This article is available online at http://www.jlr.org S. L. Hazen. Mass spectrometric profi ling of oxidized lipid products in human nonalcoholic fatty liver disease and nonalcoholic steatohepatitis. J. Lipid Res. 2010. 51: 3046-3054.Supplementary key words oxidized fatty acids • mass spectrometry • chiral mass spectrometry Nonalcoholic fatty liver disease (NAFLD) is currently the most common form of chronic liver disease affecting both adults and children, and it is strongly associated with obesity and insulin resistance ( 1, 2 ). One in three adults and one in ten children or adolescents in the United States have hepatic steatosis, a stage within the spectrum of NAFLD that is characterized by triglyceride accumulation in liver cells and follows a benign, nonprogressive clinical course ( 3, 4 ). Nonalcoholic steatohepatitis (NASH) is defi ned as lipid accumulation with evidence of cellular damage, infl ammation, and different degrees of scarring or fi brosis ( 5 ). NASH is a serious condition as approximately 25% of these patients progress to cirrhosis and its feared complications of portal hypertension, liver failure, and hepatocellular carcinoma ( 6-8 ).At present, the available noninvasive markers for NAFLD include a set of clinical signs and symptoms, nonspecifi c laboratory and radiological imaging tests, and combinations of clinical and blood test results ( 9 ). Although several of these markers are generally useful for the diagnostic evaluation of a patient with suspected Abstract Oxidative stress is a core abnormality responsible for disease progression in nonalcoholic fatty liver disease (NAFLD). However, the pathways that contribute to oxidative damage in vivo are poorly understood. Our aims were to defi ne the circulating profi le of lipid oxidation products in NAFLD patients, the source of these products, and assess whether their circulating levels refl ect histological changes in the liver. The levels of multiple structurally specifi c oxidized fatty acids, including individual hydroxy-eicosatetraenoic acids (HETE), hydroxy-octadecadenoic acids (HODE), and oxo-octadecadenoic acids (oxoODE), were measured by mass spectrometry in plasma at time of liver biopsy in an initial cohort of 73 and a validation cohort of 49 consecutive patients. Of the markers monitored, 9-and 13-HODEs and 9-and 13-oxoODEs, products of free radical-mediated oxidation of linoleic acid (LA), were signifi cantly elevated in patients with nonalcoholic steatohepatitis (NASH), compared with patients with steatosis. A strong correlation was revealed between these oxidation products and liver histopathology (infl ammation, fi brosis, and steatosis). Further analyses of HODEs showed equivalent R and S chiral distribution. A risk score for NASH (oxNASH) was developed in the initial clinical cohort and shown to have high diagnostic accuracy for NASH versus steatosis in the independent validation cohort. Subjects with elevated oxNASH levels (top tertile) were 9.7-fold ( P < 0.0001) more likely to have NASH than those with low levels (b...