Brain fasting state

The net results of these actions are most apparent in the fasting state, when the supply of glucose from gluconeogenesis, the release of amino acids from muscle catabolism, the inhibition of peripheral glucose uptake, and the stimulation of lipolysis all contribute to maintenance of an adequate glucose supply to the brain. 

Major compounds syn-thesized/used by liver, adipose, muscle, and brain during a fasting state... 

In the fasting state 75% of total body glucose disposal takes place in non-insulin dependent tissues the brain and splanchnic tissues (liver and gastrointestinal tissues). In fact, brain glucose uptake occurs at the same rate during fed and fasting periods and is not altered in type 2 diabetes. 

The metabolic problem in the fasting state is that the brain is largely dependent on glucose as its metabolic fuel, and red blood cells (which lack mitochondria) cannot utilize any metabolic fuel other than glucose. Therefore, those tissues that can utilize... 

Glucose is the main fuel for muscle in the fed state, but in the fasting state glucose is spared for use by the brain and red blood cells glycogen, fatty acids and ketone bodies are now the main fuels for muscle. 

Figure 5-5. Metabolic activities of major organs during a short-term fast. The importance of the liver in providing glucose to support the brain and other glucose-requiring organs in the post-absorptive state is illustrated. The body relies on available glycogen stores as a ready source for glucose as fuel. PPP, pentose phosphate pathway FA, fatty adds TAG, triacylglycerol.

---