Regulation of and challenges in targeting NAD + metabolism.

Nicotinamide adenine dinucleotide, in its oxidized (NAD + ) and reduced (NADH) forms, is a reduction-oxidation (redox) co-factor and substrate for signalling enzymes that have essential roles in metabolism. The recognition that NAD + levels fall in response to stress and can be readily replenished through supplementation has fostered great interest in the potential benefits of increasing or restoring NAD + levels in humans to prevent or delay diseases and degenerative processes. However, much about the biology of NAD + and related molecules remains poorly understood. In this Review, we discuss the current knowledge of NAD + metabolism, including limitations of, assumptions about and unappreciated factors that might influence the success or contribute to risks of NAD + supplementation. We highlight several ongoing controversies in the field, and discuss the role of the microbiome in modulating the availability of NAD + precursors such as nicotinamide riboside (NR) and nicotinamide mononucleotide (NMN), the presence of multiple cellular compartments that have distinct pools of NAD + and NADH, and non-canonical NAD + and NADH degradation pathways. We conclude that a substantial investment in understanding the fundamental biology of NAD + , its detection and its metabolites in specific cells and cellular compartments is needed to support current translational efforts to safely boost NAD + levels in humans.