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Alterations in cardiac energy and substrate metabolism play a critical role in the development and clinical course of heart failure. We hypothesized that the cardioprotective role of the breast cancer 1, early onset (BRCA1) gene might be mediated in part by alterations in cardiac bioenergetics.We generated cardiomyocyte-specific BRCA1 homozygous and heterozygous knockout mice using the Cre-loxP technology and evaluated the key molecules and pathways involved in glucose metabolism, fatty acid metabolism, and mitochondrial bioenergetics.Cardiomyocyte-specific BRCA1-deficient mice showed reduced cardiac expression of glucose and fatty acid transporters, reduced acetyl-coenzyme A carboxylase 2 and malonyl-coenzyme A decarboxylase (key enzymes that control malonyl coenzyme A, which in turn controls fatty acid oxidation), and reduced carnitine palmitoyltransferase I, a rate-limiting enzyme for mitochondrial fatty acid uptake. Peroxisome proliferator-activated receptor α and γ and carnitine palmitoyltransferase I levels were also downregulated in these hearts. Rates of glucose and fatty acid oxidation were reduced in the hearts of heterozygous cardiomyocyte-restricted BRCA1-deficient mice, resulting in a decrease in the rate of adenosine triphosphate production. This decrease in metabolism and adenosine triphosphate production occurred despite an increase in 5′-adenosine monophosphate-activated protein kinase and AKT activation in the heart.Cardiomyocyte-specific loss of BRCA1 alters critical pathways of fatty acid and glucose metabolism, leading to an energy starved heart. BRCA1-based cell or gene therapy might serve as a novel target to improve cardiac bioenergetics in patients with heart failure.