Chapter 4
Exercise Metabolism
Timing, Energy Use, and Control
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
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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?
Fuel During Exercise
Limited or unlimited?
Can we add more?
How do we get access?
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
Estimation of Fuel Utilization
During Exercise
First - need to understand oxygen uptake.
Oxygen Uptake
Ventilation vs. Respiration
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
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
Oxygen Uptake

.VO
– rate volume of oxygen used by the
body each minute
2
– 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
Estimation of Fuel Utilization
During Exercise

During steady state exercise
.
.
– VCO2 and VO2 reflective of O2
consumption and CO2 production at the
cellular level
Estimation of Fuel Utilization
During Exercise

Respiratory exchange ratio (RER or R)
.
– RER = VCO2
.VO
2
– Indicates fuel utilization
• 0.70 = 100% fat
• 0.85 = 50% fat, 50% CHO
• 1.00 = 100% CHO
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
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
Exercise Intensity and Fuel
Selection
.
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Low-intensity exercise (<30% VO2max)
– Fats are primary fuel
.
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High-intensity exercise (>70% VO2max)
– CHO are primary fuel
Effect of Exercise Intensity on
Muscle Fuel Source
Exercise Intensity and Fuel
Selection
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“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
Illustration of the “Crossover”
Concept
Exercise Duration and Fuel
Selection
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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
Shift From CHO to Fat Metabolism
During Prolonged Exercise
Interaction of Fat and CHO
Metabolism During Exercise
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It is not all one or all the other
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Carbohydrate is a key material
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Carbohydrate is the brain’s fuel source
– Run low = physical fatigue
– Run low = mental stress (BONK)
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.
Interaction of Fat and CHO
Metabolism During Exercise
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“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
Effect of Exercise Duration on
Muscle Fuel Source
Rest-to-Exercise Transitions
Light switch & energy example
Rest-to-Exercise Transitions
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Oxygen uptake increases
– Reaches steady state within 1-4 minutes
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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
The Oxygen Deficit
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
Recovery From Exercise:
Metabolic Responses

“Fast” portion of EPOC
– Resynthesis of stored PC
– Replacing muscle and blood O2 stores
Recovery From Exercise:
Metabolic Responses

“Slow” portion of EPOC
– Elevated body temperature and catecholamines
– Conversion of lactic acid to glucose (Cori Cycle
- gluconeogenesis)
Oxygen Deficit and Debt During
Light-Moderate and Heavy Exercise
Metabolic Response to Exercise:
Incremental Exercise
= Incremental increase in intensity
Metabolic Response to Exercise:
Incremental Exercise

Oxygen
uptake
increases
linearly
until
.
VO2max is reached
.
– No further increase in VO2 with increasing
work rate
Changes in Oxygen Uptake With
Incremental Exercise
Fate of Lactate
Where does it go?
-During anaerobic metabolism
-During aerobic metabolism
-After exercise
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.
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
Mechanisms to Explain the
Lactate Threshold
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
Lactate Threshold
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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
Identification of the Lactate
Threshold
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
Effect of Hydrogen Shuttle and
LDH on Lactate Threshold
Questions:
Can lactate be removed faster?
What does training do to OBLA / lactate
threshold?
Where does the lactate ultimately go?
Removal of Lactic Acid Following
Exercise
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
The Cori Cycle: Lactate As a Fuel
Source
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
Fat as fuel
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Why is fat an economical fuel source?
Comparing CHO and FAT
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1g of CHO = 4kcal
1g of CHO needs
3 g H2O for storage
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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
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
So……..
A 5 lb fat gain would equal a 40 lb CHO gain
1. WOW !!!!
2. Energy efficient?
Note:
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Too much of a good thing is never good,
however.
Questions?
End