Fig. 3: Summary of NAD+ and precursor metabolism in the body based on the current knowledge from preclinical tracer studies.
From: The role of NAD+ metabolism and its modulation of mitochondria in aging and disease
The figure provides an overview of NAD+ metabolism at the systemic level. Oral precursors (green arrow) are metabolized by the liver into NAM, which can be used by other tissues in the salvage pathway. Microbes in the GI tract can metabolize circulating NR into NAM, which in turn is subsequently metabolized into NA, bypassing the salvage pathway. Intravenous administration (blue arrow) of NR allows transport and utilization of the precursor by tissues through the circulatory system, while administration of NMN first requires metabolism by the liver. Once orally or intravenously administered, precursors are metabolized by the liver into NAM, which enters the circulation and can be utilized by tissues through salvage (black arrow). Further details in the main text39,69,126,127.
