Exercise Metabolism

1. Exercise Metabolism

2. Objectives • Discuss the relationship between exercise intensity/duration and the bioenergetic pathways that are most responsible for the production of ATP during various types of exercise. • Define the term oxygen deficit. • Define the term lactate threshold. • Discuss several possible mechanisms for the sudden rise in blood-lactate concentration during incremental exercise.

3. Objectives • List the factors that regulate fuel selection during different types of exercise. • Explain why fat metabolism is dependent on carbohydrate metabolism. • Define the term oxygen debt. • Give the physiological explanation for the observation that the O2 debt is greater following intense exercise when compared to the O2 debt following light exercise.

4. Energy Requirements at Rest Rest-to-Exercise Transitions Recovery from Exercise: Metabolic Responses Outline • Factors Governing Fuel Selection Exercise Intensity and Fuel Selection Exercise Duration and Fuel Selection Interaction of Fat/ Carbohydrate Metabolism Body Fuel Sources • Metabolic Responses to Exercise: Influence of Duration and Intensity Short-Term, Intense Exercise Prolonged Exercise Incremental Exercise • Estimation of Fuel Utilization During Exercise

5. Energy Requirements at Rest Energy Requirements at Rest • Almost 100% of ATP produced by aerobic metabolism • Blood lactate levels are low (<1.0 mmol/L) • Resting O2 consumption: • 0.25 L/min • 3.5 ml/kg/min

6. Rest-to-Exercise Transitions Rest-to-Exercise Transitions • ATP production increases immediately • Oxygen uptake increases rapidly • Reaches steady state within 1–4 minutes • After steady state is reached, ATP requirement is met through aerobic ATP production • Initial ATP production through anaerobic pathways • ATP-PC system • Glycolysis • Oxygen deficit • Lag in oxygen uptake at the beginning of exercise

7. Rest-to-Exercise Transitions The Oxygen Deficit Figure 4.1

8. Rest-to-Exercise Transitions Comparison of Trained and Untrained Subjects • Trained subjects have a lower oxygen deficit • Better-developed aerobic bioenergetic capacity • Due to cardiovascular or muscular adaptations • Results in less production of lactic acid

9. Rest-to-Exercise Transitions Differences in VO2 Between Trained and Untrained Subjects Figure 4.2

10. Rest-to-Exercise Transitions In Summary • In the transition from rest to light or moderate exercise, oxygen uptake increases rapidly, generally reaching a steady state within one to four minutes. • The term oxygen deficit applies to the lag in oxygen uptake in the beginning of exercise. • The failure of oxygen uptake to increase instantly at the beginning of exercise suggests that anaerobic pathways contribute to the overall production on ATP early in exercise. After a steady state is reached, the body’s ATP requirement is met via aerobic metabolism.

11. Recovery From Exercise: Metabolic Responses Recovery From Exercise • Oxygen uptake remains elevated above rest into recovery • Oxygen debt • Term used by A.V. Hill • Repayment for O2 deficit at onset of exercise • Excess post-exercise oxygen consumption (EPOC) • Terminology reflects that only ~20% elevated O2 consumption used to “repay” O2 deficit • Many scientists use these terms interchangeably

12. Recovery From Exercise: Metabolic Responses Oxygen Debt • “Rapid” portion of O2 debt • Resynthesis of stored PC • Replenishing muscle and blood O2 stores • “Slow” portion of O2 debt • Elevated heart rate and breathing =  energy need • Elevated body temperature =  metabolic rate • Elevated epinephrine and norepinephrine =  metabolic rate • Conversion of lactic acid to glucose (gluconeogenesis)

13. Recovery From Exercise: Metabolic Responses EPOC is Greater Following Higher Intensity Exercise • Higher body temperature • Greater depletion of PC • Greater blood concentrations of lactic acid • Higher levels of blood epinephrine and norepinephrine

14. Recovery From Exercise: Metabolic Responses Oxygen Deficit and Debt During Light/Moderate and Heavy Exercise Figure 4.3

15. Recovery From Exercise: Metabolic Responses A Closer Look 4.1Removal of Lactic Acid Following Exercise • Classical theory • Majority of lactic acid converted to glucose in liver • Recent evidence • 70% of lactic acid is oxidized • Used as a substrate by heart and skeletal muscle • 20% converted to glucose • 10% converted to amino acids • Lactic acid is removed more rapidly with light exercise in recovery • Optimal intensity is ~30–40% VO2 max

16. Recovery From Exercise: Metabolic Responses Blood Lactate Removal Following Strenuous Exercise Figure 4.4

17. Recovery From Exercise: Metabolic Responses Factors Contributing to EPOC Figure 4.5

18. Metabolic Responses to Exercise: Influence of Duration and Intensity Metabolic Responses to Short-Term, Intense Exercise • First 1–5 seconds of exercise • ATP through ATP-PC system • Intense exercise longer than 5 seconds • Shift to ATP production via glycolysis • Events lasting longer than 45 seconds • ATP production through ATP-PC, glycolysis, and aerobic systems • 70% anaerobic/30% aerobic at 60 seconds • 50% anaerobic/50% aerobic at 2 minutes

