Mitochondrial dysfunction is a hallmark of heart failure undetectable by current clinical techniques. We examined pigs with cardiac overload using hyperpolarized [1-13C]pyruvate magnetic resonance spectroscopy at rest and under stress. Mitochondrial function was determined in-vitro. Pyruvate oxidation rates were decreased in overloaded hearts, especially under stress. In-vitro mitochondrial respiration rates were decreased in tissue from overloaded hearts. In one group, we pharmacologically increased pyruvate oxidation, which led to decreased hypertrophy, increased contractile reserve and better mitochondrial respiration. Our work underlines the importance of metabolism in heart failure and suggests that stress hyperpolarized imaging may be a marker of mitochondrial dysfunction.