As the postnatal heart grows it also undergoes critical metabolic adaptations to use mainly lipids rather than carbohydrates as energy-providing substrates. Abnormal growth patterns and lack of metabolic flexibility are pathological responses of the heart that lead to cardiovascular diseases (CVD). Carotenoids, such as β-carotene - the most abundant dietary precursor of vitamin A - have been linked to the prevention of CVD. In the heart, β-carotene can be metabolized to retinoids and other apo-carotenoids by the action of β-carotene 9’,10’-dioxygenase (BCO2). We found that Bco2 -/- female mice have significantly smaller hearts than wild-type (WT), pre- and post-puberty. Echocardiography revealed a significant increase in aortic blood flow velocity with no change in overall cardiac output. This analysis also suggests that the mutant has a smaller left ventricle with thinner walls. In addition, expression of cardiac foetal genes (BNP, α-Skeletal actinin and β-MyHc ), typically upregulated during embryogenesis or under pathological conditions in the adult, were significantly increased in the mutant. Bco2 -/- females showed impaired exercise capacity, displaying a ~40% reduction in running distance compared to WT. These data indicate functional cardiac defects in the mutant under stress conditions. Bco2 -/- females also displayed impaired cardiac metabolic flexibility. We found that Bco2 -/- mice have reduced Pdk4 expression and thus enhanced PDH activity. Cardiac expression of Glut1 and Mct1, key glucose and lactate transporters , were significantly higher in Bco2 -/- females, while triglyceride levels were lower in the heart and serum. These findings suggest preferential use of glucose as a cardiac energy source. Interestingly, the mutants had reduced cardiac retinoic acid levels (by LC-MS analysis) compared to WT. Retinoic acid upregulates Pdk4 , thus we postulated that retinoic acid deficiency contributes to loss of cardiac metabolic flexibility in Bco2 -/- females. Ongoing studies aim to confirm this hypothesis and determine if altered retinoic acid homeostasis is also linked to impaired postnatal heart growth in mutant mice. These findings give new insights into a potential role of the β-carotene metabolic pathway in regulating adult heart function.