Quick answer: NAD+ (nicotinamide adenine dinucleotide) is a coenzyme in every cell of the body that is central to energy production (ATP synthesis), DNA repair (via PARP enzymes), sirtuin activation (longevity-associated deacetylases), and circadian rhythm regulation. NAD+ levels decline approximately 50% by age 60 compared to age 20, and this decline is associated with the hallmarks of biological aging. The two main NAD+ precursor supplements — NMN (nicotinamide mononucleotide) and NR (nicotinamide riboside) — successfully raise NAD+ levels in human trials (NMN by 38% at 250 mg/day; NR by up to 60% at 300 mg/day). However, the translation from elevated NAD+ to health outcomes in humans is still being established — the most robust human evidence is for muscle function, metabolic parameters, and cognitive performance in specific populations. The most potent free lifestyle strategy for raising NAD+ is intense exercise, which acutely increases cellular NAD+ to a greater degree than current oral supplement doses.
What NAD+ Does: The Central Role in Cellular Energy and Aging
NAD+ exists in two forms: the oxidized form (NAD+) and the reduced form (NADH). The ratio of NAD+/NADH is a critical cellular signal that reflects the cell’s energy and redox status. In the electron transport chain (the mitochondrial machinery that produces 90% of the cell’s ATP), NADH delivers electrons to Complex I, where they power ATP synthesis, and in the process NAD+ is regenerated. A healthy NAD+/NADH ratio is essential for efficient mitochondrial energy production — when this ratio falls (as it does with aging, obesity, and metabolic dysfunction), mitochondrial efficiency decreases and fatigue ensues.
Beyond energy metabolism, NAD+ is the required substrate for sirtuins (SIRT1-7) — a family of NAD+-dependent deacetylases that regulate gene expression, DNA repair, inflammation, and metabolic adaptation. Sirtuins are considered central longevity regulators: SIRT1 activates PGC-1α (mitochondrial biogenesis), regulates the circadian clock, and suppresses inflammatory NF-κB signaling; SIRT3 protects mitochondria from oxidative stress; SIRT6 is essential for DNA double-strand break repair. When NAD+ falls with aging, sirtuin activity declines proportionally — this is one mechanism linking NAD+ decline to the accumulation of DNA damage, mitochondrial dysfunction, and inflammation characteristic of biological aging.
NAD+ is also consumed by PARP enzymes (poly(ADP-ribose) polymerases) — DNA repair enzymes that are activated by DNA damage and require NAD+ for their activity. As DNA damage accumulates with aging (from ultraviolet exposure, oxidative stress, replication errors), PARP activity increases and consumes NAD+, depleting the pool available for sirtuins and mitochondrial function. This creates a positive feedback loop: DNA damage → PARP activation → NAD+ depletion → reduced sirtuin activity → less DNA repair capacity → more DNA damage. CD38, an NADase enzyme that degrades NAD+ (its activity increases with age and chronic inflammation), is now recognized as a major contributor to age-related NAD+ decline and a target for therapeutic intervention.
Why NAD+ Declines With Age
The mechanisms driving the ~50% decline in NAD+ from young adulthood to age 60 include: increased PARP activity from accumulated DNA damage, increased CD38 activity (CD38 expression rises with age and inflammation — it is the dominant NADase in mammals), decreased expression of NAMPT (nicotinamide phosphoribosyltransferase — the rate-limiting enzyme in the NAD+ salvage pathway, which regenerates NAD+ from nicotinamide), and age-related changes in the gut microbiome that reduce nicotinamide absorption. Obesity and type 2 diabetes accelerate NAD+ decline through chronic PARP activation from oxidative stress and NLRP3 inflammasome activation increasing CD38 expression.
NMN vs. NR: The Evidence Comparison
Nicotinamide Riboside (NR)
NR was the first NAD+ precursor to achieve clinical evidence in humans. It is a naturally occurring form of vitamin B3 found in small amounts in milk and other foods. NR is taken up by cells through specific NR transporters and converted to NMN, then to NAD+. Multiple human trials have confirmed that NR raises whole-blood NAD+ levels — the CALERIE-related study showed 300 mg NR twice daily raised blood NAD+ by approximately 60%. NR is converted to NMN and then to NAD+ — it takes an extra enzymatic step compared to NMN, but this doesn’t appear to limit its efficacy in practice. The most compelling clinical evidence for NR is a 2018 University of Colorado study showing that NR (500 mg/day) improved certain cardiovascular function parameters in healthy older adults, and a 2022 trial showing cognitive benefits in older adults with mild memory concerns.
