Many learn about NAD+ in our cellular metabolism and how it decreases with age from the book Lifespan by David Sinclair. In it, he promotes NMN, an anti-aging supplement that can boost NAD+ to youthful levels. As such, NMN is by far the most popular NAD+ booster. However, it is not the only one. There is also NR and Niacin (Vitamin B3). Of the three, NMN is actually the one with the worst bioavailability and the least amount of science to support it.
As a general rule of thumb, the bigger and more complex a molecule, the less likely it is to be absorbed in our intestine, to pass unaltered through our liver, and to enter our cells. This is the reason we cannot consume NAD+ directly and have to resort to precursors. Of the three precursors NMN, NR, and Niacin, Niacin is the smallest and the most bioavailable. Unfortunately, Niacin causes skin flushing at high doses.
This leaves us with NMN and NR to boost NAD+. Of the two, NMN is larger and less bioavailable. It is actually not clear if NMN can even enter the cell or if it has to be broken down to NR first. As such, NR is most likely the best choice. NR also has the most studies backing up its claim of increasing NAD+ in different parts of the body.
When choosing a supplement, it is important not to be distracted by hearsay, popularity, or experts' opinions (no matter how prominent). The only thing that matters is data. Right now, there is much more data on NR than on NMN. There is data from human trials that shows NR is safe, it actually boosts NAD+ in different tissues, and hints at the improvement of functional measures. On the flip side, for NMN, there are hardly any human studies, and the ones that do exist are small and flawed.