Key Takeaways:
- Aim for regular moderate intensity exercise:
- Uses and builds low twitch muscle fibers.
- These fibers have the highest number of mitochondria – you want more of them.
- These fibers also help clear lactate.
- More low twitch fibers = better mitochondrial health = better fuel partitioning.
- Rely more on burning fat.
- Save your glycogen stores.
- Also known as “Zone 2” training.
- Zone 2 = “you are still able to have a conversation” (or more sophisticated heart rate or lactate levels measurements).
- 2 times a week for about one hour.
- Uses and builds low twitch muscle fibers.
Introduction
- All tissues require adenosine triphosphate (ATP) to function normally
- ATP = universal energy currency.
- Adenosine plus three phosphate groups.
- Phosphate groups have high energy (electron) bonds between them.
- ATP = universal energy currency.
- The release of phosphate groups by ATP creates free energy to be used by the tissue.
- Losing one phosphate group at a time (see “AMPK” for more detail):
- ATP -> ADP -> AMP.
- Needs an enzyme to get the process started.
- ATPase.
- Losing one phosphate group at a time (see “AMPK” for more detail):
- ATP can be produced:
- With or without oxygen (aerobically or anaerobically).
- From various substrates (mainly fats or carbohydrates).
- Inside the cell’s cytosol or mitochondria.
- Three main energy systems generate energy and replenish ATP.
- Phosphagen system.
- When: for immediate, ultra-short term ATP demand.
- How: anaerobic.
- Using: no substrate (uses existing ATP stores in the muscles).
- Where: mitochondria.
- Glycolysis.
- When: for short-term (high intensity) ATP demand.
- How: anaerobic.
- Using: glucose.
- Where: cytosol.
- Mitochondrial respiration.
- When: for longer-term (low intensity) ATP demand.
- How: aerobic.
- Using: glucose or fats.
- Where: mitochondria.
- Phosphagen system.
- All energy systems contribute to varying degrees to the coordinated metabolic cellular response.
- Depending on intensity and duration of the exercise.
Phosphagen system: anaerobic use of existing ATP stores in the muscle.
- Immediate response to high ATP demand.
- Able to support extremely high muscle force application and power outputs.
- ATP demands exceed the capacity of mitochondrial oxidation.
- Short-term singular or a limited number of repeated intense muscle contractions.
- Throwing, hitting, jumping, sprinting.
- Limited capacity.
- Able to support extremely high muscle force application and power outputs.
- Existing ATP or phosphocreatine stores in the muscle.
- ATP store:
- Small, allows for 1-2 seconds of maximum effort.
- Phosphocreatine store:
- Another 5-8 seconds of exercise.
- Together 10-15 seconds of exercise of maximal activity.
- Seldom near complete reliance on phosphagen system.
- Rapidly replenished during recovery.
- ATP store:
Glycolysis: anaerobic use of glucose in the cytosol.
- High intensity ATP demand.
- When requirements exceed maximum oxygen uptake.
- Kicks in after the first few seconds and up to 2-3 minutes.
- Breaks up blood glucose.
- Glycogen stores in the muscles (80%) and in the liver (15%)
- One glucose molecule breaks into two pyruvate molecules.
- Produces pyruvate and NADH.
- Pyruvate: produced through the breakdown of glucose.
- NADH: produced through the reduction of NAD+.
- Produces modest amount of ATP:
- Net yield of 2 ATP produced in two phases:
- Phase 1: costs 2 ATP (breaking glucose into 2 intermediate molecules).
- Phase 2: generates 4 ATP (formation of 2 pyruvate molecules).
- Net yield of 2 ATP produced in two phases:
- In presence of oxygen:
- Pyruvate and NADH absorbed by mitochondria for further ATP production.
- If oxygen is absent:
- NADH donates its electrons to (excess) pyruvate.
- Pyruvate converts into lactate.
- This process allows NADH to be onverted back into NAD+.
- (Similar to process of fermentation)
- Production of lactate is important.
- Helps to remove excess pyruvate.
- Helps to regenerate NAD+.
- NAD+ allows process of glycolysis to continue.
- Lactic acid taken back to the liver.
- Through the bloodstream where it is converted back to pyruvate.
- Perhaps, some lactic acid taken back to the mitochondria.
- Lactate shuttle theory (see below).
Mitochondrial respiration: aerobic using fats and carbs in the mitochondria.
- When ATP demand is slow enough.
- Cellular respiration or oxidative phosphorylation.
- Combustion of fuel in the presence of sufficient oxygen.
- In multiple steps, oxygen takes electrons from fuel to generate free energy.
- Energy is then stored in the form of ATP.
- Step 1: fuel enters the mitochondria.
- Fatty acids, glucose / pyruvate enter the matrix of the mitochondria.
- Step 2: taking electrons from fuel.
- Taking away electrons.
- Oxidation of pyruvate.
- Beta-oxidation of fatty acids.
- Giving electrons.
- Reduction of NAD+ into NADH (see “NAD” for more detail)
- Reduction of FAD into FADH2.
- Formation of acetyl CoA.
- Taking away electrons.
- Step 3: taking electrons from acetyl CoA.
- A series of reactions called the Krebs or Citric Acid Cycle.
- Oxidation of acetyl CoA.
- Again, reduction of NAD+ and FAD into NADH and FADH2.
- Generation of 2 ATP.
- A series of reactions called the Krebs or Citric Acid Cycle.
- Step 4: release of electrons.
- Process takes places at the inner membrane space.
- The “electron transport chain”.
- NADH and FADH2 are ready to release their electrons.
- Oxidative phosphorylation.
- Oxidation of NADH (into NAD+).
- Oxidation of FADH2.
- Oxidative phosphorylation.
- Electrons are gradually released through the transport chain.
