The NAD literature is large. It is also uneven, and the unevenness is rarely explained to customers.
Where the evidence is strong
The biochemistry. NAD's structure, its role in metabolism, the enzymes that use it, the pathways that make it. This is textbook material established over decades.
Where it is developing
Precursor supplementation studies. There is a real and growing body of work on NMN and NR in humans, much of it small, short, and focused on blood markers rather than outcomes people care about. Some of it is promising. None of it is conclusive.
Where it is thin
Direct oral NAD+ specifically, as opposed to its precursors. There is considerably less human data on this.
Where it is essentially absent
Long-term outcome studies in healthy people. The questions most customers actually have — will this make me feel better in a year, will I age differently — have not been answered by trials of the kind that could answer them.
How to use this
Treat any brand claiming certainty about the last two categories with caution. The honest position is that this is an area of genuine scientific interest where the consumer-facing claims have run well ahead of the evidence.
