• Digested by bacteria in stomach - NAD+ and precursors are mostly digested in the stomach, with new research pointing to bacteria as voracious consumers. The chart above shows huge increase in NMN after antibiotics.
• Digested by enzymes in intestines - "Dietary NAD was hydrolyzed primarily in the small intestine...to NMN... was rapidly hydrolized to NR... and NR more slowly to NAM"
• Metabolized to NAM in Liver - Any that makes it past the gauntlet of the GI tract to the liver get mostly metabolized to the less effective Nicotinamide (NAM) before being excreted to the bloodstream.
• NR quickly degraded in bloodstream - NR has an even bigger bio-availabilty problem than NMN and NAD+.  It is very unstable in the bloodstream, so it is never found at more than trace levels,  Even supplementation of 1,000 mg a day of NR in humans does not increase NR levels in the blood.

Quotes from some research below:

Unlike in cell culture where NR and NMN are readily incorporated into NAD, oral administration fails to deliver NR or NMN to tissues ⁽⁴⁾

Interestingly, we found that neither compound was able to enter the circulation intact in substantial quantities when delivered orally ⁽⁴⁾

Orally delivered NR and NMN are converted into NAM before reaching the systemic circulation. ⁽⁴⁾