Control of Blood Glucose

Posted on May 6, 2007

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The Body Alternates Between Anabolic & Catabolic States

  • Anabolism: energy storage & growth
  • Catabolism: mobilization of stored energy to support body functions
  • We use energy continuously but take it in only during meals
  • During and immediately following a meal the body goes into an anabolic state & stores energy:
  • Anabolic activities are supported by the hormone insulin
  • Between meals the body goes into the state of catabolism: it breaks down glycogen, triglycerides and sometimes protein to provide energy
  • Catabolism is supported by the hormones glucagon and growth hormone
  • In exercise or stressful situations epinephrine and cortisol increase the rate of catabolism

Blood Glucose is Controlled Mainly by the Liver
  • Immediately after meals blood glucose comes from meal
  • Between meals blood glucose comes from the liver & kidneys
    • Only the liver and kidneys can release glucose into the blood (requires a special enzyme that converts glucose metabolites to glucose, which can be transported across the cell membrane)
    • Liver provides 80% of blood glucose supply between meals, the kidney 20%
  • Liver makes glucose in 2 ways:
    • Breaks down liver glycogen (glycogenolysis)
    • Converts other types of small molecules into glucose (gluconeogenesis)
      • Gluconeogenesis requires 4 special enzymes which can reverse some of the reactions of glycolysis
      • Many small molecules can be converted to glucose:
        • Lactate & pyruvate: mainly come from muscles
        • Glycerol: supplied by adipose tissue when triglycerides are broken down-
          • Fatty acids cannot be converted into glucose, but:
          • Fatty acid metabolism indirectly supports gluconeogenesis by producing AcetylCoA
          • AcetylCoA activates and inhibits key enzymes, promoting glucose formation
        • Amino acids: 20 of the 22 amino acids can be coverted to glucose
  • Figure shows rate of glucose delivery to the blood by the liver (Data from: Keith N. Frayn. Metabolic Regulation, 1997)
  • Delivery of glucose to the blood is controlled by insulin, glucagon and other hormones
  • Brain receives a constant 80 mg/min (doesn’t change much in exercise)
  • Amount delivered to other tissues depends upon body state (rest vs exercise)
    • At rest total glucose output to the blood is about 130 mg/min (80 to brain, 50 to other tissues)
    • During exercise total can increase to about 2500 mg/min

When the Body Can’t Burn Glucose it Switches to Fat Metabolism
  • If muscle cells can’t get enough glucose body will increase fat breakdown to provide free fatty acids (FFAs) as an energy source
  • Remember: brain cannot use FFAs
  • A side reaction of excess lipid metabolism is the build-up of ketoacids in the blood
    • Ketoacids can be smelled on breath: odor similar to acetone
    • Ketoacids disturb mental functions and lower blood pH

Diabetic Ketoacidosis is a Medical Emergency
  • If a diabetic lets his blood sugar get too high he may develop ketoacidosis, a life-threatening emergency
  • Most of the problems in ketoacidosis are due to:
    • Dehydration (due to excessive urine production)
    • Low pH (due to excessive lipid metabolism)
    • Large amounts of ketoacids in the blood (also due to excessive lipid metabolism)

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