Metabolism during the Post Absorptive State
About 4 hours after a meal, absorption in the small intestine is almost complete. Blood glucose levels start to fall because blood glucose continues to leave the bloodstream and enter body cells while none is being absorbed from the GI tract. Therefore, as mentioned before, the challenge is to maintain a steady blood glucose level. Blood glucose concentration is important for the nervous system and RBCs due to the following: The dominant fuel molecule for ATP production in the nervous system is glucose, because fatty acids cannot pass through the blood-brain barrier. Your brain needs glucose! RBCs derive all their ATP from glycolysis of glucose because they have no mitochondria. Krebs and ETC are not available.
Post Absorptive State Reactions:
1. Breakdown of liver glycogen: during fasting this is a major source of blood glucose and can provide a 4-hour supply. 2. Lipolysis: glycerol produced by the breakdown of triglycerides in adipose tissue is also used to form glucose. 3. Gluconeogenesis using Lactic Acid: In the liver, lactic acid can be used for gluconeogenesis producing glucose released in the blood.
4. Gluconeogenesis using Amino Acids: can be converted to glucose in the liver.
Despite all the ways of making glucose, blood glucose cannot be maintained for very long. Therefore, a major adjustment is made during this state to produce ATP while conserving glucose. These are the following: 5. Oxidation of Fatty Acids: Most cells can oxidize fatty acids directly, feed them into the Krebs cycle as Acetyl-CoA and produce ATP through the ETC.
6. Oxidation of Lactic Acid: Cardiac muscles can produce ATP aerobically from lactic acid.
7. Oxidation of Amino Acids: amino acids may be oxidized directly in hepatocytes to produce ATP.
8. Oxidation of Ketone Bodies: Hepatocytes can convert fatty acids to ketone bodies which can be used by the heart, kidneys and other tissues for ATP production. 9. Breakdown of Muscle Glycogen: skeletal muscle breaks down glycogen to G6P which undergoes glycolysis and provides ATP for muscle contraction.
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