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Background: Impaired energy metabolism is a possible mechanism that contributes to insulin resistance and ectopic fat storage. Objective: We examined whether meal ingestion differently affects hepatic phosphorus metabolites in insulin-sensitive and insulin-resistant humans. Design: Young, lean, insulin-sensitive humans (CONs) [mean ± SD body mass index (BMI; in kg/m²): 23.2 ± 1.5]; insulin-resistant, glucose-tolerant, obese humans (OBEs) (BMI: 34.3 ± 1.7); and type 2 diabetes patients (T2Ds) (BMI: 32.0 ± 2.4) were studied (n = 10/group). T2Ds (61 ± 7 y old) were older (P < 0.001) than were OBEs (31 ± 7 y old) and CONs (28 ± 3 y old). We quantified hepatic γATP, inorganic phosphate (Pi), and the fat content [hepa-tocellular lipids (HCLs)] with the use of 31P/¹H magnetic resonance spectroscopy before and at 160 and 240 min after a high-caloric mixed meal. In a subset of volunteers, we measured the skeletal muscle oxidative capacity with the use of high-resolution respirom-etry. Whole-body insulin sensitivity (M value) was assessed with the use of hyperinsulinemic-euglycemic clamps. Results: OBEs and T2Ds were similarly insulin resistant (M value: 3.5 ± 1.4 and 1.9 ± 2.5 mg ⋅ kg-1 ⋅ min-1, respectively; P = 0.9) and had 12-fold (P = 0.01) and 17-fold (P = 0.002) higher HCLs, respectively, than those of lean persons. Despite comparable fasting hepatic γATP concentrations, the maximum postprandial increase of γATP was 6-fold higher in OBEs (0.7 ± 0.2 mmol/L; P = 0.03) but only tended to be higher in T2Ds (0.6 ± 0.2 mmol/L; P = 0.09) than in CONs (0.1 ± 0.1 mmol/L). However, in the fasted state, muscle complex I activity was 53% lower (P = 0.01) in T2Ds but not in OBEs (P = 0.15) than in CONs. Conclusions: Young, obese, nondiabetic humans exhibit augmented postprandial hepatic energy metabolism, whereas elderly T2Ds have impaired fasting muscle energy metabolism. These findings support the concept of a differential and tissue-specific regulation of energy metabolism, which can occur independently of insulin resistance. This trial was registered at clinicaltrials.gov as NCT01229059. [ABSTRACT FROM AUTHOR]