Nicotinamide Mononucleotide (NMN)
NMN is one step closer to NAD+ in the biosynthesis pathway. It was catapulted to mainstream awareness by David Sinclair’s research at Harvard demonstrating remarkable anti-aging effects in mice — restoring muscle function, vascular endothelial function, and mitochondrial biogenesis in older mice to levels resembling young mice. The key human trial: a 2022 RCT by Washington University showed that 250 mg/day NMN for 10 weeks raised blood NAD+ by 38% and improved muscle insulin sensitivity and skeletal muscle function in postmenopausal women with prediabetes. A 2023 Japanese trial showed that 250 mg/day NMN improved sleep quality and gait speed in healthy older men. The route of absorption debate: whether NMN is converted to NR before intestinal absorption or absorbed directly as NMN was recently resolved — human studies show NMN is absorbed intact and rapidly raises blood NMN levels, supporting direct absorption.
NMN vs. NR: Practical Summary
Both NMN and NR effectively raise NAD+ levels in humans. NR has more clinical trial data (it has been studied since 2016 in humans vs. 2020 for NMN). NMN has more compelling preclinical data and is more directly on the biosynthetic pathway to NAD+. For practical purposes, the cost of NMN has decreased substantially since 2020 (from $50+/dose to $0.50–1.50/dose for quality products), making it more accessible. The primary advantage of NMN over NR is sublingual or liposomal delivery options that bypass gut conversion — potentially achieving higher peak NAD+ elevation. Practical recommendation: either NMN (250–500 mg/day) or NR (300–600 mg/day) is reasonable; the difference in human outcomes between the two is not clearly established.
CD38 Inhibitors: The Underappreciated Part of the NAD+ Strategy
CD38 is an NADase enzyme that degrades NAD+ — its expression increases dramatically with aging and inflammation, and it is now considered one of the primary reasons NAD+ supplements have less impact in older, more inflamed individuals than in younger ones. Blocking CD38 prevents NAD+ degradation, allowing NAD+ levels to rise more effectively — analogous to reducing a water leak rather than simply adding more water. Apigenin (a flavonoid found in celery, parsley, and chamomile tea) inhibits CD38 in cell studies and has become a popular NAD+ stack addition at 50–100 mg/day. Quercetin also inhibits CD38 at pharmacological doses. Luteolin is a related CD38 inhibitor. These are food-derived compounds with favorable safety profiles. The evidence for CD38 inhibition in humans as a NAD+-boosting strategy is primarily theoretical at this point — but the mechanism is well-established and the combination of NMN/NR + CD38 inhibitor is a rational and commonly used stack.
The Most Effective Lifestyle NAD+ Boosters
High-intensity exercise is the most potent free NAD+ booster. Exercise activates AMPK (which stimulates NAMPT, the rate-limiting NAD+ biosynthesis enzyme), increases mitochondrial demand for NAD+, and triggers adaptations that chronically increase NAD+ biosynthesis capacity. Acute exercise raises skeletal muscle NAD+ to levels that current oral supplement doses may not match. Zone 2 aerobic training and resistance training both increase NAD+ through AMPK activation, with high-intensity interval training producing the most acute NAD+ elevation per session.
Intermittent fasting and caloric restriction increase NAD+ through multiple mechanisms: reduced PARP activation (less oxidative stress with caloric restriction), increased SIRT1 activity, and upregulation of NAMPT. Berberine activates AMPK and increases NAMPT expression — a mechanism shared with NMN. Time-restricted eating (16:8 or 18:6 intermittent fasting) consistently raises SIRT1 activity and NAD+ levels in animal models. Adequate sleep (7–9 hours) is important because NAD+ and the circadian clock are intimately linked — SIRT1 and CLOCK/BMAL1 form a feedback loop, and disrupted sleep disrupts NAD+ cycling.
The Safety Profile: Known and Unknown
NR and NMN have favorable short-term safety profiles in the clinical trials conducted to date (typically up to 12 weeks at doses of 250–1,000 mg/day). Common mild side effects: nausea, bloating, and flushing (less flushing than with nicotinic acid/niacin, which is a different B3 form). No serious adverse events have been reported in trials to date. The critical unknown: long-term safety. NAD+ supplements activate mTOR and mTORC1 in some contexts — the same pathway that promotes cellular growth but also suppresses autophagy when chronically elevated. In the context of cancer, chronically elevated NAD+ is theoretically capable of supporting tumor cell energy metabolism. This concern remains theoretical at standard supplemental doses in healthy people without cancer history, but is a reasonable caution. Niacin (nicotinamide) at very high doses (3g+/day) can cause liver toxicity — NR and NMN do not appear to share this risk at standard doses, but long-term data is still limited.
