Medium-chain acyl–coenzyme A (CoA) dehydrogenase (MCAD) catalyzes crucial steps in mitochondrial fatty acid oxidation, a process that is of key relevance for maintenance of energy homeostasis, especially during high metabolic demand. To gain insight into the metabolic consequences of MCAD deficiency under these conditions, we compared hepatic carbohydrate metabolismin vivoin wild-type and MCAD−/− mice during fasting and during a lipopolysaccharide (LPS)-induced acute phase response (APR). MCAD−/− mice did not become more hypoglycemic on fasting or during the APR than wild-type mice did. Nevertheless, microarray analyses revealed increased hepatic peroxisome proliferator-activated receptor gamma coactivator-1α (Pgc-1α) and decreased peroxisome proliferator-activated receptor alpha (Ppar α) and pyruvate dehydrogenase kinase 4 (Pdk4) expression in MCAD−/− mice in both conditions, suggesting altered control of hepatic glucose metabolism. Quantitative flux measurements revealed that thede novosynthesis of glucose-6-phosphate (G6P) was not affected on fasting in MCAD−/− mice. During the APR, however, this flux was significantly decreased (−20%) in MCAD−/− mice compared with wild-type mice. Remarkably, newly formed G6P was preferentially directed toward glycogen in MCAD−/− mice under both conditions. Together with diminishedde novosynthesis of G6P, this led to a decreased hepatic glucose output during the APR in MCAD−/− mice;de novosynthesis of G6P and hepatic glucose output were maintained in wild-type mice under both conditions. APR-associated hypoglycemia, which was observed in wild-type mice as well as MCAD−/− mice, was mainly due to enhanced peripheral glucose uptake.Conclusion:Our data demonstrate that MCAD deficiency in mice leads to specific changes in hepatic carbohydrate management on exposure to metabolic stress. This deficiency, however, does not lead to reducedde novosynthesis of G6P during fasting alone, which may be due to the existence of compensatory mechanisms or limited rate control of MCAD in murine mitochondrial fatty acid oxidation.