Oral administration of nicotinamide mononucleotide (NMN) is a prominent strategy to elevate nicotinamide adenine dinucleotide (NAD+) levels to treat age-related pathologies, where it is assumed to be directly incorporated into the NAD+ metabolome through the canonical recycling pathway. During oral delivery, NMN is exposed to the gut microbiome, which can modify the NAD+ metabolome through enzyme activities that are not present in mammals. Here, we show that orally delivered NMN can undergo direct deamidation and incorporation in mammalian tissue via the de novo pathway, and that this deamidation is reduced in animals treated with antibiotics to ablate the gut microbiome. Further, we show that antibiotics treatment increases the overall availability of NAD+ metabolites in the gut epithelium, with one possibility that the gut microbiome could be in competition with the host for dietary NAD+ precursors. Together, these data highlight previously undescribed interactions between orally delivered NMN and the gut microbiome.