19. Metabolic Responses to Exercise: Influence of Duration and Intensity In Summary • During high-intensity, short-term exercise (i.e., two to twenty seconds), the muscle’s ATP production is dominated by the ATP-PC system. • Intense exercise lasting more than twenty seconds relies more on anaerobic glycolysis to produce much of the needed ATP. • Finally, high-intensity events lasting longer than forty-five seconds use a combination of the ATP-PC system, glycolysis, and the aerobic system to produce the needed ATP for muscular contraction.

20. Metabolic Responses to Exercise: Influence of Duration and Intensity Metabolic Responses to Prolonged Exercise • Prolonged exercise (>10 minutes) • ATP production primarily from aerobic metabolism • Steady-state oxygen uptake can generally be maintained during submaximal exercise • Prolonged exercise in a hot/humid environment or at high intensity • Upward drift in oxygen uptake over time • Due to body temperature and rising epinephrine and norepinephrine

21. Metabolic Responses to Exercise: Influence of Duration and Intensity Upward Drift in Oxygen Uptake During Prolonged Exercise Figure 4.6

22. Metabolic Responses to Exercise: Influence of Duration and Intensity Metabolic Responses to Incremental Exercise • Oxygen uptake increases linearly until maximal oxygen uptake (VO2 max) is reached • No further increase in VO2 with increasing work rate • VO2 max • “Physiological ceiling” for delivery of O2 to muscle • Affected by genetics and training • Physiological factors influencing VO2 max • Maximum ability of cardiorespiratory system to deliver oxygen to the muscle • Ability of muscles to use oxygen and produce ATP aerobically

23. Metabolic Responses to Exercise: Influence of Duration and Intensity Changes in Oxygen Uptake During Incremental Exercise Figure 4.7

24. Metabolic Responses to Exercise: Influence of Duration and Intensity Lactate Threshold • The point at which blood lactic acid rises systematically during incremental exercise • Appears at ~50–60% VO2 max in untrained subjects • At higher work rates (65–80% VO2 max) in trained subjects • Also called: • Anaerobic threshold • Onset of blood lactate accumulation (OBLA) • Blood lactate levels reach 4 mmol/L

25. Metabolic Responses to Exercise: Influence of Duration and Intensity Changes in Blood Lactate Concentration During Incremental Exercise Figure 4.8

26. Metabolic Responses to Exercise: Influence of Duration and Intensity Explanations for the Lactate Threshold • Low muscle oxygen (hypoxia) • Accelerated glycolysis • NADH produced faster than it is shuttled into mitochondria • Excess NADH in cytoplasm converts pyruvic acid to lactic acid • Recruitment of fast-twitch muscle fibers • LDH isozyme in fast fibers promotes lactic acid formation • Reduced rate of lactate removal from the blood

27. Metabolic Responses to Exercise: Influence of Duration and Intensity Effect of Hydrogen Shuttle on Lactic Acid Formation Figure 4.9

28. Metabolic Responses to Exercise: Influence of Duration and Intensity Mechanisms to Explain the Lactate Threshold Figure 4.10

29. Metabolic Responses to Exercise: Influence of Duration and Intensity Practical Uses of the Lactate Threshold • Prediction of performance • Combined with VO2 max • Planning training programs • Marker of training intensity

30. Metabolic Responses to Exercise: Influence of Duration and Intensity In Summary • Oxygen uptake increases in a linear fashion during incremental exercise until VO2 max is reached. • The point at which blood lactic acid rises systematically during graded exercise is termed the lactate threshold or anaerobic threshold. • Controversy exists over the mechanism to explain the sudden rise in blood lactic acid concentrations during incremental exercise. It is possible that any one or a combination of the following factors might provide an explanation for the lactate threshold: (1) low muscle oxygen, (2) accelerated glycolysis, (3) recruitment of fast fibers, and (4) a reduced rate of lactate removal. • The lactate threshold has practical uses such as in performance prediction and as a marker of training intensity.

31. Estimation of Fuel Utilization During Exercise VCO2 R = VO2 VCO2 16 CO2 R= = = 0.70 VO2 23O2 VCO2 6 CO2 R = = = 1.00 VO2 6 O2 Estimation of Fuel Utilization During Exercise • Respiratory exchange ratio (RER or R) • R for fat (palmitic acid) • R for carbohydrate (glucose) C16H32O2 + 23 O2 16 CO2 + 16 H2O C6H12O6 + 6 O2 6 CO2 + 6 H2O

32. Estimation of Fuel Utilization During Exercise Estimation of Fuel Utilization During Exercise

33. Estimation of Fuel Utilization During Exercise In Summary • The respiratory exchange ratio (R) is the ratio of carbon dioxide produced to the oxygen consumed (VCO2/VO2). • In order for R to be used as an estimate of substrate utilization during exercise, the subject must have reached steady state. This is important because only during steady-state exercise are the VCO2 and VO2 reflective of metabolic exchange of gases in tissues.

