If fat breakdown is predominating over carbohydrate metabolism, then there will be insufficient oxaloacetate (formed from pyruvate) to maintain the citric acid cycle at the necessary rate. In that case the acetyl Coa is diverted in the liver to the formation of ketone bodies. One of the ketone bodies, acetoacetate, is a β-keto acid, so it readily decarboxylates, forming acetone, which can be detected in the breath of a person with high levels of acetoacetate in the blood. Believe it or not, this has been on the MCAT.