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Feed-Fast Cycle
FEED CYCLE (ABSORPTIVE CYCLE)
- 2-4 hours after a meal
- Insulin: glucagon ratio is high
- Anabolic processes dominate
- Most tissues use glucose for energy in this state
- Excess carbohydrates stored as: glycogen (liver & muscle) and body fat (liver & adipose)
- Short term or long term (starvation)
- Occurs after absorptive phase
- Insulin: glucagon ratio is low
- Catabolic processes dominate (nutrients mobilized from storage)
Body's objectives:
1. Deliver glucose to brain and red blood cells
2. Distribute remainder to the rest of the tissues (TAG & ketone bodies)
Timeline
- Hour 0: ingest meal
- Hour 4: glucose depletes, glycogen stores mobilized
- Hour 18: liver glycogen depletes --> gluconeogenesis --> TAG mobilization --> ketogenesis
- Hour 24: TAG mobilization & ketogenesis peak
Primary functions of each organ italicized for clarity.
LIVER: CENTRAL ROLE IN METABOLISM
- Nutrients drain from gut (primary nutrient source) to liver via hepatic portal vein
- Liver distributes nutrients to remaining organs
- Responds to needs of all organs
Fed state
Most anabolic and storage pathways
- Glucose (glycolysis, HMP shunt-->NADPH)): stored as glycogen
- Fatty acid synthesis (from glucose --> acetyl CoA)
- Chylomicron remnants: TAG synthesized and packaged into VLDL
- Amino acids (synthesis of proteins, pyruvate, acetyl CoA & CAC intermediates)
Fasting state
Responds to body's glucose needs
i. Glycogen --> glucose
ii. Gluconeogenesis: liver only
iii. Ketogenesis: liver only (cannot be used by liver for energy)
- Ketogenesis prevents degradation of essential proteins for gluconeogenesis
- Hepatocyte energy requirements met by fatty acid oxidation
- TAG --> Free fatty acids (--> ATP) + glycerol (--> gluconeogenesis)
Fed state
Glucose: primary energy source (glycolysis --> ATP)
Fasting state
- Glucose still primary energy source
- Extended fast: ketone bodies (can cross blood brain barrier)
Fed state
Fatty acids stored as TAG
- Glucose (glycolysis, HMP shunt-->NADPH)
- Fatty acid synthesis
- Chylomicrons (dietary TAG): degrade to chylomicron remnants
Fasting state
- TAG --> Free fatty acids (--> ATP in liver/muscle) + glycerol (--> gluconeogenesis in liver)
Fed state
Glucose (glycolysis): stored as glycogen
Branched chain amino acids (preferentially): protein synthesis
- Chylomicrons (dietary TAG): degrade to chylomicron remnants
- Fatty acid oxidation (secondary energy sources)
- Energy needs change during exercise: primary metabolic function depends on own needs
Fasting state
- Muscle glycogen --> glucose (glycolysis)
- Muscle protein mobilized (gluconeogenic amino acids --> liver)
- As starvation progresses: fatty acid oxidation, ketone bodies