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
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)
C16H32O O CO H2O
RER= VCO2 / VO2 = 16 CO2 / 23 O2
= 0.70 . .

13. Estimation of Fuel Utilization During Exercise
Glucose Oxidation (metabolism)
C6H12O6 + 6 O 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. Comparing CHO and FAT 1g of CHO = 4kcal 1g of fat = 9kcal
1g of CHO needs g H2O for storage
1g of fat = 9kcal
1g of fat needs no H2O for storage
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

51. So……..
A 5 lb fat gain would equal a 40 lb CHO gain
1. WOW !!!!
2. Energy efficient?

52. Note:
Too much of a good thing is never good, however.

53. Questions?

54. End