Well, because certain cells actually prefer to use ketone bodies as the major energy source. In fact, the cells of the renal cortex and the heart prefer to use ketone bodies over glucose molecules. On the other hand, cells found in the brain prefer to use glucose over ketone bodies. But if we're under certain conditions, for example, starvation conditions, the brain will actually use ketone bodies. In fact, 75% of the energy needs of the brain will come from the breaking down of ketone bodies. So we see that normally, acetoacetate and D three hydroxybutyrate are fuel molecules that are normally used by the heart and the renal cortex over glucose.