Resveratrol and Pterostilbene: The Sirtuin Activators
Resveratrol gained fame as a potential longevity molecule after David Sinclair’s discovery that it activates SIRT1 (the primary longevity sirtuin) in yeast. The subsequent mouse studies showed remarkable life extension. Human translation has been more limited — resveratrol has poor bioavailability (rapidly metabolized to inactive conjugates after oral ingestion) and the direct SIRT1 activation mechanism remains debated. Pterostilbene is a naturally methylated analog of resveratrol with significantly better bioavailability (~80% vs. ~20% for resveratrol) and appears more potent at the same dose. The combination of NMN or NR (which raises NAD+ substrate for sirtuins) with resveratrol or pterostilbene (which may enhance sirtuin activity) is the basis of the “Sinclair stack” popularized in the longevity community. The clinical evidence for this combination in humans remains preliminary — but the mechanistic rationale is sound, and the safety profile is favorable at standard doses (resveratrol 250–500 mg/day, pterostilbene 100–200 mg/day).
The Bottom Line
NAD+ decline is a real and significant hallmark of aging with clear mechanistic connections to mitochondrial dysfunction, DNA repair failure, and sirtuin activity loss. NMN (250–500 mg/day) and NR (300–600 mg/day) successfully raise NAD+ levels in humans and show promising early results for muscle function, metabolic parameters, and cognitive performance in specific populations. The translation from raised NAD+ to measurable longevity outcomes in humans is still being established — we don’t yet have the long-term human trials that animal studies would predict. The most robust NAD+-boosting strategy combines supplementation with lifestyle interventions: intense exercise (the most potent acute NAD+ booster), intermittent fasting, adequate sleep, and CD38 inhibitors (apigenin, quercetin). The long-term safety profile of NMN and NR is favorable in available data but limited to short-term trials — people with cancer or cancer history should discuss with their oncologist before use.
If you are interested in a comprehensive longevity and cellular health evaluation — including assessment of biological aging markers, metabolic function, mitochondrial health, and a personalized supplement and lifestyle protocol — call our office at (810) 206-1402 to schedule a functional medicine longevity consultation.
Frequently Asked Questions
Does NMN actually work in humans?
Yes — NMN successfully raises blood NAD+ levels in humans (38% at 250 mg/day in the Washington University trial). The 2022 Washington University RCT showed NMN improved skeletal muscle insulin sensitivity and function in postmenopausal women with prediabetes. A 2023 Japanese trial showed improved sleep quality and gait speed in healthy older men. NMN has compelling preclinical data and growing human trial evidence. However, the most important caveat: demonstrating that NMN raises NAD+ is not the same as demonstrating that it extends lifespan or prevents major diseases in humans — those long-term outcomes are not yet established in human RCTs.
What is better — NMN or NR?
Both NMN and NR effectively raise blood NAD+ levels and have similar safety profiles. NR has more years of human clinical data (studied since 2016). NMN is one step closer to NAD+ in the biosynthesis pathway and has shown muscle-specific benefits in the most recent RCTs. NMN is now available at similar cost to NR. For sublingual delivery (which bypasses gut conversion and may provide faster absorption), NMN has an advantage — sublingual NMN tablets are available. For someone starting NAD+ supplementation, either is reasonable; the difference in clinical outcomes between the two in humans is not clearly established.
What is the best dose of NMN?
The most studied dose range is 250–500 mg/day. The Washington University trial used 250 mg/day. A 2021 Keio University trial used 250 mg/day and showed safety across a range of doses. Going higher (1,000 mg/day) may provide additional NAD+ elevation but the additional clinical benefit over 500 mg/day is not established in humans. Taking NMN in the morning with resveratrol/pterostilbene and a small amount of fat (to improve resveratrol absorption) is the standard protocol in the longevity community. Cycling is sometimes recommended (5 days on, 2 days off) to allow autophagy to run during off days, though the evidence for cycling vs. continuous dosing is not established.
Can you get NAD+ from food?
NAD+ itself degrades in the digestive tract and cannot be absorbed from food. NAD+ precursors are found in food: niacin (vitamin B3/nicotinic acid) in meat, fish, and legumes; nicotinamide in a wide variety of foods; NR in small amounts in milk; tryptophan (which can be converted to NAD+ via the de novo pathway) in protein-rich foods. However, dietary NAD+ precursors are present in amounts far smaller than the doses used in supplements — food alone is not sufficient to meaningfully raise NAD+ levels in aging individuals with significant NAD+ decline. Lifestyle interventions (exercise, fasting) are the most potent free approaches to raising NAD+.