Answer
In the post-absorptive state, the body tries to keep plasma glucose levels within the range 90-100 mg/dL. It is especially necessary to control the level of plasma level of sugar (CHO) because the brain needs glucose to use as a fuel. If enough dietary carbohydrates are not available, carbohydrates are derived from body stores by glycogenolysis. Glucose can also be synthesized from other compounds (aa's) by the process of gluconeogenesis.
Adipocytes and hepatocytes hydrolyze fats in the post-absorptive state. The glycerol metabolite derived is converted to glucose and the fatty acids are used as an energy source through the metabolism of ketone bodies. In this way, fats spare glucose for use by the brain-- the glucose-sparing effect.
The body catabolizes proteins for use as fuel if sugars and fats are not available as energy sources.
Work Step by Step
Although, under normal circumstances, the brain uses only glucose as fuel, in the case where one fasts for several days-- an extreme case of the post-absorptive state --the brain will begin to use ketone bodies as fuel.
Some proteins are more easily catabolized-- used as energy source --than others. For example, collagen is resistant to catabolism, while the protein of skeletal muscle is easily broken down.
