On: Mitochondria, exercise, and metabolic health

Episode: 85

Date: December 2019

Background: Researcher in sports medicine and performance, diabetes, cancer and critical care medicine.

Key Subjects:

• Breakdown into six different metabolic training zones based on:
  • Type of muscle fiber recruited: slow or fast twitch.
  • Type of cellular fuel use: fatty acids, or glucose.
  • Energy system: existing ATP stores (anaerobic), glycolysis (anaerobic), mitochondrial respiration (aerobic).
  • See “Exercise – Energy Systems and Exercise Zones“.
• A lot of exercise is low intensity and involves the aerobic burning of fat.
  • Known as zone 2 (when you can still carry on a conversation).
• At training intensity increases, we start using glucose as fuel and use less fat.
  • Zone 3: oxidize both fats and glucose in mitochondria (aerobic).
  • Zone 4: oxidize glucose in mitochondria (aerobic).
  • Zone 5: glycolysis of glucose in cytosol (anaerobic).
• At higher intensity, use more glucose, create more lactate.
  • Lactate is produced by Type II fibers during anaerobic exercise (glycolysis).
    • Glucose -> pyruvate -> lactate (similar to fermentation pathway).
• Old thesis: lactate is a toxic metabolic by-product and makes you tired.
  • During anaerobic exercise, excess lactate goes into the bloodstream.
  • Goes back to the liver where it is cleared.
  • Causes fatigue and muscle pain.
• Complementary new thesis: lactate shuttle.
  • Lactate is formed not only under anaerobic conditions, but also in the presence of oxygen.
  • Lactate is not only cleared by going into the blood stream, it is also transported into mitochondria for fuel.
    • Done by Type I fibers.
  • The oxidation of lactate in the mitochondria can be a major energy source.
• Elite athletes: high ability to burn fat and clear lactate.
  • Indicates strong mitochondrial health.
    • Elite athletes have more and larger mitochondria.
  • High ability to burn fat = don’t rely on glucose as fuel too early = don’t produce a lot of lactate.
  • High ability to clear lactate = mitochondria can use it as fuel = blood glucose levels don’t rise too early.
• Diabetes is the opposite: not good at burning fat or clearing lactate.
  • Rely more on glucose as fuel, burn little fat, produce a lot of lactate, blood lactate levels rise quickly.
• Heart rate training.
  • May make sense.
  • Heart rate is a physiologic parameter and responds to physiologic and metabolic stress.
  • Heart rate increases in line with lactate increases.
• Exercise and restricting carbs.
  • May not make sense.
  • Especially in situations requiring glycolysis, ie where you need glucose as a fuel.
  • Tends to lead to lower power output, lower lactate, lower heart rate.
  • There may be longer-term metabolic adaptation, but most athletes can’t wait for that to happen.
• Exercise and insulin sensitivity.
  • Exercise increases insulin sensitivity.
    • More efficient uptake of glucose by the muscles.
  • Exercise also increases non-insulin dependent glucose uptake.
    • Muscle contraction has similar impact on glucose uptake as insulin.
• Regular low intensity Zone 2 training improves metabolic flexibility.
  • Use and build Type 1 fibers.
    • Better mitochondria (Type I has highest density of mitochondria).
    • Better fuel partitioning (burn more fat, preserve glycogen, produce less lactate).
    • Clear more lactate (done by Type I fibers).
  • 2x a week for about an hour.
• Altitude training:
  • Improves oxygen carrying capacity.
  • But may reduce glycolytic capacity.