NAD+ Precursors: NMN, NR, and the Biology of Cellular Energy Decline

What NAD+ is and what it does

Nicotinamide adenine dinucleotide is a coenzyme present in all cells, where it serves as an electron carrier in cellular respiration. In the most basic description, NAD+ accepts electrons from metabolic reactions and passes them along the mitochondrial electron transport chain, a process that drives the synthesis of ATP. Without adequate NAD+, the cell cannot run oxidative phosphorylation efficiently.

Beyond energy metabolism, NAD+ is the required substrate for two classes of enzymes that have attracted substantial research attention. Sirtuins, a family of seven proteins in humans (SIRT1 through SIRT7), use NAD+ as a cofactor for deacylation reactions that regulate gene expression, stress responses, and mitochondrial biogenesis. PARP enzymes, which are involved in detecting and signaling DNA damage, also consume NAD+ as their substrate. Both systems draw on the same cellular NAD+ pool, which means that high DNA damage burden or chronic inflammatory signaling can deplete NAD+ and reduce what is available for other functions.

How NAD+ levels change with age

Multiple research groups have measured NAD+ and its metabolites in tissue samples from younger and older animals and humans. The consistent finding is that NAD+ levels decline substantially with age. Studies in rodents showed marked decreases in NAD+ in liver, muscle, and brain tissue between young and old animals. Human data from blood and tissue samples points in the same direction.

The reasons for this decline are not fully resolved. Proposed mechanisms include increased PARP activity from accumulated DNA damage, higher expression of a NAD+-consuming enzyme called CD38 in aging tissue, reduced activity of the salvage pathway enzymes that recycle NAD+ precursors, and potential changes in dietary niacin utilization. The likely answer involves a combination of these factors rather than a single cause.

• •Whole blood and peripheral blood mononuclear cell NAD+ has been measured as substantially lower in older adults compared with younger cohorts in multiple human studies
• •Skeletal muscle NAD+ content appears to decline with age based on biopsy data from human studies
• •CD38, an enzyme that degrades NAD+, shows increased expression in aging tissue and in the context of chronic low-grade inflammation
• •Caloric restriction, which extends healthy lifespan in rodent models, has been associated with maintained NAD+ levels and sirtuin activity
• •Exercise has been reported to acutely raise NAD+ levels in some tissues, providing a proposed mechanistic link between physical activity and metabolic aging

NMN as a precursor molecule

Nicotinamide mononucleotide is a nucleotide naturally present in small amounts in food and in the body. It sits one step upstream of NAD+ in the primary biosynthesis pathway and can be converted to NAD+ by the enzyme NMNAT. The rationale for using NMN as a supplement is that supplying more of this intermediate could support higher NAD+ synthesis, bypassing potential bottlenecks earlier in the pathway.

Research from David Sinclair's laboratory at Harvard and from Shin-ichiro Imai's laboratory at Washington University in St. Louis produced a series of widely cited rodent studies between 2013 and 2016 reporting that NMN supplementation raised NAD+ levels and produced effects on metabolic and physiological markers in aging mice. These papers contributed significantly to public awareness of NMN as a concept, though rodent studies cannot be taken as direct evidence of effects in humans.

Human trial data on NMN began appearing in peer-reviewed literature around 2020. A safety and pharmacokinetics trial by Irie and colleagues published in Endocrine Journal in 2020 enrolled healthy adult men and reported that oral NMN administration was well tolerated at doses up to 500 mg and measurably raised blood NMN and related metabolite levels. This was a small, short-duration trial focused primarily on tolerability rather than clinical outcomes.

NR as a precursor molecule

Nicotinamide riboside is a form of vitamin B3 and another NAD+ precursor. Chemically, it differs from NMN in that it lacks a phosphate group. NR enters cells via specific transporters and is phosphorylated to NMN before being converted to NAD+. It can also be dephosphorylated from NMN that is present in the extracellular environment.

NR became commercially available as a supplement product, sold under trade names including Tru Niagen, before NMN products reached the market at scale. This partly accounts for why some of the earlier and larger human pharmacokinetics studies used NR rather than NMN. A study by Martens and colleagues published in Nature Communications in 2018 enrolled healthy middle-aged and older adults and found that NR supplementation raised blood NAD+ metabolite levels in a dose-dependent manner and was well tolerated over 12 weeks. This is one of the better-controlled early trials in the field.

A follow-on study from the same group examined whether raising NAD+ with NR produced measurable changes in physiological markers like blood pressure and arterial stiffness in older adults. The results were mixed: some markers changed while others did not, and the effect sizes where changes were reported were modest. This pattern, of clear NAD+ elevation without obvious large-scale physiological effects, has been a recurring feature of NR trials.

What published human trials actually show

The honest summary of the human trial literature as of 2024 is that NMN and NR can reliably raise NAD+ levels in blood, that they appear safe and well tolerated in the doses studied, and that the functional consequences of that NAD+ elevation in healthy humans are not yet clearly established.

A notable trial published in Science in 2021 by Yoshino and colleagues studied NMN supplementation in postmenopausal women with prediabetes. The trial reported that NMN raised blood NAD+ and improved muscle insulin signaling, as measured by gene expression in muscle biopsy samples. However, whole-body insulin sensitivity measured by gold-standard clamp methods did not reach statistical significance versus placebo. The paper illustrates the gap between molecular biomarker changes and clinically measurable outcomes that appears throughout this literature.

Safety, dosing context, and open questions

NMN and NR have both shown acceptable tolerability in the trials published to date. Common adverse events reported have been mild and gastrointestinal in nature. Neither compound carries the flushing side effect associated with pharmacological doses of niacin (nicotinic acid). Doses studied in human trials have generally ranged from 250 mg to 1000 mg per day for NMN, and from 100 mg to 2000 mg per day for NR.

Neither NMN nor NR is an approved drug. NMN was listed by the FDA as a new dietary ingredient with a history of use in dietary supplements, though its regulatory status has been subject to some evolving agency guidance. NR is sold as a dietary supplement under FDA oversight. Neither compound has an approved clinical indication.

Open research questions include which tissues benefit most from oral NAD+ precursor supplementation, whether the route of delivery matters for specific outcomes, how baseline NAD+ status and age affect the response, how NMN and NR compare directly in adequately powered head-to-head trials, and whether any functional health outcomes in humans are large enough and consistent enough to support clinical guidance. These questions are active areas of research in academic and commercial settings.

The research chemical and supplement landscape

NMN and NR are available both as dietary supplements from consumer brands and, in some cases, from research chemical suppliers. The supplement market for these compounds grew rapidly following the publication of the rodent studies in the 2013 to 2016 period and has continued to expand as human trial data has accumulated. Consumer products vary widely in purity, formulation, and third-party verification.

Because these compounds are not prescription medications, the standards for purity verification, dosing accuracy, and label claims differ from what applies to regulated drugs. For research purposes, sourcing from suppliers who provide certificates of analysis from third-party laboratories is the standard approach to verifying identity and purity.

Compound reference pages