanded rapidly in recent years, with researchers mapping how the body synthesizes it, what drains it, and whether diet, exercise, or supplements can meaningfully shift the balance.
How Your Body Builds NAD
Cells have three separate routes for making NAD, and each one starts with a different raw material. The most ancient of these is the de novo pathway, which begins with the amino acid tryptophan, something you get from protein-rich foods. Tryptophan is broken down through a series of steps called the kynurenine pathway, eventually producing a compound called quinolinic acid, which is then converted into NAD.1PubMed Central. Kynurenine pathway metabolism and neuroinflammatory disease This route is relatively slow and inefficient compared to the other two, but it is the only one that can build NAD from scratch without recycling any part of the molecule. It appears to be especially important in certain tissues like the liver and kidneys.2PubMed. Kynurenine 3-monooxygenase limits de novo NAD(+) synthesis through dietary tryptophan in renal proximal tubule epithelial cell models
The second route is the Preiss-Handler pathway, which uses nicotinic acid, one of the forms of vitamin B3 (also called niacin). Nicotinic acid gets converted into nicotinic acid mononucleotide and then into NAD through a couple of enzymatic steps. This pathway was among the first to be described, dating back to Elvehjem’s work in the 1930s linking niacin to the prevention of pellagra.3PubMed. Discoveries of nicotinamide riboside as a nutrient and conserved NRK genes establish a Preiss-Handler independent route to NAD+ in fungi and humans
The third route, and the one most cells lean on most heavily, is the salvage pathway. Rather than building NAD from scratch, this pathway recycles nicotinamide, the byproduct left behind every time an enzyme uses up NAD. The bottleneck enzyme here is called NAMPT (nicotinamide phosphoribosyltransferase), which converts nicotinamide into nicotinamide mononucleotide (NMN), and that NMN is then quickly turned back into NAD.4PubMed Central. Inhibition of nicotinamide phosphoribosyltransferase (NAMPT), the rate-limiting enzyme of the nicotinamide adenine dinucleotide (NAD) salvage pathway, to target glioma heterogeneity through mitochondrial oxidative stress Because cells constantly break down NAD in the course of normal operations, the salvage pathway is essentially running all the time, making it the dominant source of NAD in most adult tissues.5PubMed Central. The role of nicotinamide adenine dinucleotide salvage enzymes in cardioprotection
A fourth precursor, nicotinamide riboside (NR), was discovered more recently. NR feeds into the salvage pathway through its own set of enzymes called NRK1 and NRK2, bypassing the NAMPT step entirely.3PubMed. Discoveries of nicotinamide riboside as a nutrient and conserved NRK genes establish a Preiss-Handler independent route to NAD+ in fungi and humans This discovery was significant because it added an entirely new entry point for NAD production, one that eventually became the basis for several supplement products.
What Consumes NAD
NAD is not just an electron shuttle. It is also consumed outright by several families of enzymes, meaning the molecule is broken apart and must be rebuilt. Understanding what uses up NAD helps explain why levels can fall even when supply seems adequate.
Sirtuins are a family of seven enzymes (SIRT1 through SIRT7) that remove chemical tags from proteins, a process that influences metabolism, stress resistance, and gene expression. Every time a sirtuin removes an acetyl group from a protein, it uses up one molecule of NAD in the process.6PubMed Central. The role of sirtuins in the control of metabolic homeostasis SIRT1, the most studied member, plays roles in glucose and fat metabolism across many tissues, while SIRT3 operates inside mitochondria, helping regulate the energy-production machinery there.7PubMed. Sirtuins: NAD(+)-dependent deacetylase mechanism and regulation Because sirtuin activity depends directly on how much NAD is available, these enzymes essentially act as sensors of the cell’s energy status.8PubMed Central. NAD metabolism and sirtuins: metabolic regulation of protein deacetylation in stress and toxicity
PARP1 (poly-ADP-ribose polymerase 1) is a DNA damage sensor that rushes to broken DNA strands and uses NAD to build repair scaffolds. When damage is light, PARP1 uses modest amounts of NAD. But when damage is heavy, such as after exposure to radiation or certain toxins, PARP1 can chew through NAD so quickly that it triggers a cell-wide metabolic shift.9PubMed Central. NAD+ consumption by PARP1 in response to DNA damage triggers metabolic shift critical for damaged cell survival PARP1 is actually considered the single largest NAD-consuming activity in the nucleus.10PubMed Central. The taming of PARP1 and its impact on NAD(+) metabolism
CD38 is an enzyme found on the surface of many cell types, especially immune cells, and it breaks NAD into smaller signaling molecules. CD38 has emerged as one of the main NAD-degrading enzymes in mammalian tissues, and its activity appears to regulate NAD levels across multiple organs.11PubMed Central. CD38 as a regulator of cellular NAD: a novel potential pharmacological target for metabolic conditions As we will see, CD38 plays a central role in the story of why NAD declines with age.
