The brain and heart can take advantage of ketone bodies when the amount of glucose is low. These are byproducts of fat metabolism and can be converted to acetyl-coA via the citric acid cycle.
Overproduction of these products can cause pathological conditions:
When the rate of synthesis of ketone bodies exceeds the rate of utilization ,their concentration in blood increases , this is known as ketonemia. This is followed by ketonuria- excretion of ketone bodies in urine. The overall picture of ketonemia and ketouria is commonly referred as ketosis. Smell of acetone in breath is a common feature in ketosis.