Summary Background Obesity can influence hepatic mitochondrial function, and cause non‐alcoholic steatohepatitis (NASH). Diagnosis and follow‐up rely on invasive liver biopsy so blood‐based markers are urgently required. Aim To investigate whether values of circulating metabolites from energy and one‐carbon (1‐C) metabolism may: (a) reflect hepatic mitochondrial flexibility failure and (b) act as NASH biomarkers. Methods Patients with severe obesity undergoing bariatric surgery (n = 270) were investigated using quantitative targeted plasma metabolomics. Comparisons were with non‐obese controls without liver disease (n = 50). Obese patients with NASH (n = 53) and without NASH (n = 130) representing extreme groups of liver disease were assessed to test the diagnostic ability of the measured circulating metabolites. Paired liver biopsy and plasma samples from NASH patients were available 1 year post‐surgery and were evaluated to monitor metabolomic changes with liver damage resolution. Results We identified correlations between human liver metabolism and obesity. High‐plasma α‐ketoglutarate (α‐KG) and lactate concentrations in NASH patients indicating citric acid cycle replenishment via glutaminolysis might also be a crucial point in NASH onset. Plasma measurements of α‐KG, β‐hydroxybutyrate, pyruvate and oxaloacetate reduced the uncertainty in clinical diagnosis of NASH area under receiver operating characteristic curve (AUC) of 0.826 and predicted NASH resolution without ambiguity (AUC of 0.999). Conclusion Changes in plasma mitochondrial metabolites appear to be associated with NASH. These metabolic responses may be dynamically remodelled following resolution of liver damage through massive weight loss.