NAD+

NAD+ (nicotinamide adenine dinucleotide) is a coenzyme present in every living cell. It sits at the center of energy metabolism, carrying electrons through the mitochondrial reactions that convert food into ATP, and it serves as a substrate for enzymes that repair DNA, regulate gene expression, and manage cellular stress. Without adequate NAD+, cells cannot perform these functions efficiently.

NAD+ levels fall substantially with age — by middle age, tissue concentrations may be roughly half of what they were in youth. This decline has attracted significant research attention as a potential driver of age-related metabolic dysfunction. Rather than supplementing NAD+ itself (which is poorly absorbed orally), most protocols use oral precursors — nicotinamide riboside (NR) or nicotinamide mononucleotide (NMN) — or deliver NAD+ directly by IV or subcutaneous injection.

People seek NAD+ therapy for energy, mental focus, recovery support, and general longevity. It is important to frame expectations honestly: NAD+ is not a weight-loss drug, the human clinical trial evidence is still early, and the dramatic anti-aging effects seen in animal models have not been replicated in large human RCTs. The biology is compelling; the clinical proof of benefit in healthy adults remains incomplete.

NAD+ is consumed as a cofactor by two major enzyme families. Sirtuins — a class of deacylases involved in gene silencing, DNA repair, and metabolic regulation — require NAD+ to function; when NAD+ falls, sirtuin activity declines. PARPs (poly ADP-ribose polymerases) also consume NAD+ during single-strand DNA break repair: high DNA damage accelerates NAD+ depletion, potentially creating a feedback loop that impairs repair capacity over time [1][2].

Oral precursors NR and NMN enter cells via distinct transporter pathways and are enzymatically converted to NAD+ intracellularly. Both have been shown in human studies to raise blood and tissue NAD+ levels. Intravenous NAD+ delivers the coenzyme directly into circulation, bypassing gut absorption, but whether this route produces meaningfully higher tissue NAD+ — or better clinical outcomes — compared to oral precursors has not been established in published human trials.

The foundational human bioavailability data come from Trammell et al. (2016, Nat Commun), who showed that a single dose of NR raised whole-blood NAD+ metabolites in healthy men, confirming that an oral precursor can meaningfully elevate NAD+ in humans [3]. This was a pharmacokinetic study, not a clinical outcomes trial, but it validated the precursor approach.

Martens et al. (2018, Nat Commun) conducted a randomized, placebo-controlled crossover trial of NR in healthy middle-aged and older adults for six weeks. NR reliably elevated blood NAD+ metabolites. However, it produced no significant improvements in blood pressure, arterial stiffness, or resting metabolic rate versus placebo — an honest null result for clinical endpoints in healthy people [4]. Similarly, Dollerup et al. (2018, Am J Clin Nutr) found in a 12-week placebo-controlled RCT that NR supplementation in obese men did not improve insulin sensitivity or skeletal muscle mitochondrial function despite raising NAD+ [7].

More promising signals emerge from specific populations. Elhassan et al. (2019, Cell Rep) found that NR supplementation augmented the NAD+ metabolome in aged human skeletal muscle and was associated with anti-inflammatory transcriptomic changes, though functional performance improvements were not statistically significant [5]. Yoshino et al. (2021, Science) reported that oral NMN (250 mg/day for 10 weeks) improved muscle insulin signaling in postmenopausal women with prediabetes and overweight — one of the most clinically specific signals in the human literature to date. Body weight did not change, and the sample size was small [6].

A broader mechanistic and preclinical review by Verdin (2015, Science) and an in vivo evidence review by Rajman et al. (2018, Cell Metab) synthesize the animal and early human data: NAD+ boosting extends healthspan in multiple animal models and attenuates metabolic disease phenotypes, but translation to humans is uncertain [1][2]. The evidence grade is B — plausible mechanism, consistent NAD+-raising effect in humans, inconsistent downstream clinical benefit in healthy adults. IV NAD+ specifically has no published controlled human trials; its use is based on mechanism and clinical anecdote.

For oral precursors, clinical trials have used NR at 250–1,000 mg/day and NMN at 250–500 mg/day, typically in single or divided doses with a meal. Providers commonly use these dose ranges when prescribing oral NAD+ support. For compounded injectable or IV NAD+, clinical protocols vary widely; providers commonly use 0.5–4 mg/kg per session administered slowly to reduce infusion reactions, but no dose has been validated in a large published RCT.

Duration is undefined by evidence: most published human trials ran 4–12 weeks. Long-term maintenance dosing is a matter of clinical judgment rather than established evidence. Patients should discuss dosing with a licensed provider who can account for their individual health status.

Oral NR and NMN have been well tolerated in clinical trials at doses up to 1,000–2,000 mg/day for periods of weeks to months. Mild gastrointestinal effects — nausea, loose stools, or mild flushing — are the most commonly reported adverse events and typically resolve with dose adjustment. No serious adverse events have been attributed to oral NAD+ precursors in published human trials to date.

Compounded injectable and IV NAD+ are frequently reported to cause flushing, chest tightness, palpitations, and nausea, particularly when infused rapidly. These effects generally resolve with slower infusion rates. Serious adverse events from IV NAD+ have not been systematically characterized in large published trials, so the full risk profile is unknown. Compounded injectable/IV NAD+ is not FDA-approved as a drug. Use is not recommended during pregnancy or breastfeeding, or in individuals with active cancer (NAD+ pathway upregulation is theorized to support tumor metabolism in some cancers, though this remains under study). Anyone with cardiac conditions or significant metabolic disease should be evaluated by a physician before starting NAD+ therapy.