Obesity is associated with nonalcoholic fatty liver disease (NAFLD), which represents a spectrum of liver abnormalities that are characterized by an increase in intrahepatic triacylglycerol (TAG) content (i.e., steatosis) with or without infl ammation and fi brosis (i.e., steatohepatitis). The presence of steatosis is an important marker of metabolic dysfunction and correlates closely with multiorgan insulin resistance and dyslipidemia ( 1 ), which are key risk factors for diabetes and coronary artery disease. However, the mechanisms responsible for the link between increased intrahepatic TAG content and metabolic disease are not clear.It has been hypothesized that the hepatocellular accumulation of specifi c lipid intermediates, including diacylglycerol (DAG), acyl-CoA, and ceramide, in people with NAFLD is the molecular mechanism driving insulin resistance ( 2, 3 ). Accordingly, an understanding of the metabolic interrelationships among hepatic glycerolipid synthesis and lipid intermediates has important physiological and clinical implications. There are two pathways for diacylglycerol synthesis. In the liver, most DAG and TAG are synthesized from the sequential acylation of glycerol-3-phosphate. However, these lipids can also be synthesized from monoacylglycerol through the monoacylglycerol acyltransferase (MGAT) pathway ( Fig. 1 ) ( 4, 5 ). The MGAT and glycerol-3-phosphate pathways are convergent and each results in the synthesis of DAG, which is then acylated to form TAG by the DAG acyltransferase (DGAT) enzymes ( Fig. 1 ). The MGAT pathway is an important TAG Abstract Intrahepatic lipid accumulation is extremely common in obese subjects and is associated with the development of insulin resistance and diabetes. Hepatic diacylglycerol and triacylglycerol synthesis predominantly occurs through acylation of glycerol-3-phosphate. However, an alternative pathway for synthesizing diacylglycerol from monoacylglycerol acyltransferases (MGAT) could also contribute to hepatic glyceride pools. MGAT activity and the expression of the three genes encoding MGAT enzymes ( MOGAT1 , MOGAT2 , and MOGAT3 ) were determined in liver biopsies from obese human subjects before and after gastric bypass surgery. MOGAT expression was also assessed in liver of subjects with nonalcoholic fatty liver disease (NAFLD) or control livers. All MOGAT genes were expressed in liver, and hepatic MGAT activity was readily detectable in liver lysates. The hepatic expression of MOGAT3 was highly correlated with MGAT activity, whereas MOGAT1 and MOGAT2 expression was not, and knockdown of MOGAT3 expression attenuated MGAT activity in a liver-derived cell line. Marked weight loss following gastric bypass surgery was associated with a signifi cant reduction in MOGAT2 and MOGAT3 expression, which were also overexpressed in NAFLD subjects. These data suggest that the MGAT pathway is active and dynamically regulated in human liver and could be an important target for pharmacologic intervention for the treatment of obesity-related insulin resistance and ...