Does NR reach my cells, or does it get degraded first?
One study says its data "exclude the possibility that oral NR is used exclusively as Nam."
Another study reports the opposite, that first pass metabolism of NR to NAM in the liver makes NR "similar to or indistinguishable from oral NAM."
A third study reports that its data "conclusively demonstrate that NR contributes to NAD mainly via NA" in the gut.
A fourth study says that there are two phases to oral NR absorption -- first it gets distributed as NR, next it gets broken down to NA in the gut and NAM in the liver.
They can't all be right.
And there are LOTS of studies that show NR working differently from either NA and NAM.
It's much more likely that all of the studies have a piece of the truth: some NR gets through as NR, some gets degraded to NA in the gut, and some gets turned into NAM in the liver, and therefore the overall effect of oral NR is to light up all the NAD biosynthetic pathways at once.
Collectively, that's what the data says.
And it's also what one of the earliest studies concluded: "NR produced or tended to produce dose-dependent elevation of the entire NAD+ metabolome."
You can read a summary of all these studies here. Read together, they make far more sense than when you read their apparently conflicting conclusions in isolation: