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Exercise Metabolism

Chapter 4. Exercise Metabolism. Timing, Energy Use, and Control. What are the steps that occur going from rest to exercise back to rest? How do we know what’s happening inside the body - external indicators? How are the energy demands supplied in the time necessary?

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Exercise Metabolism

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  1. Chapter 4 Exercise Metabolism

  2. Timing, Energy Use, and Control • What are the steps that occur going from rest to exercise back to rest? • How do we know what’s happening inside the body - external indicators? • How are the energy demands supplied in the time necessary? • What are the limitations that prevent us all from being world record holders?

  3. Fuel During Exercise Limited or unlimited? Can we add more? How do we get access?

  4. Sources of Fuel During Exercise • Carbohydrate • Blood glucose • Muscle glycogen • Fat • Plasma FFA (from adipose tissue lipolysis) • Intramuscular triglycerides • Protein • Only a small contribution to total energy production (only ~2%) • May increase to 5-15% late in prolonged exercise • Blood lactate • Gluconeogenesis via the Cori cycle

  5. Estimation of Fuel Utilization During Exercise First - need to understand oxygen uptake.

  6. Oxygen Uptake Ventilation vs. Respiration

  7. Oxygen Uptake • Ventilation - moving air into and out of the lungs – breathing • Stimulated by CO2 in the blood • Air exhaled from the lungs is missing some oxygen and has new CO2 added

  8. Oxygen Uptake • Respiration – movement of gasses – oxygen and carbon dioxide • Pulmonary – lungs to blood, blood to lungs • Cellular – blood to tissues, tissues to blood • Tied to metabolism • O2 needed for metabolism • CO2 made from metabolism

  9. Oxygen Uptake . • VO2 – rate volume of oxygen used by the body each minute • Absolute units = liters/min • Relative units = ml/kg/min • VCO2 – rate volume of carbon dioxide produced by the body each minute • Absolute units = liters/min • Relative units = ml/kg/min • VE – rate volume of air exhaled each minute • Absolute units = liters/minute . .

  10. Estimation of Fuel Utilization During Exercise • During steady state exercise • VCO2 and VO2 reflective of O2 consumption and CO2 production at the cellular level . .

  11. Estimation of Fuel Utilization During Exercise • Respiratory exchange ratio (RER or R) • RER = VCO2 VO2 • Indicates fuel utilization • 0.70 = 100% fat • 0.85 = 50% fat, 50% CHO • 1.00 = 100% CHO . .

  12. Estimation of Fuel Utilization During Exercise Fat Oxidation (metabolism) C16H32O2 + 23 O2 16 CO2 + 16 H2O RER= VCO2 / VO2 = 16 CO2 / 23 O2 = 0.70 . .

  13. Estimation of Fuel Utilization During Exercise Glucose Oxidation (metabolism) C6H12O6 + 6 O2 6 CO2 + 6 H2O RER = VCO2 / VO2 = 6 CO2 / 6 O2 = 1.0 . .

  14. Exercise Intensity and Fuel Selection . • Low-intensity exercise (<30% VO2max) • Fats are primary fuel • High-intensity exercise (>70% VO2max) • CHO are primary fuel .

  15. Effect of Exercise Intensity on Muscle Fuel Source

  16. Exercise Intensity and Fuel Selection • “Crossover” concept • Describes the shift from fat to CHO metabolism as exercise intensity increases • Due to: • Recruitment of muscle fibers that need fuel quickly • Increasing blood levels of epinephrine that stimulates glycogenolysis

  17. Illustration of the “Crossover” Concept

  18. Exercise Duration and Fuel Selection • During prolonged exercise there is a shift from CHO metabolism toward fat metabolism • Increased rate of lipolysis • Breakdown of triglycerides into glycerol and free fatty acids (FFA) • Stimulated by rising blood levels of epinephrine

  19. Shift From CHO to Fat Metabolism During Prolonged Exercise

  20. Interaction of Fat and CHO Metabolism During Exercise • It is not all one or all the other • Carbohydrate is a key material • Carbohydrate is the brain’s fuel source • Run low = physical fatigue • Run low = mental stress (BONK)

  21. Interaction of Fat and CHO Metabolism During Exercise Some carbohydrate must be present in order for fat to be metabolized. Physiologic strategy: do what is necessary to “spare” carbohydrate.

