NAM has long been recognized for its metabolic benefits due to its ability of boosting the NAD+ level.51,52 In mammals, NAM is primarily metabolized in the liver through the catalytic actions of key enzymes, namely NNMT and AOX1, ultimately leading to the formation of N-Me-6-PY and N-Me-4-PY for urinary excretion.53,54 Recent studies announced that overactivation of NNMT contributes to the progression of liver steatosis,55,56 while inhibition of AOX1 decreased hepatic TG in NAFLD rats,25 both suggesting that dysregulation of NAM metabolism might engage in the pathological process of NAFLD. Here, NNMT is linked to fatty liver disease.