Fatty acids during fasting

The hver makes ketone bodies from fatty acids during fasting. Fortunately, after 2 days of fasting, the brain adapts to use the ketone bodies as fuel, reducing the need for glucose and therefore decreasing the need for gluconeogenesis. 

B) oxidizes fatty acids during overnight fasting. 

Smith and Stirton applied this mechanism to the sulfonation of long-chain fatty acid esters [31]. Instead of forming the well-defined mixed anhydride during the reaction of fatty acids with S03, the acid esters form a complex less defined in structure and composition. In this complex the a-hydrogen is activated, so that a second molecule of S03 can react. These two addition steps are fast. The final step is again a slow rearrangement of the intermediate with a loss of one molecule of S03. 

The regulation of fat metabolism is relatively simple. During fasting, the rising glucagon levels inactivate fatty acid synthesis at the level of acetyl-CoA carboxylase and induce the lipolysis of triglycerides in the adipose tissue by stimulation of a hormone-sensitive lipase. This hormone-sensitive lipase is activated by glucagon and epinephrine (via a cAMP mechanism). This releases fatty acids into the blood. These are transported to the various tissues, where they are used. 

As we have seen, normally pyruvate would be the substrate for pyruvate dehydrogenase complex to form acetyl-CoA, but during fasting in the absence of glucose, acetyl -CoA for the TCA cycle is derived from fatty acid (3-oxidation (see Section 7.5.2) so pyruvate is diverted into oxaloacetate by the enzyme pyruvate carboxylase. Thus any amino acids whose carbon skeletons can be converted into pyruvate, OAA or another substrate of the TCA cycle, can be used for glucose synthesis. 

During fasting, the decrease in insulin and the increase in epinephrine activate hormone-sensitive lipase in fat cells, allowing fatty acids to be released into the circulation. 

Table 7.5 Arteriovenous differences of oxygen, glucose, fatty acids, acetoacetate and hydroxybutyrate across the brain of obese females during prolonged fasting...

In an adult, it is only after about 24 hours of fasting or during prolonged physical activity that fatty acid oxidation plays a major role in energy provision. Neither condition is common in developed countries, so that an inability to generate ATP from fat oxidation is not normally apparent. 

During the fasting state, the energy needs of the liver are provided by fatty acid catabolism ( 3-oxidation), which spares further glucose for export to peripheral tissues. 

D. Skeletal muscle in its resting state can satisfy most of its energy needs by oxidation of fatty acids taken up from blood, and during the early stages of fasting, protein degradation in the muscle is increased. 

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