Link to bioRxiv paper: http://biorxiv.org/cgi/content/short/2020.09.10.289561v1?rss=1 Authors: Kim, L.-J., Chalmers, T. J., Smith, G. C., Das, A., Poon, E. W. K., Wang, J., Tucker, S. P., Sinclair, D. A., Quek, L.-E., Wu, L. E. Abstract: Nicotinamide mononucleotide (NMN) is a prominent strategy to raise nicotinamide adenine dinucleotide (NAD+) levels, where it is assumed that exogenous NMN is directly incorporated into NAD+ by the canonical recycling pathway. This redox cofactor is required across evolution, including bacteria in the gut microbiome, which can de-amidate NMN to nicotinic acid mononucleotide (NaMN). Here, we sought to determine whether orally delivered NMN undergoes microbial de-amidation into NaMN prior to its uptake. We designed NMN isotopologues with 13C and 15N labelling at strategic positions to indicate incorporation into the NAD+ metabolome via the amidated or deamidated pathways, which were delivered to animals treated with antibiotics to deplete the gut microbiome. We provide evidence for NMN de-amidation by the microbiome prior to its incorporation in vivo. Microbiome depletion increased the overall abundance of NAD+ metabolites, suggesting a competition relationship. Strikingly, treatment with labelled NMN profoundly increased the production of unlabelled NAD+ precursors, suggesting that exogenous NMN impacts the NAD metabolome through indirect means, rather than through its direct incorporation. These results suggest the gut microbiome moderates and alters the assimilation of orally delivered NMN. Copy rights belong to original authors. Visit the link for more info