  22. Interaction of Fat and CHO Metabolism During Exercise • “Fat burns in the flame of carbohydrates” • When glycogen is depleted during prolonged high-intensity exercise • Reduced rate of glycolysis and production of pyruvate • Reduced Krebs cycle intermediates • Reduced fat oxidation • Fats are metabolized by Krebs cycle

  23. Effect of Exercise Duration on Muscle Fuel Source

  24. Rest-to-Exercise Transitions Light switch & energy example

  25. Rest-to-Exercise Transitions • Oxygen uptake increases • Reaches steady state within 1-4 minutes • Oxygen deficit • Lag in oxygen uptake at the beginning of exercise • Suggests anaerobic pathways contribute to total ATP production • After reaching steady state, ATP requirement is met primarily aerobically

  26. The Oxygen Deficit

  27. Recovery From Exercise: Metabolic Responses • EPOC (formerly known as oxygen debt) • Excess post-exercise oxygen consumption • Elevated VO2 for several minutes immediately following exercise • Aerobic metabolism provides the energy to recycle ADP to ATP .

  28. Recovery From Exercise: Metabolic Responses • “Fast” portion of EPOC • Resynthesis of stored PC • Replacing muscle and blood O2 stores

  29. Recovery From Exercise: Metabolic Responses • “Slow” portion of EPOC • Elevated body temperature and catecholamines • Conversion of lactic acid to glucose (Cori Cycle - gluconeogenesis)

  30. Oxygen Deficit and Debt During Light-Moderate and Heavy Exercise

  31. Metabolic Response to Exercise: Incremental Exercise = Incremental increase in intensity

  32. Metabolic Response to Exercise: Incremental Exercise • Oxygen uptake increases linearly until VO2max is reached • No further increase in VO2 with increasing work rate . .

  33. Changes in Oxygen Uptake With Incremental Exercise

  34. Fate of Lactate Where does it go? -During anaerobic metabolism -During aerobic metabolism -After exercise

  35. Fate of Lactate Anerobic metabolism • Lactate can stay in the cell and build up • Hydogens inhibit PFK – slow glycolysis • Fatigue and discomfort • Decreased performance • Lactate can leave the cell and go into the blood • Measurable in millimoles per liter (mM/liter) • Lactate can be used by “aerobic” cells – reconvert to pyruvate, etc.

  36. Lactate Threshold • The point at which blood lactic acid suddenly rises during incremental exercise • Also called the anaerobic threshold • Also called OBLA – onset of blood lactic acid

  37. Mechanisms to Explain the Lactate Threshold

  38. Lactate Threshold • Practical use as a marker of exercise intensity • High intensity needs anaerobic metab =OBLA • The intensity cannot last for long • inhibit PFK with lactic acid – high intensity • run out of glycogen/glucose – high inten. + duration

  39. Lactate Threshold • Practical uses in prediction of performance • High threshold compared to max capacity = ability to remain at “low intensity” when others might be at “high intensity” • Ex. Top marathoners remain “aerobic” while running faster than 5 min/mile

  40. Identification of the Lactate Threshold

  41. Other Mechanisms for the Lactate Threshold • Failure of the mitochondrial hydrogen shuttle to keep pace with glycolysis • Excess NADH in sarcoplasm favors conversion of pyruvic acid to lactic acid • Type of LDH • Enzyme that converts pyruvic acid to lactic acid • LDH in fast-twitch muscle fibers favors formation of lactic acid

  42. Effect of Hydrogen Shuttle and LDH on Lactate Threshold

  43. Questions: Can lactate be removed faster? What does training do to OBLA / lactate threshold? Where does the lactate ultimately go?

  44. Removal of Lactic Acid Following Exercise

  45. Training Effect on OBLA • Endurance Training • “Grow” more mitochondria • Use aerobic metab. to supply ATP at higher intensities (less lactate produced) • More places for lactate to go • Greater and thus faster LA removal / use

  46. The Cori Cycle: Lactate As a Fuel Source

  47. Lactate as Fuel for the Heart • Heart is the ultimate “aerobic” muscle • Converts lactate to pyruvate easily • LDH in slow-twitch muscle fibers favors formation of pyruvate • Makes ATP through aerobic metabolism with pyruvate as the substrate

  48. Fat as fuel • Why is fat an economical fuel source?

  49. 1g of CHO = 4kcal 1g of CHO needs 3 g H2O for storage 1g of fat = 9kcal 1g of fat needs no H2O for storage Comparing CHO and FAT • So the energy equivalent of CHO in 1g of fat requires 2g CHO and 6g H2O • This is 8g of weight

  50. So…….. To gain the energy equivalent of 1 lb of fat it would take 2 lb CHO + 6 lb H2O = 8 total lb of weight

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