- As electrons are released, protons are pumped across the membrane.
- At the end of the chain, electrons are donated to oxygen (forming water).
- Oxygen is needed as a final acceptor of electrons.
- Without oxygen, the process would back up.
- As electrons are released, protons are pumped across the membrane.
- A proton gradient forms on one side of the membrane (see also “The Vital Question”).
- The flow-back of protons across the membrane back to the matrix helps form ATP.
- ADP -> ATP.
- Produces about 27-38 ATP.
- Depends on efficiency of the cell.
- Process takes places at the inner membrane space.
- See also MedCram video for explanation of mitochondrial process.
Examples of estimated contribution of energy systems:
- 10 second sprint estimated ATP contribution:
- Phosphagen: 53%
- Glycolysis: 44%
- Mitochondrial respiration: 3%
- 30 second sprint estimated ATP contribution:
- Phosphagen: 23%
- Glycolysis: 49%
- Mitochondrial respiration: 28%
- 75 second sprint estimated ATP contribution:
- Anaerobic (phosphagen & glycolysis): 50%
- Mitochondrial respiration: 50%
Exercise intensity and energy systems and fuels
- Exercise = sustained muscle contraction = sustained need for ATP.
- Muscle metabolic rates vary to a greater extent than any other tissue.
- High intensity exercise -> 1,000-fold increase in ATP demand compared to at rest.
- Muscle tissue can vary its metabolic rate to a greater extent than any other tissue.
- Depending on the demands placed upon it.
- Low to moderate intensity exercise (majority).
- Up to 55-75% of VO2max.
- Mitochondrial respiration.
- Burning mostly fats, limited carbohydrates.
- Higher exercise intensity exercise.
- Beyond 75% of VO2max.
- Mitochondrial respiration.
- Less fats (ATP generation too slow), more carbohydrates.
- Very high intensity.
- Beyond 100% of VO2max.
- Anaerobic (glycolysis and phosphagen system).
- Burn glucose.
- Importance of lactate.
- We could not sustain high-intensity exercise for more than 15 seconds without lactate production.
Exercise intensity and muscle fibers.
- Two kinds of muscle fibers.
- Type I or slow twitch (less forceful muscle contractions, fatigue slower).
- Type II or fast twitch (more forceful, fatigue faster).
- Sub-divided into Type IIa and IIb.
- Sequential recruitment.
- First Type I, then Type IIa, then Type IIb.
- Slow twitch, Type 1:
- Used in low intensity exercise.
- Have the highest mitochondrial density.
- High oxidative capacity (mitochondrial respiration), using fatty acids.
- Low glycolytic capacity.
- Fast twitch, Type 2A:
- Used in higher intensity exercise.
- Lower mitochondrial density.
- Fatigue faster, but still relatively slowly.
- High oxidative and high glycolytic capacity.
- Fast twitch, Type 2B:
- Used in highest intensity (short duration) exercise.
- Little mitochondrial density.
- Fatigues fastest.
- Low oxidative and high glycolytic capacity (also using existing ATP stores).
Exercise zones: combining substrates and muscle fibers
- Iñigo San Millan’s six training zones (image credit: Trainingpeaks.com)
Exercise: benefits of zone 2 training
- Stimulates Type I fibers.
- Have the highest density of mitochondria.
- Clear lactate.
- Stimulates mitochondrial growth and function.
- This in turn will improve the ability to utilize fat.
- Key in athletic performance.
- By improving fat utilization, preserve glycogen utilization for longer duration.
- Beneficial at later stages of exercise as intensity increases and glucose utilization is needed.
- Provides better lactate clearance.
- Type II fibers create lactate (during glycolysis).
- Type I fibers clear lactate (by transporting it back to mitochondria for further use)
- Frequency and duration:
- 3-4 days a week of zone 2 training in the first 2-3 months of pre-season training.
- 2-3 days a week as the season gets closer.
- 2 days of maintenance once the season is in full blown.
Exercise: lactate as fuel
- Old thesis: lactate is a toxic metabolic by-product.
- Excess lactate goes into the bloodstream, then back to the liver where it is cleared.
- Lactate gives rise to fatigue and muscle pain during anaerobic exercise.
- Complementary new thesis: lactate shuttle.
- The accumulation of Hydrogen ions drives fatigue, not lactate.
- Lactate is formed under both aerobic and anaerobic conditions.
- Type I fibers clear lactate by transporting it back to mitochondria.
- The oxidation of lactate (back to pyruvate) in the mitochondria can be a major energy source.
- So, lactate can be a major fuel source.
- Faster fuel than glucose.
Exercise and fatigue.
- Decrease in force production during muscle contraction despite constant or increasing effort.
- Preventive mechanism.
- Stops ATP from falling to levels that could cause muscle rigor or irreversible muscle damage.
Sidenote: aerobic glycolysis (as opposed to “usual” anaerobic glycolysis).
- Glucose can also be converted to lactate in the presence of oxygen.
- Usually it happens anaerobically.
- Cancer stem cells within a tumor are notorious for aerobic glycolysis
- Warburg Effect.
- Even in the presence of sufficient oxygen, cancer cells use glucose for fuel.
- Cancer cells produce a lot of lactate due to:
- Injured mitochondria -> increased glycolysis -> increased lactate.
- High cell growth drives high metabolic demand -> increased lactate.
- Lactate is a signaling model -> over-express oncogenes.
Key Sources
- Interaction among Skeletal Muscle Metabolic Energy Systems during Intense Exercise, Journal of Nutrition and Metabolism, Volume 2010, Julien S. Baker, Marie Clare McCormick, and Robert A. Robergs
-
Zone 2 Training For Endurance Athletes, Iñigo San Millan, April 2, 2014.
- The Drive with Peter Attia — Iñigo San Millan, December 23, 2019