NAD and the Body Clock
NAD levels are not static throughout the day. In mice, both NAMPT (the rate-limiting salvage enzyme) and NAD itself rise and fall in a roughly 24-hour cycle driven by the core circadian clock machinery.12PubMed Central. Circadian clock feedback cycle through NAMPT-mediated NAD+ biosynthesis This creates a feedback loop: the clock genes control NAMPT expression, NAMPT controls how much NAD is available, and NAD levels in turn influence the activity of sirtuins and other enzymes that feed back into clock gene regulation.
This oscillation is not just a curiosity. It means the cell’s repair capacity, metabolic flexibility, and gene regulation shift across the day in a coordinated rhythm. Research has shown that this NAD oscillation operates in a tissue-specific manner, with different organs peaking at different times.13PubMed Central. NAMPT-dependent NAD(+) biosynthesis controls circadian metabolism in a tissue-specific manner Disrupted sleep schedules and shift work, which desynchronize circadian clocks, may therefore have downstream effects on NAD-dependent processes, though the human data on this specific connection remain limited.
Why NAD Drops With Age
One of the findings that brought NAD into mainstream health conversations is that tissue NAD levels decline as people and animals get older. The question is why. The answer is not simply that production slows down, though that happens to some degree. A major driver appears to be rising CD38 activity.
A landmark study showed that CD38 expression and activity increase during aging, and that this increase is both necessary and sufficient to explain much of the age-related NAD decline. The researchers found that CD38 drives mitochondrial dysfunction in aging through a pathway that involves reduced SIRT3 activity.14PubMed Central. CD38 Dictates Age-Related NAD Decline and Mitochondrial Dysfunction through an SIRT3-Dependent Mechanism In other words, the problem is not just that NAD drops; it is that the drop disables a protective enzyme (SIRT3) inside mitochondria, leading to a cascade of metabolic deterioration.
A follow-up line of research identified where the extra CD38 comes from. As tissues age, they accumulate senescent cells, which release inflammatory signals. These signals recruit a particular type of inflammatory macrophage that expresses high levels of CD38. These CD38-rich macrophages then act as NAD sinks, actively degrading NAD in the surrounding tissue.15Nature Metabolism. Senescent cells promote tissue NAD+ decline during ageing via the activation of CD38+ macrophages This finding connected several threads in aging research: cellular senescence, chronic inflammation, immune cell behavior, and metabolic decline all converge on this one molecule.
Exercise, Fasting, and NAD
If rising CD38 and declining NAMPT both contribute to falling NAD levels, it is natural to ask whether anything reverses those trends without taking a supplement. Exercise turns out to be one of the most reliable interventions studied so far.
In sedentary but otherwise healthy people, just three weeks of exercise training increased skeletal muscle NAMPT protein by about 127%.16PubMed Central. Skeletal muscle NAMPT is induced by exercise in humans Both aerobic and resistance training appear to work. A study comparing the two found that aerobic exercise boosted NAMPT by roughly 12 to 28% and resistance exercise by about 25 to 30%, with older adults seeing some of the largest gains.17PubMed Central. Aerobic and resistance exercise training reverses age‐dependent decline in NAD+ salvage capacity in human skeletal muscle
A study in middle-aged, overweight, untrained individuals found that resistance training boosted muscle NAD levels by about 127%, bringing their post-training levels up to match those of college-aged participants. NAMPT protein rose about 15%, and overall sirtuin activity increased about 13%.18PubMed Central. Resistance training increases muscle NAD+ and NADH concentrations as well as NAMPT protein levels and global sirtuin activity in middle-aged, overweight, untrained individuals That last detail is worth pausing on: it suggests that exercise does not just raise NAD levels in the abstract but functionally reactivates the NAD-dependent protective enzymes that aging had suppressed.
