Prolonged fasting starvation

Muscle decreases its use of ketone bodies and oxidizes fatty acids as its primary energy source. 

Because of decreased use by muscle, blood levels of ketone bodies rise. 

The brain then takes up and oxidizes ketone bodies to derive energy. Consequently, the brain decreases its use of glucose, although glucose is still a major fuel for the brain. 

Muscle protein is spared (i.e., less muscle protein is degraded to provide amino acids for gluconeogenesis). 

Because of decreased conversion of amino acids to glucose, less urea is produced from amino acid nitrogen in starvation than after an overnight fast. 

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In prolonged fasting (starvation), changes occur in the use of fuel stores that permit survival for extended periods of time. 

C. Blood Glucose Levels during Prolonged Fasting (Starvation)... 

Even during a prolonged fast, blood glucose levels do not decrease dramatically. After 5 to 6 weeks of starvation, blood glucose levels decrease to only approximately 65 mg/dL (Table 31.2). 

Hanus, L, Avraham, Y, Ben-Shushan, D., Zolotarev, O., Berry, E.M., and Mechoulam, R. (2003). Short term fasting and prolonged semi starvation have opposite effect on 2-AG levels in mouse brain. Brain Res., 983 144-151. 

Among the difficulties caused by prolonged fasting are metabolic disorders caused by vitamin deficiencies. What vitamins are needed during starvation to ensure that cells can continue to carry out the metabolic adaptations discussed in Section 30.3.1 of the text ... 

As discussed in section 5.4.4.2 and section 9-3.2, many of the products of amino acid metabolism can also be used for gluconeogenesis, as they are sources of pyruvate or one of the intermediates in the citric acid cycle, and hence give rise to oxaloacetate. The requirement for gluconeogenesis from amino acids in order to maintain a supply of glucose explains why there is often a considerable loss of muscle in prolonged fasting or starvation, even if there are apparently adequate reserves of adipose tissue to meet energy needs. 

The use of prolonged starvation for treatment of obesity has posed a fascinating problem that man is capable of fasting for periods of time beyond which he would have used all his carbohydrate resources and all his protein for gluconeogenesis in order to provide adequate calories as glucose for the central nervous system. 

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