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Metabolism in the Fed and Fasting States
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Fuel source kJ gramsTG’s 590,000 15,000Glycogen in liver 1500 90Glycogen in muscle 6000 350Free glucose 320 20Protein 100,000 6000
Fuel Reserves in a 70 kg man
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Metabolism of Carbohydrate, Lipid, & Protein
• TCA & ETC - common to all 3.
• All 3 give Acetyl CoA
• This catabolic pathway also:– Produces CO2
– Produces ATP – Produces NADH & FADH– Produces more ATP in ETC.
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The principal fuels are:GlucoseLactateFatty acidsKetone bodiesAmino acids
Glycerol and ethanol can be used only to a minor degree.
Glycogen is not a fuel, it is a storage molecule. Its breakdown product, glucose, is a fuel.Circulating triacylglycerols are not fuel. Fatty acids derived from their hydrolysis are.Not all cells use all fuels at all times and not all cells can process all fuels.
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Principal Fuel Metabolism of Tissues
Fuel used and/or stored and the fuel released form:
1) Brain
2) Liver
3) Adipose
4) Muscle
5) RBC
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• Brain Glucose None• Ketone bodies
• Liver Glucose, Glucose from amino acids by gluconeogenesis• Glycogen for raising blood glucose• Proteins form amino acids • Lipoproteins & fatty acids.
• Adipose tissue Glucose Fatty acids, glycerol, Triglycerides• Branched-chain • fatty acids• amino acids
• Muscle Glucose, Lactate, alanine, and glutamine• Fatty acids (anaerobic gives lactate, transported as• Ketones alanine. Glutamate from catabolism of aa)• Amion acids • Glycogen is for use of muscle only• • RBC Glucose only (No mitochondria) •
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(FED & Fasting States)Five Phases of Glucose Homeostasis
• Absorptive, postabsorptive, and early starvation occur sequentially over ~2 days.
• Intermediate, and prolonged starvation are over 38 subsequent days and beyond
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The Fed-Fast Cycle• Stages of fed-fast cycle
Fed state lasts 3 hours after meal ingestion.Postabsorptive/early fasting state 3 to 12-18 hours after mealFasting state 18 hours to 2 days after meal when nothing else eatenStarvation/long-term fast deprivation (Weeks)
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• The fed stateGlucose glycogen (Glycogenesis).Fatty acids synthesis (Lipogenesis).aa Protein (Protein synthesis).Glucose ATP (Glycolysis). ATP from glucose is used by:
nervous tissues (Brain)RBCsAdipose tissuesMuscle tissues
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Fuels in Fed State
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Metabolism in Fed State
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• The postabsorptive/early fasting state
Glycogen glucose (Glycogenolysis). (in liver & muscle)Amino acids (primarily) 3 used for (Gluconeogenesis).GlycerolLactate
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Glucogenic amino acids
1. Serine 2. Valine 3. Histidine 4. Arginine 5. Cysteine 6. Proline 7. Alanine 8. Glutamate 9. Glutamine 10.Aspartate 11.Asparagine 12.Methionine
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• The fasting stateAmino acids (primarily) (Protein Breakdown)GlycerolLactate Above 3 used for (Gluconeogenesis)Fatty acids acetyl CoA Ketone formation
(Lipolysis & Ketogenesis)Ketone bodies are:
Acetoacetate Beta hydroxybutyrateAcetone.
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FATTY ACIDS
FATTY ACYL CoA
CoA
β-OXIDATION
ACETYL CoA
KETONES
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• The starvation state:
Fatty acids used to greater extent
Glycerol major glucose source
Ketosis after oxaloacetate depletion
Because oxaloactate comes from glucose which is deficient in starving state.
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Amino Acid Metabolism
• Fed state: used for synthesis of proteins & excess is degraded (Protein synthesis & Urea cycle)
• Fasting state:Catabolism for energy produces quantities of N
(Excreted in urea), (Urea cycle)
Gluconeogensis in liver & kidneys
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Five Phases of Glucose Homeostasis
• Absorptive, postabsorptive, and early starvation occur sequentially over ~2 days.
• Intermediate, and prolonged starvation are over 38 subsequent days and beyond
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In general
• Brain is simply a fuel consumer• Liver and adipose tissue function as fuel
storage tissues and reservoirs• Muscle is both source and consumer leaning
heavily toward consumption• Kidney is only a significant source under
starvation
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The Central Role of the Liver in Metabolism
• Glycolysis • Glycogenesis• Glycogenolysis• Gluconeogenesis• Urea formation (cycle)• Protein synthesis• Lipoprotein synthesis• Catabolism, conversion to other compounds
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