Caloric restriction and fasting also influence the system. In mice, a 24-hour fast raised liver NAD levels along with NAMPT activity, and this triggered increased SIRT1 expression.19PubMed. Fasting promotes the expression of SIRT1, an NAD+ -dependent protein deacetylase, via activation of PPARalpha in mice The mechanism makes intuitive sense: when food is scarce, cells ramp up the recycling pathway to maintain NAD from the nicotinamide they already have. How precisely this translates to human intermittent fasting protocols is still being studied, but the directional finding is consistent across multiple animal models.
The Gut Microbiome’s Role in NAD Metabolism
One of the more surprising findings in NAD biology is that gut bacteria are active participants in NAD precursor metabolism. The relationship works like a two-way exchange. The host produces nicotinamide as a byproduct of normal NAD turnover, and some of that nicotinamide enters the gut. Gut microbes then convert that nicotinamide into nicotinic acid, which re-enters the bloodstream and is used by host tissues, especially the liver, to make NAD through the Preiss-Handler pathway.20PubMed Central. NAD precursors cycle between host tissues and the gut microbiome
This microbial conversion is robust enough that it maintains circulating nicotinic acid levels even when none is being consumed in the diet.21PubMed. Sharing the load in NAD metabolism In effect, the gut microbiome acts as a metabolic relay station, converting one form of vitamin B3 into another and cycling it back to the host. The practical implication is that gut health, antibiotic use, and the composition of your microbiome may all influence how efficiently your body maintains NAD levels, though this area is still early in its research arc.
This same microbial conversion has consequences for supplements. Research has shown that most orally taken NMN and NR does not get absorbed directly from the small intestine in its original form. Instead, gut bacteria break these molecules down and convert them to nicotinic acid, which then enters the liver through what is called enterohepatic circulation.22PubMed Central. Nicotinamide riboside and nicotinamide mononucleotide facilitate NAD(+) synthesis via enterohepatic circulation Even intravenously administered NMN and NR were rapidly broken down into nicotinamide, secreted into bile, and then converted to nicotinic acid by gut bacteria. This means the microbiome is not just a bystander in supplement metabolism; it is a central processing hub.
NAD in Immune Function
Immune cells, particularly macrophages, rely heavily on NAD for their basic functions. Research has shown that macrophages generate NAD through the de novo pathway (the tryptophan-to-kynurenine route) and that blocking this pathway depletes NAD, suppresses mitochondrial respiration, and impairs the ability of macrophages to engulf pathogens and resolve inflammation.23PubMed Central. Macrophage de novo NAD(+) synthesis specifies immune function in aging and inflammation
There is an ironic twist. When the immune system is activated by infection or injury, it triggers the early steps of the kynurenine pathway but actually suppresses the final step that converts quinolinic acid into NAD. The result is that actively fighting macrophages generate less NAD from their own internal pathway at the very moment they need it most. Aging macrophages show a similar profile: upstream pathway activation but downstream NAD synthesis failure.23PubMed Central. Macrophage de novo NAD(+) synthesis specifies immune function in aging and inflammation Experimentally, restoring NAD synthesis in these aged or inflamed macrophages brought back normal mitochondrial function and immune responses. This positions NAD not just as a metabolic cofactor but as a genuine switch that determines whether an immune cell can do its job properly.
NAD and Nerve Health
Nerve cells, with their extraordinarily long axons, face a unique challenge in maintaining NAD levels far from the cell body. Axonal survival depends on a continuous supply of NAD produced by an enzyme called NMNAT2, which is made in the cell body and shipped down the axon. If NMNAT2 delivery is interrupted, as happens after nerve injury, the axon loses its NAD supply.
When NAD drops in the axon, a destructive enzyme called SARM1 activates. SARM1 is itself an NADase: once switched on, it rapidly destroys whatever remaining NAD the axon has, triggering a self-destruction program called Wallerian degeneration.24PubMed Central. An NAD+/NMN balancing act by SARM1 and NMNAT2 controls axonal degeneration The balance between NMNAT2 (which builds NAD) and SARM1 (which destroys it) effectively determines whether an axon lives or dies.25PubMed Central. SARM1-Dependent Axon Degeneration: Nucleotide Signaling, Neurodegenerative Disorders, Toxicity, and Therapeutic Opportunities
This discovery has opened a new therapeutic angle. If SARM1 could be kept inactive, or if NMNAT2 activity could be sustained, damaged nerves might survive longer, giving the body more time to repair them. Several drug development programs are now targeting SARM1 for conditions involving nerve degeneration, from peripheral neuropathy to traumatic brain injury. The underlying biology here is distinct from the metabolic and aging stories discussed earlier, but the central molecule is the same: NAD.
