NAD+ salvage and replacement
Cells recover and rebuild this coenzyme. Adding a precursor changes the supply; the human trials ask what follows.
The salvage pathway
A cell has several ways to make NAD+, including rebuilding it from nicotinamide. This salvage pathway is the part of NAD+ biology I enjoy most: a molecule essential to energy metabolism is continually recovered and remade. NAD+ means nicotinamide adenine dinucleotide, a coenzyme that helps enzymes work. Its chemistry puts it outside the peptide family.
The metabolism review by Covarrubias and colleagues follows NAD+ through its different jobs. In energy metabolism it carries electrons and cycles between NAD+ and NADH. Sirtuins, PARPs and CD38 consume it during signalling and repair processes, creating a continuing demand for replacement.
CD38 activity rises with age and inflammation. A falling NAD+ pool therefore raises questions about consumption as well as supply. I picture a busy cell making, recycling and spending a shared resource.
Blood NAD+ in the NR trial
In healthy overweight adults, a randomised, double-blind, placebo-controlled trial gave nicotinamide riboside, or NR, for eight weeks. The study's oral doses of 100, 300 and 1000 mg per day raised whole-blood NAD+ by 22%, 51% and 142%, respectively. It found no flushing and no significant adverse-event differences from placebo. Those are the NR trial’s measured blood changes, under its particular conditions.
The rise is impressive on its own terms. It establishes that a precursor can move this marker. Whether the relevant tissue receives a useful change, and whether a person benefits, are further experimental questions. I resist treating the largest bar as a picture of the largest health gain.
NMN and muscle insulin sensitivity
NMN, nicotinamide mononucleotide, is another precursor. A trial in prediabetic postmenopausal women reported improved muscle insulin sensitivity after ten weeks, assessed by a clamp procedure that measures the response to insulin under controlled conditions. Body composition and HbA1c did not change.
A separate 60-day NMN trial in middle-aged adults raised blood NAD+ across its dose groups and reported improvements in walking distance and quality-of-life scores. These deserve space alongside the biochemical marker. They concern particular populations and endpoints, however, and leave the wider longevity claim open.
The 2025 review of precursor supplementation in human ageing describes limited efficacy across human trials and sparse data on tissue-specific NAD+ dynamics. An experiment connecting those tissue measurements to an outcome would be the next advance I could get excited about.
NAD+ preparations and cautions
The route changes the question substantially. Oral precursor trials dominate the controlled human evidence discussed here. Intact NAD+ is poorly taken up by most cells, and intravenous NAD+ wellness therapy has little controlled evidence. Rapid infusions have been associated with chest or abdominal discomfort, flushing and nausea; FDA recalled a compounded injectable NAD+ preparation for elevated bacterial endotoxin, categorising the recall as Class I.
NAD+ and its precursors are marketed as dietary supplements. FDA has contested NMN's supplement status on the ground that it was first investigated as a drug. Injectable and intravenous NAD+ preparations are commonly compounded following a licensed provider's prescribing decision; compounded preparations are not FDA-approved.
The oncology concern is theoretical and context dependent: existing cancers might also make use of a larger NAD+ supply. Product purity is another uncertainty. The human ageing review’s cautious assessment is a sound place to return before making a larger claim about benefit. The cellular maintenance story has won me over; its connection to long-term human outcomes still needs considerable work.