It has long been known that when swallowed, NAD+, NMN and NR are mostly hydrolyzed to NAM in the stomach.
This study showed supplementation with NAM increases NAD+ levels just as much as NR and NMN, and it is not known how much more effective an oral supplement of NR, NMN or NAD+ is as compared to regular old NAM.
More surprising, was the Liu study published in Mar 2018 that clearly proved nearly all supplements of NR and NMN are digested to NAM in the stomach or liver, with very little reaching past the liver.
So while researchers have had success using oral supplementation with NAD+ precursors such as NR and NMN to increase NAD+ levels inside the body, they are all clearly degraded when passing through the stomach.
Taken sublingually (under the tongue) NAD+, NMN and NR may be more effective as they can be rapidly absorbed directly to the bloodstream, bypassing the GI tract entirely.