NAD+

NAD+ decline with age is well-documented

Multiple studies across tissues and species confirm that NAD+ levels drop significantly with age. This decline is associated with mitochondrial dysfunction, impaired DNA repair, increased inflammation, and metabolic deterioration. The decline is real -- it is the most agreed-upon finding in the NAD+ field. What remains debated is whether supplementation reverses these downstream effects in humans.

NMN and NR do raise blood NAD+ levels in humans

Yoshino et al. (2021, Science) showed that oral NMN supplementation (250mg/day for 10 weeks) increased NAD+ metabolites and improved muscle insulin sensitivity in 25 prediabetic, postmenopausal women. NR (nicotinamide riboside) has also been shown to raise blood NAD+ levels in multiple human trials (Martens et al., 2018). The precursors do get absorbed and do raise NAD+ -- the claim that oral supplements "do nothing" is incorrect. But raising blood NAD+ is the easy part. The hard question is: does that translate to anti-aging outcomes?

Downstream anti-aging effects are NOT yet proven in humans

Animal data (mostly mice) shows impressive results: improved mitochondrial function, extended healthspan, better metabolic markers. But mice are not humans. No human trial has demonstrated that NAD+ supplementation extends lifespan, reverses aging biomarkers at a clinically meaningful level, or prevents age-related disease. The data so far shows biochemical changes (NAD+ goes up) without confirmed clinical endpoints (you live longer, get fewer diseases, or measurably age slower).

IV NAD+ infusions: expensive, bioavailability advantage debated

IV NAD+ infusions cost $500-1,500 per session and are marketed as superior to oral precursors. The theoretical advantage is bypassing digestion and delivering NAD+ directly to the bloodstream. However, NAD+ is a large molecule and its cellular uptake from the blood is not straightforward -- cells may still need to break it down and rebuild it internally. The clinical evidence that IV infusions produce meaningfully better outcomes than oral NMN/NR ($30-60/month) is thin. Many of the reported benefits (energy, mental clarity) occur during the infusion and may involve placebo effects, niacin flush, or simple hydration.

Sinclair's claims drew legitimate peer criticism

David Sinclair (Harvard) popularized NAD+ supplementation through his book Lifespan and his research on sirtuins and NMN in mice. His mouse data is real and published. However, his public claims about NAD+ as an aging cure have been criticized by peers (notably Charles Brenner, who discovered NR) for extrapolating animal data to human recommendations and for commercial conflicts of interest (Sinclair co-founded and held equity in NMN supplement companies). The science is promising; the certainty with which it has been communicated to consumers has been a problem.

CD38: the major NAD+ consumer you rarely hear about

The enzyme CD38 increases with age and chronic inflammation, and it is one of the biggest consumers of NAD+ in the body. Some researchers argue that addressing CD38 overactivity (through lifestyle, inflammation reduction, or targeted compounds like apigenin or quercetin) may be as important as supplementing NAD+ precursors. Simply flooding the system with more NAD+ while CD38 is aggressively consuming it may be fighting the symptom rather than the cause.

CALERIE trial and the caloric restriction connection

The CALERIE trial (the only long-term caloric restriction trial in healthy humans) showed that moderate caloric restriction improved metabolic markers and slowed some measures of biological aging. Caloric restriction is known to activate sirtuins and increase NAD+ availability. This raises the question: are the benefits of NAD+ supplementation trying to replicate what caloric restriction does naturally? And if so, is the supplement a shortcut that works, or a partial mimic that misses the full picture?

The tryptophan de novo pathway -- often overlooked

Most of the NAD+ conversation focuses on the salvage pathway (NMN/NR). But the body also makes NAD+ from scratch via tryptophan (an amino acid from protein-rich foods) through the de novo pathway. This pathway is often overlooked in supplement marketing because it does not involve a product you can sell. Adequate protein intake, particularly foods high in tryptophan (turkey, chicken, fish, eggs), supports NAD+ synthesis without supplementation.

The biology is real. NAD+ matters for cellular energy, DNA repair, and metabolic regulation. Its decline with age is well-documented. The precursors (NMN, NR) do raise blood NAD+ levels in humans.

But "reverses aging" is a leap the human data does not yet support. No human trial has shown lifespan extension, disease prevention, or clinically meaningful aging reversal from NAD+ supplementation. The animal data is promising; the human data is early.

If you are supplementing, track your biomarkers over time rather than trusting how you "feel" on day one. And understand that a $40/month NMN capsule may be doing the same thing as a $1,000 IV drip -- the evidence that IV is meaningfully superior does not exist yet.

Randomized, placebo-controlled trial in 25 prediabetic, postmenopausal women. 250mg/day NMN for 10 weeks increased NAD+ metabolites in blood and improved muscle insulin sensitivity and insulin signaling. Small sample, but the first rigorous human trial showing a metabolic endpoint from NMN.

Crossover trial showing that NR (1,000mg/day for 6 weeks) safely raised NAD+ levels by ~60% in blood. Also showed trends toward lower blood pressure and reduced aortic stiffness, though the trial was not powered for clinical endpoints.

The only long-term (2-year) caloric restriction trial in healthy, non-obese humans. 25% caloric restriction improved metabolic health markers and slowed biological aging by epigenetic clock measures. Relevant because caloric restriction works partly through NAD+/sirtuin pathways -- the same pathways NAD+ supplements target.

Identified CD38 enzyme as a primary driver of age-related NAD+ decline. CD38 expression increases with inflammation and aging, consuming NAD+ faster than it can be replenished. Raised the question of whether reducing CD38 activity is more important than simply supplementing more NAD+.

In late 2022, the FDA determined that NMN may qualify as a new drug under investigation, which technically bars its sale as a dietary supplement in the US. Enforcement has been inconsistent and NMN remains widely available from supplement companies, but the regulatory status is unsettled. NR does not face this issue.