Josh Rabinowitz has published a paper on NAD oral vs. IV
https://reader.elsevier.com/reader/sd/pii/S1550413118301967?token=C4C5E802F943BC5F630EFF514A754D30E930B6C1179B92D6CAD323E31D830E1B0B24DED0557B297D334E1B3C5946B509