NAD+ (nicotinamide adenine dinucleotide) is a coenzyme present in every cell, central to energy metabolism and DNA repair. Its concentration falls with age, roughly 50% lower by the sixth decade compared to the twenties, and this decline is linked to reduced cellular energy production, impaired DNA repair capacity, and decreased activity of sirtuins, proteins that regulate cellular stress responses and metabolic function. NMN and NR are oral precursors that raise intracellular NAD+ levels, and both have moved through early human trials with reasonable safety profiles. The translation of those increases into meaningful health benefits in humans is less certain, and commercial enthusiasm around NAD+ supplements currently outruns the clinical evidence.

How NAD+ Works in Cells

NAD+ functions as an electron carrier in cellular respiration, accepting electrons during glycolysis and the citric acid cycle and shuttling them to the mitochondrial electron transport chain where ATP is produced. Beyond energy metabolism, NAD+ is consumed by:

  • Sirtuins (SIRT1-7): Proteins that regulate gene expression, mitochondrial biogenesis, inflammation, and DNA repair. Sirtuins require NAD+ as a substrate and are inactive without it.
  • PARPs (poly-ADP-ribose polymerases): DNA damage repair enzymes that consume NAD+ to detect and repair strand breaks. DNA damage increases with age, consuming more NAD+.
  • CD38: An enzyme that degrades NAD+, whose activity increases with age and chronic inflammation, contributing to the age-related NAD+ decline.

The result is a self-reinforcing pattern: aging increases NAD+ consumption through DNA damage and inflammation while reducing biosynthesis capacity.

NMN vs NR: The Two Main Precursors

Nicotinamide riboside (NR): NR enters cells and is phosphorylated to NMN, then to NAD+. Multiple human trials have confirmed that oral NR raises blood NAD+ levels. A 2018 randomized trial in Nature Communications found that NR at 1,000 mg/day increased whole-blood NAD+ by approximately 60% versus placebo in healthy adults. ChromaDex, the company behind the NR product Tru Niagen, has funded the largest body of NR research.

Nicotinamide mononucleotide (NMN): NMN is one step closer to NAD+ in the biosynthesis pathway. A 2022 randomized clinical trial in NPJ Aging found oral NMN 250 mg/day increased NAD+ levels in blood and skeletal muscle of older adults over 12 weeks. David Sinclair’s lab at Harvard has been the most prominent proponent of NMN, though most of his team’s mechanistic work has been conducted in animal models.

Both compounds raise blood NAD+ levels reliably. Whether blood NAD+ accurately reflects intracellular NAD+ in specific tissues such as brain, muscle, or liver is debated, and the measurement challenge complicates interpretation of most NAD+ precursor trials.

What Human Trials Have Shown

Muscle function and metabolism: A 2023 trial in Cell Metabolism found NMN supplementation improved muscle insulin sensitivity and increased skeletal muscle NAD+ in overweight middle-aged women, with activation of SIRT1 and SIRT3 in muscle tissue. It did not improve aerobic capacity as measured by VO2 max.

Physical function in older adults: A 2021 trial in Nature Aging found 300 mg NMN daily for 8 weeks improved walking speed and grip strength in older adults compared to placebo, suggesting some functional benefit, though the mechanism and durability are not established.

Metabolic markers: Several small trials show modest improvements in insulin sensitivity and lipid profiles, but sample sizes of 20-50 participants and durations of 8-12 weeks are insufficient to draw conclusions about long-term metabolic health outcomes.

Cognitive function: Animal models show NAD+ precursors improve memory and reduce neuroinflammation. No adequately powered clinical trial has tested NR or NMN for cognitive outcomes in humans. The mechanistic rationale is there; the clinical data is not yet.

Cardiovascular endpoints: Not yet tested in published human RCTs.

The Measurement Problem

Most NAD+ supplementation trials measure blood NAD+ as the primary outcome. Blood is accessible and standardized but may not reflect what is happening in tissues where NAD+ most matters, particularly skeletal muscle, liver, and brain. A supplement that doubles blood NAD+ without affecting muscle NAD+ may have minimal impact on the pathways that concern most users.

This is an active methodological problem in the field. Muscle biopsies allow direct measurement of skeletal muscle NAD+, which the 2022 NMN trial and 2023 Cell Metabolism trial used, but this approach is invasive and limits trial scale. Until better non-invasive tissue measures are available, the field will face this uncertainty.

NMN vs NR: Is One Better?

No head-to-head randomized trial in humans has directly compared NMN and NR at equivalent doses. Preclinical data suggests NMN may have slightly higher cellular uptake efficiency due to a dedicated NMN transporter (Slc12a8) identified in mouse intestinal cells, though whether this transporter functions the same way in humans is under investigation.

Practical differences:

  • NR is generally less expensive per dose
  • NMN is available in sublingual forms, which some manufacturers claim improve bioavailability by bypassing first-pass liver metabolism — one pharmacokinetic study supports faster absorption via sublingual delivery, but whether this translates to higher tissue NAD+ is not established
  • Both are well tolerated in published trials

Dosing

Doses used in human trials:

  • NR: 500-1,000 mg/day
  • NMN: 250-500 mg/day

Both are generally well tolerated. The most common side effects are mild digestive symptoms at higher doses. No serious adverse events have appeared in published trials at these doses.

Lifestyle Factors That Support NAD+ Biosynthesis

Before considering supplementation, several well-documented lifestyle factors raise endogenous NAD+ levels through the salvage pathway:

Exercise: High-intensity interval training activates AMPK and increases NAMPT expression, the rate-limiting enzyme in NAD+ salvage. A 2020 study in Cell Metabolism found that exercise training increased skeletal muscle NAD+ independent of supplementation.

Fasting and caloric restriction: Both reduce NAD+ consumption by lowering PARP activation (less oxidative stress) and increase NAMPT activity. Even 24-hour fasting increases blood NAD+ measurably.

Adequate sleep: Sleep deprivation increases inflammatory signaling and PARP activity, consuming NAD+. Consistent sleep timing supports NAD+ maintenance.

These interventions have a more established human evidence base than supplementation and produce overlapping benefits through the same sirtuin and PARP pathways.

What the Evidence Supports Doing

Taking NR or NMN at studied doses is safe and will raise blood NAD+ levels. Whether that increase produces meaningful health outcomes in middle-aged or older humans, beyond what is demonstrable in animal studies, remains genuinely open. The claim that NAD+ supplementation reverses aging is not supported by current human trial data. The evidence that it supports metabolic function in deficient or aging populations is more plausible but still emerging.

For people optimizing for cognitive performance and metabolic health, the lifestyle foundations of exercise, adequate sleep, and dietary quality have a stronger human evidence base than any supplement. NAD+ precursors are a reasonable addition for people who have those foundations in place and want to explore what the emerging science supports.

For more on evidence-based approaches to cognitive performance, see Caffeine and Cognition: What the Research Shows and Omega-3 and Brain Health.