In short
NR (nicotinamide riboside) is a form of vitamin B3 and a precursor in the NAD+ salvage pathway, absorbed via well-characterized nucleoside transporters and converted to NAD+ through fewer, better-established enzymatic steps than NMN.
NR is phosphorylated by the enzyme NRK1 (or NRK2 in some tissues) to become NMN, which is then converted to NAD+ — one step upstream of where NMN itself enters the pathway. Because NR uses established nucleoside transporters for gut absorption, its oral bioavailability pathway is somewhat better characterized in the literature than NMN's, though direct human head-to-head bioavailability comparisons between the two are limited.
Human trials of NR (marketed under the brand name Niagen, among others) have similarly and consistently shown it raises blood NAD+ levels reliably at doses commonly studied between 250 mg and 1,000 mg per day, and is generally well tolerated. Some trials have reported specific findings: a 2018 trial (Martens et al., Nature Communications) in healthy middle-aged and older adults found NR supplementation reduced blood pressure and arterial stiffness, though other trials measuring different outcomes (such as insulin sensitivity or muscle function) have found no significant effect, illustrating that results vary by outcome measured and study population rather than showing uniform benefit.
As with NMN, no human trial has demonstrated that NR extends lifespan or reverses a hallmark of aging — human evidence remains limited to NAD+ level changes and a mixed, outcome-specific set of biomarker findings across different small trials. NR has a more settled U.S. regulatory position than NMN, having received a formal Generally Recognized as Safe (GRAS) notification acknowledged by the FDA and continuing to be sold as a dietary supplement.