The only commercially available form of NR, NIAGEN®, has twice been successfully reviewed under FDA's new dietary ingredient (“NDI”) notification program and has also been successfully notified to the FDA as generally recognized as safe (“GRAS”). NMN has not yet been notified to the United States (as NDIN or GRAS).*
In this 2018 study, 1000 mg/day of NR increased NAD levels by 60% on average in older adults. There is no published data to show how NMN affects human NAD levels.*
Scientists studying how precursors enter cells have made some interesting observations about NR and NMN. For example, NR uses a unique set of transporters and enzymes to convert directly into NAD within the cell, while NMN is broken down into NR first, outside of cells, before being used to increase NAD. Scientists also found NMN can’t increase NAD as efficiently without those unique NR transporters or enzymes. [1-4]
These experimental findings suggest that NMN, in its supplement form, becomes NR first before entering the cell. Then once inside the cell, it converts back into NMN to make NAD. This is a three-step and rather inefficient process. NR can directly access the cell, so it only requires two steps to begin creating NAD.