Objective: Aging is an important risk factor of nonalcoholic fatty liver disease (NAFLD).Here, we investigated whether the deficiency ofnicotinamide adenine dinucleotide (NAD+), a ubiquitous coenzyme, links aging with NAFLD.Methods: Hepatic NAD+ concentration,together with the protein levels of nicotinamide phosphoribosyltransferase (NAMPT) and several other critical enzymes regulating NAD+ biosynthesis, were compared between middleaged and aged mice or patients.