From the new review paper:
"What is really known about the effects of nicotinamide riboside supplementation in humans"
"...the primary circulating NAD+ precursors resulting from oral NR administration are NA and NAM (22), and what little orally administered NR enters the blood in its native form is rapidly converted to NAM in whole blood (half-life of ~3 min), so the concentration of NR in the blood is very low (15)."
https://www.science..../sciadv.adi4862