The differential impact of three different NAD+ boosters on circulatory NAD and microbial metabolism in humans

Nicotinamide adenine dinucleotide (NAD(H)) and its phosphorylated form NADP(H) are vitamin B3-derived redox cofactors essential for numerous metabolic reactions and protein modifications. Various health conditions are associated with disturbances in NAD+ homeostasis. To restore NAD+ levels, the main biosynthetic pathways have been targeted, with nicotinamide (Nam), nicotinamide riboside (NR) and nicotinamide mononucleotide (NMN) being the most prominent boosters. However, while many preclinical studies have examined the effects of these precursors, a direct comparison in humans is lacking, and recent rodent research suggests that the NAD+-boosting effects of NR and NMN may depend on their microbial conversion to nicotinic acid (NA), a mechanism not yet confirmed in humans. Here we show in a randomized, open-label, placebo-controlled study in 65 healthy participants that 14 days of supplementation with NR and NMN, but not Nam, comparably increases circulatory NAD+ concentrations in healthy adults. Unlike the chronic effect, only Nam acutely and transiently affects the whole-blood NAD+ metabolome. Using ex vivo fermentation with human microbiota, we identify that NR and NMN give rise to NA and specifically enhance microbial growth and metabolism. We further demonstrate ex vivo in whole blood that NA is a potent NAD+ booster, while NMN, NR and Nam are not. Ultimately, we propose a gut-dependent model for the modes of action of the three NAD+ precursors with NR and NMN elevating circulatory NAD+ via the Preiss–Handler pathway, while rapidly absorbed Nam acutely affects NAD+ levels via the salvage pathway. Overall, these results indicate a dual effect of NR and NMN and their microbially produced metabolite NA: a sustained increase in systemic NAD+ levels and a potent modulator of gut health.

Nicotinamide adenine dinucleotide (NAD(H)) and its phosphorylated form NADP(H) are vitamin B3-derived redox cofactors that have a central role in hundreds of metabolic reactions and post-translational modifications of proteins1,2,3,4,5,6. Given its essentiality, disturbances in NAD+ homeostasis, and hence, a decline in NAD+ levels, have been associated with multiple health conditions7,8,9,10. Several strategies have been deployed to replenish NAD+ levels through feeding with precursors from its three canonical biosynthetic routes: the salvage pathway from nicotinamide (Nam), the Preiss–Handler pathway from nicotinic acid (NA) and the de novo biosynthesis from the essential amino acid tryptophan, which converges on the Preiss–Handler pathway4,7,8,9,11 .

The most prominent NAD+ boosters are Nam, nicotinamide riboside (NR) and nicotinamide mononucleotide (NMN), while the use of NA is limited by skin flushing and gastrointestinal (GI) symptoms at high dose4,7,11,12. Although several studies have examined the acute and chronic effects of these precursors after oral administration4,7,13,14,15,16,17,18,19,20,21,22,23,24, a head-to-head comparison of their impact on NAD+ levels in humans is lacking. Moreover, the oral bioavailability of polar and charged nucleotides and nucleosides similar to NR and NMN is notoriously poor25, and recent studies in rodents suggest that a substantial part of the NAD+-boosting effect of NR and NMN is mediated via their microbial conversion to NA26,27,28,29,30. Whether such gut microbial activities have a role in these precursor modes of action in a human clinical set-up remains elusive.

To compare the effects of the three NAD+ precursors NR, NMN and Nam versus placebo in healthy adults, a randomized, open-label, placebo-controlled four-arm study was conducted (NCT05517122. The primary endpoint of the study was the change in the whole-blood baseline level of NAD+ after 14 days of once-daily dosing. The acute (in the 4-h period after supplementation) and chronic (from baseline on day 1 to baseline on day 14) effects on the NAD+ metabolome in whole blood were secondary endpoints. The NAD+ metabolomes in plasma and urine were exploratory endpoints. Additional exploratory endpoints included targeted and untargeted metabolomics in plasma and urine, respectively.

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https://www.nature.com/articles/s42255-025-01421-8