NAD+
“Using isotope tracers that allowed them to distinguish NAD+ made directly from NR or NMN versus NAD made from nicotinamide, NR, or NMN, they found that a low oral dose of the precursors (50 milligrams per kilogram body weight, mg/kg/bw) produced very low levels of NAD+ made directly from NR and NMN in the liver but not in other tissues.”