Non-alcoholic fatty liver disease (NAFLD) Is the Ieading cause of chronic liver disease in Children and adults and is strongly associated with obesity and insulin resistance. NAFLD is defined by fatty infiltration of :: 5% of liver volume. Once considered a benign condition, untreated NAFLD can lead to progressive hepatic damage, non-alcoholic steatohepatitis (NASH), and eventually cirrhosis. Hispanics have the highest prevalence of NAFLD, although the reasons for racial/ethnic differences are mostly unknown. Recent studies showed that liver fat percent is an independent risk factor for disease progression. There is a lack of clinically accessible and inexpensive tools to diagnose and monitor the liver fat changes over time. Jsing magnetic resonance imaging, we have demonstrated that 40% of children with obesity who have entered a clinical weight management program at the Arkansas Children's Hospital had NAFLD with varying degrees of liver fat percent at baseline. Children with NAFLD, in this cohort, had higher circulating Fibroblast Growth Factor (FGF)-21 and lower adiponectin concentrations compared to children without NAFLD. Furthermore, after 6-months of unsupervised lifestyle modifications and repeated liver MRI scans, % change in liver fat percent was significantly correlated with % change in FGF21 to Adiponectin Ratio :FAR) in those with NAFLD at baseline. This finding is clinically relevant because FAR may be an Adjunct tool to monitor liver fat content in response to treatments such as diet and exercise. FGF21 is a hepatokine md metabolic regulator that improves the whole body and hepatic lipid metabolism (i.e., FGF-21 induces 1epatic fatty acid oxidation and inhibits de novo lipogenesis, thus favoring decreased hepatic lipid content). nterestingly, obesity is characterized by an FGF21 resistance state. Some of the positive effects of exercise on metabolic health are hypothesized to be related to the FGF21-adiponectin axis. Adults with obesity and type 2 diabetes who were treated with exogenous FGF21 analogs had an elevation in their serum concentrations of adiponectin, suggesting that adiponectin mediates some of the beneficial effects of =-GF21. We hypothesize that the HIIT-driven improvements in hepatic steatosis will be associated with the changes in FGF21-adiponectin and insulin resistance at different degrees among ethnicities. The liver fat content will be quantified, and changes will be monitored via Fibroscan® (Echosens, Paris). The long-term oal is to determine whether HIIT is beneficial for treating NAFLD in adolescents and, if so, to identify (i) the characteristics of the participants with a positive response and (ii) the mechanistic pathways including FGF21-adiponectin and insulin-glucose axis by which HIIT improves NAFLD. Preliminary data generated 'rom this translational clinical study will guide studies to compare the effects of different exercise regimens md to examine if there are differenti...