The Rabinowitz study suggested that not much NR made it out of the liver, with nicotinamide (NAM) levels increasing in the blood but not nicotinamide riboside (NR).

“the study didn't find that much NR made it out of the liver in fact what the study if I recall and I know it's been so long since I look at it but I think that they saw na M nicotinamide was up in the blood but not nicotinamide rai beside which you presumably will still wanted some of that leaving the liver to go to get into other cells”

“there was a paper that came out from Princeton Josh Rabinowitz his lab that looked Aurel nicotinamide ride beside it was a tracer study that looked at mice where they gave them oral and are and basically the question was what is the fate of this where is it going and what that paper showed was the liver took because this was oral of course so that stuff gets the NR gets absorbed out of the gut presumably and very quickly everything in the gut makes its way to the liver first hence it's called this first pass effect and it was in the liver that most of that nr got turned into nad”