Aims: Low Carbohydrate Diets (LCD) are a popular intervention for weight loss, but the effect of such diets on myocardial ischemia is not known. Myocardial energy substrates and insulin signaling pathways may be affected by these diets, and both may play a role in protection of ischemic myocardium. We investigated whether LCD increases susceptibility to cardiac injury during ischemia and reperfusion in the isolated rat heart. Main methods: Rats were fed LCD (60% kcal from fat/30% protein/10% carbohydrate) or a control diet (CONT; 16%/19%/65%) for 2 weeks. Hearts from rats fed with LCD or CONT were isolated and subjected to normal perfusion in Langendorff mode, with 30 min global low flow ischemia (LFI; 0.3 ml/min) followed by 60 min reperfusion, or 60 min LFI followed by 120 min reperfusion. Key findings: LCD diet led to an increase in 3-hydroxybutyrate and lower circulating insulin. LCD diet also resulted in impaired left ventricular performance during LFI, reduced recovery of function following LFI and reperfusion, and 10- to 20-fold increased injury as measured by lactate dehydrogenase release and histologic infarct area. LCD diet also led to lower myocardial glycogen stores and glycogen utilization during LFI, and lower insulin signaling as assessed by Akt phosphorylation at the end of LFI and reperfusion, but no differences in ERK 1/2 phosphorylation. Significance: These results demonstrate that LCD affects myocardial energy substrates, affects insulin signaling, and increases myocardial injury following ischemia-reperfusion in the isolated heart. © 2008 Elsevier Inc. All rights reserved.