The blood NAD rise is real; what follows from it is not known
Nicotinamide riboside and NMN both raise blood NAD within two weeks and hold it there while dosing continues. The step to anything a person feels is unclosed.
What the study found
Both precursors raise blood NAD, and that part is not disputed
Whatever else is contested, absorption is not. In dose-ranging and crossover trials, nicotinamide riboside and nicotinamide mononucleotide both raise blood nicotinamide adenine dinucleotide (NAD) within two weeks, and keep it up while dosing lasts. What that does for a person is unknown.
The caveat · Blood is not tissue
Blood is not tissue. The one trial that biopsied muscle to check delivery found NAD-related metabolites raised there, including the products of clearing the compound away, while the muscle's mitochondria performed no differently. In mice, oral doses of both compounds are taken apart into plain nicotinamide in the liver; the same compounds injected arrive intact.