When mice take NMN or NR supplements by mouth, most of it gets changed into another form—nicotinic acid—by gut bacteria before the body uses it. The liver mostly uses this changed form to make an important molecule called NAD+, and only a little bit gets used directly.

What we've found so far is that when mice take oral NMN or NR, gut bacteria appear to convert much of it into nicotinic acid before it’s used by the liver to make NAD+ [1]. Our analysis of the available research suggests this conversion happens early in the digestive process, meaning the liver relies more on this altered form than on the original supplements. Based on what we've reviewed so far, the evidence leans toward the idea that NMN and NR don’t reach the liver unchanged after being taken by mouth [1]. Instead, gut bacteria play a key role in transforming these compounds into nicotinic acid, which the liver then uses to produce NAD+ [1]. Only a small amount seems to be used directly in its original form [1]. This suggests that the gut microbiome may be more involved in NAD+ metabolism than previously thought—at least in mice. The current analysis is based on a single assertion drawn from multiple supporting studies—nine in total—all pointing in the same direction, with none refuting the idea [1]. Still, we’re cautious about drawing firm conclusions from evidence focused on one animal model. Since these findings are in mice, we can’t assume the same process happens in humans. Also, we don’t yet know how variations in gut bacteria might affect this conversion between individuals. Our current analysis shows that in mice, oral NMN or NR likely becomes nicotinic acid thanks to gut bacteria before being used for NAD+ production in the liver. But we’re still building this picture—future research may change how we understand this pathway. Practical takeaway: In mice, the benefits of NMN or NR supplements may actually come from a byproduct made by gut bacteria, not the supplements themselves.