NAD, Liver Health, and Metabolic Disease
The liver is one of the most metabolically active organs and depends heavily on NAD for processing fats, detoxifying substances, and regulating blood sugar. When researchers engineered mice with reduced NAMPT activity (and therefore lower NAD levels), even on a normal diet those animals developed features of non-alcoholic fatty liver disease, including fat accumulation, oxidative stress, inflammation, and impaired insulin sensitivity.26PubMed Central. Hepatic NAD(+) deficiency as a therapeutic target for non-alcoholic fatty liver disease in ageing The implication is that NAD depletion itself, independent of a high-fat diet, can push the liver toward disease.
The connection extends to insulin resistance more broadly. SIRT2, one of the NAD-dependent sirtuins, has been linked to insulin sensitivity in liver cells. When SIRT2 levels drop, oxidative stress and mitochondrial dysfunction increase, and insulin signaling deteriorates.27Human Molecular Genetics. The NAD+-dependent deacetylase SIRT2 attenuates oxidative stress and mitochondrial dysfunction and improves insulin sensitivity in hepatocytes This creates a vicious cycle: low NAD reduces sirtuin activity, which worsens mitochondrial function, which further strains metabolic health. Much of the current interest in NAD-boosting strategies for metabolic disease centers on breaking this cycle.
NAD Precursor Supplements
Given everything above, it is not surprising that supplement companies have seized on NAD precursors as anti-aging products. The two most commercially available are nicotinamide riboside (NR) and nicotinamide mononucleotide (NMN). Both raise blood NAD levels in humans, and both appear to be well tolerated in clinical trials conducted so far.
In an eight-week trial of healthy overweight adults, NR at doses of 100, 300, and 1,000 mg raised whole blood NAD in a dose-dependent manner, with increases of roughly 22%, 51%, and 142% respectively. These increases appeared within two weeks and held steady throughout the study. No flushing (a common side effect of plain niacin) was reported, and there were no significant differences in adverse events between NR and placebo groups.28Scientific Reports. Safety and Metabolism of Long-term Administration of NIAGEN (Nicotinamide Riboside Chloride) in a Randomized, Double-Blind, Placebo-controlled Clinical Trial of Healthy Overweight Adults
For NMN, a 12-week trial in healthy subjects found that oral supplementation safely raised blood NAD levels with no abnormalities on laboratory tests and no obvious adverse effects.29PubMed Central. Oral Administration of Nicotinamide Mononucleotide Is Safe and Efficiently Increases Blood Nicotinamide Adenine Dinucleotide Levels in Healthy Subjects
Raising blood NAD is one thing; demonstrating that this translates to meaningful health benefits is another. Most of the dramatic findings linking NAD restoration to improved metabolism, reduced inflammation, or extended healthspan come from animal models. Human trials on hard clinical endpoints are still in their early stages. The gut microbiome processing described earlier also complicates the picture: if most oral NR and NMN gets converted to nicotinic acid by gut bacteria before reaching tissues, the mechanistic story behind these supplements may be less direct than originally thought. This does not mean they do not work, but it suggests the pathway from pill to benefit is more roundabout than supplement marketing tends to imply.
How Energy Production Actually Depends on NAD
At the most fundamental level, NAD participates in nearly every step of converting food into usable energy. It picks up electrons from the breakdown of sugars, fats, and amino acids (becoming NADH in the process), then delivers those electrons to the mitochondrial machinery that generates ATP. The ratio between NAD and NADH in the cell controls the rate at which this whole system operates, regulating both the enzymes that feed electrons into the chain and the NAD-dependent enzymes that modify proteins in response to energy status.30PubMed Central. NAD(H) in mitochondrial energy transduction: implications for health and disease
This is why NAD sits at such a critical intersection. It is simultaneously the currency that energy metabolism runs on and the signal that tells the cell how much energy is available. When NAD is abundant, the cell reads that as a sign of metabolic health and activates protective pathways through sirtuins and other sensors. When NAD is depleted, whether by age-related CD38 increases, heavy DNA damage consuming it through PARP1, or simple nutritional deficiency, those protective systems go quiet. The result is not a sudden failure but a gradual erosion: mitochondria become less efficient, DNA damage accumulates a little faster, inflammatory signaling gets a little louder, and metabolic flexibility narrows. The research community is still working out which of these downstream consequences are most responsive to intervention and whether raising NAD in humans can produce the kind of rejuvenation seen in animal experiments. But the centrality of this single molecule to so many aspects of cellular health is no longer in dispute.