34. Factors Governing Fuel Selection Exercise Intensity and Fuel Selection • Low-intensity exercise (<30% VO2 max) • Fats are primary fuel • High-intensity exercise (>70% VO2 max) • Carbohydrates are primary fuel • “Crossover” concept • Describes the shift from fat to CHO metabolism as exercise intensity increases • Due to: • Recruitment of fast muscle fibers • Increasing blood levels of epinephrine

35. Factors Governing Fuel Selection Illustration of the “Crossover” Concept Figure 4.11

36. Factors Governing Fuel Selection The Regulation of Glycogen Breakdown During Exercise • Dependent on the enzyme phosphorylase • Activation of phosphorylase • Calmodulin activated by calcium released from sarcoplasmic reticulum • Active calmodulin activates phosphorylase • Epinephrine binding to receptor results in formation of cyclic AMP • Cyclic AMP activates phosphorylase

37. Factors Governing Fuel Selection The Regulation of Muscle Glycogen Breakdown During Exercise Figure 4.12

38. Factors Governing Fuel Selection McArdle’s Syndrome: A Genetic Error in Muscle Glycogen Metabolism • Cannot synthesize the enzyme phosphorylase • Due to a gene mutation • Inability to break down muscle glycogen • Also prevents lactate production • Blood lactate levels do not rise during high-intensity exercise • Patients complain of exercise intolerance and muscle pain

39. Factors Governing Fuel Selection A Closer Look 4.3Is Low-Intensity Exercise Best for Burning Fat? • At low exercise intensities (~20% VO2 max) • High percentage of energy expenditure (~60%) derived from fat • However, total energy expended is low • Total fat oxidation is also low • At higher exercise intensities (~50% VO2 max) • Lower percentage of energy (~40%) from fat • Total energy expended is higher • Total fat oxidation is also higher

40. Factors Governing Fuel Selection Rate of Fat Metabolism at Different Exercise Intensities Figure 4.14

41. Factors Governing Fuel Selection Exercise Duration and Fuel Selection • Prolonged, low-intensity exercise • Shift from carbohydrate metabolism toward fat metabolism • Due to an increased rate of lipolysis • Breakdown of triglycerides  glycerol + FFA • By enzymes called lipases • Stimulated by rising blood levels of epinephrine

42. Factors Governing Fuel Selection Shift From Carbohydrate to Fat Metabolism During Prolonged Exercise Figure 4.13

43. Factors Governing Fuel Selection Interaction of Fat and CHO Metabolism During Exercise • “Fats burn in the flame of carbohydrates” • 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

44. Factors Governing Fuel Selection Carbohydrate Feeding via Sports Drinks Improves Endurance Performance • The depletion of muscle and blood carbohydrate stores contributes to fatigue • Ingestion of carbohydrates can improve endurance performance • During submaximal (<70% VO2 max), long-duration (>90 minutes) exercise • 30–60 g of carbohydrate per hour are required • May also improve performance in shorter, higher intensity events

45. Factors Governing Fuel Selection Sources of Carbohydrate During Exercise • Muscle glycogen • Primary source of carbohydrate during high-intensity exercise • Supplies much of the carbohydrate in the first hour of exercise • Blood glucose • From liver glycogenolysis • Primary source of carbohydrate during low-intensity exercise • Important during long-duration exercise • As muscle glycogen levels decline

46. Factors Governing Fuel Selection Sources of Fat During Exercise • Intramuscular triglycerides • Primary source of fat during higher intensity exercise • Plasma FFA • From adipose tissue lipolysis • Triglycerides  glycerol + FFA • FFA converted to acetyl-CoA and enters Krebs cycle • Primary source of fat during low-intensity exercise • Becomes more important as muscle triglyceride levels decline in long-duration exercise

47. Factors Governing Fuel Selection Influence of Exercise Intensity on Muscle Fuel Source Figure 4.15

48. Factors Governing Fuel Selection Effect of Exercise Duration on Muscle Fuel Source Figure 4.16

49. Factors Governing Fuel Selection Sources of Protein During Exercise • Proteins broken down into amino acids • Muscle can directly metabolize branch chain amino acids and alanine • Liver can convert alanine to glucose • Only a small contribution (~2%) to total energy production during exercise • May increase to 5–10% late in prolonged-duration exercise

50. Factors Governing Fuel Selection Lactate as a Fuel Source During Exercise • Can be used as a fuel source by skeletal muscle and the heart • Converted to acetyl-CoA and enters Krebs cycle • Can be converted to glucose in the liver • Cori cycle • Lactate shuttle • Lactate produced in one tissue and transported to another