<P> Glucose can be obtained directly from dietary sugars and by the breakdown of other carbohydrates . In the absence of dietary sugars and carbohydrates, glucose is obtained from the breakdown of stored glycogen . Glycogen is a readily - accessible storage form of glucose, stored in notable quantities in the liver and in small quantities in the muscles . </P> <P> When the glycogen reserve is depleted, glucose can be obtained from the breakdown of fats from adipose tissue . Fats are broken down into glycerol and free fatty acids, with the glycerol being utilized in the liver as a substrate for gluconeogenesis . </P> <P> When even glycerol reserves are depleted, or sooner, the liver starts producing ketone bodies . Ketone bodies are short - chain derivatives of fatty acids, which, since they can cross the blood--brain barrier, can be used by the brain as an alternative metabolic fuel . Fatty acids can be used directly as an energy source by most tissues in the body . </P> <P> After the exhaustion of the glycogen reserve, and for the next 2--3 days, fatty acids are the principal metabolic fuel . At first, the brain continues to use glucose, because, if a non-brain tissue is using fatty acids as its metabolic fuel, the use of glucose in the same tissue is switched off . Thus, when fatty acids are being broken down for energy, all of the remaining glucose is made available for use by the brain . </P>

Where does the body use fat from first