Thus, circulating host NAM enters the gut lumen and is used by the microbiome for both NAD and NA synthesis.
Thus, gut microbiota provide an alternative route from NAM to NAD, bypassing the salvage synthesis pathway, in intestine and liver
Orally delivered NR was found in the GI tract but not in other peripheral tissues, indicating rapid metabolism of NR before it enters the circulation (Figure S4A). Only a small fraction of NAD in the liver and small intestine was M+9, indicating minimal direct assimilation of NR