Twenty-five percent of the United States population has nonalcoholic fatty liver disease (NAFLD), a disease that includes hepatic fatty infiltration alone (simple steatosis) or steatosis plus inflammation, liver cell injury and death (nonalcoholic steatohepatitis \[NASH\]). NAFLD involving hepatic steatosis alone can be a stable clinical condition existing as the hepatic manifestation of insulin resistance. NASH develops in 25% of patients with NAFLD, and likely has additional pathologic underpinnings compared to steatosis alone. NASH leads to liver fibrosis, an elevated risk of cirrhosis and hepatocellular carcinoma and is likely to be the leading cause of liver transplantation in the U.S. The gold standard to distinguish probable uncomplicated NAFLD from NASH can be made using the histopathological nonalcoholic fatty liver disease score (NAS) score in which the sum of the categorical severity of steatosis, inflammation, and hepatocyte ballooning injury is ≥ 4, i.e., suggestive of NASH. Patients with obesity (BMI³30 kg/m2), type 2 diabetes (T2DM), age older than 45 years, and certain ethnicities are at high risk for developing NASH. In the U.S., the cost of management of NASH and its complications is $32 billion annually. Durable therapies are lacking for the NASH spectrum and an acceptable pharmaceutical intervention is not approved. Weight loss, achieved through lifestyle modification, is the cornerstone of therapy. Improvements in NAS are proportional to weight loss but weight loss does not reliably surpass 10% with lifestyle modification. Weight loss surgery (bariatric surgery) achieves dramatic weight loss. The vertical sleeve gastrectomy (VSG), a bariatric surgical procedure, can dramatically reduce (NAS) and favor NASH remission. Importantly, not all patients with NASH demonstrate histologic improvements following bariatric surgery, and in a small percentage of patients the disease may progress. Presently, it is not known what mechanistic biomarkers might prioritize steatosis, inflammation, or hepatocyte ballooning injury, and furthermore there are no known antecedent biomarkers of outcome of NAFLD with bariatric surgery. As the primary host for glucose and fat metabolism, the liver forms the critical nexus for whole body metabolism. In the setting of insulin resistance and NAFLD, glucose production becomes less responsive to the suppressive effects of insulin while de novo fat synthesis is enhanced in a paradoxical setting in which fat oxidation is not decreased. Mitochondrial b-oxidation of fatty acids normally produces acetyl-CoA which is terminally oxidized via the tricarboxylic acid (TCA) cycle, producing reducing equivalents needed for gluconeogenesis. While measured in uncomplicated human NAFLD, hepatic energy fluxes have never been formally measured in human NASH, and thus there is an unmet need to determine whether mitochondrial metabolism drives and/or predicts NAFLD progression/resolution, potentially yielding quantifiable predictive value over NAS alone. The research group has developed magnetic resonance spectroscopy (MRS)- based methods for quantifying hepatic oxidative energy fluxes and glucose metabolism ('hepatic energy fluxes') using administered dual 2H and 13C isotope tracers non-invasively and without imaging, requiring only collections of peripheral venous blood. These flux measurements will be performed at baseline in obese patients (BMI of 30-39.9 kg/m2) prior to VSG, who have all received preoperative liver biopsies for histopathological determination of NAS score, liver MRI-derived proton density fat fraction (PDFF) and elastography (MRE) assessment of fibrosis, and intravenous glucose tolerance tests (IVGTT) as a crude measure of insulin resistance. Then it will be determined how preoperative energy flux indices correlate with pre- and post-operative NAS, PDFF, and MRE indices. Thus, the premise of this study is that deranged hepatic mitochondrial metabolism is a key biomarker and mediator of the NAFLD/NASH continuum, and the central hypothesis the investigators will test is that preoperative hepatic fat oxidation and glucose production flux parameters differ between low versus high NAS, and response of the liver to bariatric surgery can be predicted by preoperative fluxes. It is anticipated that these preliminary observations will serve as proof-of-concept datasets supporting future R01 funding that comprehensively determine the role of hepatic oxidative fluxes in human NASH evolution and response to bariatric surgery. This will be first study to quantify hepatic metabolic fluxes in obese (BMI 30-39.9 kg/m2) adult patients with biopsy-proven NASH compared to NAFLD without NASH. This will also be the first study to correlate these findings to histopathological, radiographic, and clinical outcomes following VSG. Central parameters of hepatic energy fluxes, and the relative contributions of the liver's three carbon sources to endogenous glucose production (i.e., from PEP, glycogen, or glycerol), have not been quantified for in human NASH. Contributions of these metabolic indices preoperatively to VSG outcomes have not been interrogated.