C H A P T E R 4

1
C H A P T E R 4
METABOLISM, ENERGY, AND THE BASIC ENERGY SYSTEMS
2
Learning Objectives
w Learn how our bodies change the food we eat
into ATP to provide our muscles with the energy
they need to move.
w Examine three systems that generate energy
for muscles.
3
Learning Objectives
w Learn how exercise affects metabolism and how
metabolism can be monitored to determine energy
expenditure.
w Discover the underlying causes and sites of
fatigue in muscles.
4
Calorie and Kilocalorie
w Energy in biological systems is measured in
calories (cal).

• 1 cal is the amount of heat energy needed to
  raise 1 g of water 1C from 14.5C to 15.5C.

w In humans, energy is expressed in kilocalories
(kcal), where 1 kcal equals 1,000 cal.
w People often mistakenly say calories when
they mean kilocalories. When we speak of someone
expending 3,000 cal per day, we really mean that
person is expending 3,000 kcal per day.
5
Energy for Cellular Activity
w Food sources are processed via catabolismthe
process of breaking down.
w Energy is derived from food sources and stored
as adenosine triphosphate (ATP).
w ATP is a high-energy compound stored in our
cells and is the immediate source of all energy
used at rest and during exercise.
6
Energy Sources
w At rest and during low intensity exercise, the
body primarily uses fats for energy.
w Protein provides little energy for cellular
activity, but serves as building blocks for the
body's tissues.
w During moderate to severe muscular effort, the
body relies mostly on carbohydrate for fuel.
7
Carbohydrate
w Readily available from the diet in starches,
sugars, etc. which are digested to glucose, which
is then metabolized by most tissues, including
muscle and brain
w Glucose (C6H12O6) is transported in the blood
and can also be taken up by muscles and liver and
converted to glycogen, which is a polymer of
glucose, or taken up by fat cells and converted
to fat and stored as fat
w Glycogen stored in the liver can be converted
back to glucose as needed and transported by the
blood to the tissues where it is used to form ATP
w Unlike fat stores, glycogen stores are limited,
which can affect performance
8
Fat
w Cells catabolize free fatty acids (FFAs) to
produce ATP fats provide substantial energy at
rest and during prolonged, low-intensity activity
w FFAs are stored as triglycerides (3 FFAs
attached to a glycerol molecule) in cells,
particularly adipocytes - body stores of fat are
much larger than for carbohydrate (i.e., muscle
and liver glycogen)
w Fat is limited as an energy source by its
slower rate of energy release its metabolism
requires oxygen thus, it becomes less important
as a fuel at higher exercise intensities
9
Total Body Stores of Fuels and Energy
10
Fuel use during exercise
Capillary
1

1. Erythrocyte (RBC)
2. Endothelial cell (boundary of the capillary
3. Mitochondria
4. Myofibril

2
3
4
Muscle Fiber
Weibel, Symmorphosis, 2000
11
Fuel Use during Exercise
Weibel, Symmorphosis, 2000
12
Fuel use during exercise
Romijn et al. Am J Physiol E389, 1993
13
Protein
w Can be used as an energy source if broken down
to amino acids, which are then converted in the
liver to glucose via gluconeogenesis (important
during starvation)
w Only the basic units of protein amino acids
can be used for energy 4.1 kcal of energy per
g of protein
14
Enzymes
w Specific protein molecules that control the
rate of breakdown of chemical compounds
w Names always end in ase
w Work at different rates and can limit flux
through a pathway
w Glycolytic enzymes are located in the
cytoplasm, while oxidative enzymes are located
primarily in the mitochondria
15
ACTION OF ENZYMES
For example, ATP ADP Pi Energy
Work ATPase Enzyme ATPase Molecule AB
ATP Molecule A ADP Molecule B Pi
16
Key Points
Energy for Cellular Metabolism
w Carbohydrate and protein provide about 4.1
kcal/g while fat provides about 9.4 kcal/g.
17
Energy for Cellular Activity
Carbohydrates (glucose), Fats (FFA), Proteins
Cellular Catabolic Pathways
Energy to synthesize ATP from ADP and Pi
(Mitochondria)
Energy ADP Pi ATP
ATP ? ADP Pi Work (30) Heat (70)
Contraction, protein synthesis, ion pumping, etc.
18
Basic Energy Systems
1. ATP-PCr system (phosphagen system)cytoplasm
2. Glycolytic systemcytoplasm
3. Oxidative systemmitochondria (powerhouses
of cell)
19
ATP MOLECULE
20
ATP-PCr System Creatine Kinase reaction
w This system can prevent energy depletion by
quickly resynthesizing ATP from ADP and Pi.
w This process is anaerobicit occurs without
oxygen.
w 1 mole of ATP is produced per 1 mole of
phosphocreatine (PCr), or creatine phosphate
(CP) . The energy from the breakdown of PCr is
not used for cellular work, but solely for
regenerating ATP.
21
Resynthesis of ATP from PCr
This occurs in the cytoplasm proximal to the
myosin ATPase on the thick filaments
22
ATP AND PCr DURING SPRINTING
The muscle fiber does everything it can to
maintain its ATP stores!
23
Glycogen
Brooks et al., Exercise Physiology, 2000
24
Glycogen Breakdown and Synthesis
GlycogenesisProcess by which glycogen is
synthesized from glucose to be stored in the
muscle (or liver)
GlycogenolysisProcess by which glycogen is
broken down into single glucose-1-phosphate
molecules for catabolism in glycolysis in the
muscles (or release into the blood from the liver)
25
Glycolysis
w Requires a series of enzymatic reactions to
sequentially break down glucose into pyruvic acid
or lactic acid

• Glycolysis produces several important products
• A small amount of ATP (substrate-level
  phosphorylation
• Pyruvic acid or lactic acid, which can
  subsequently be used as an oxidative substrate in
  the mitochondria
• A small amount of H (reducing equivalents),
  that are used in the electron transport chain in
  the mitochondria to produce ATP

26
Glycolysis

• Net ATP production
• 2. Net H production
• (NAD 2H ? NADH H)
• 3. Net pyruvic acid or lactic acid production
  during anaerobic conditions -

27
Lactic acid or Lactate?
An acid is a compound that dissociates into H
and a salt when placed in water Thus, when
lactic acid occurs in the aqueous environment of
the cells or blood, lactic acid ? lactate
H This increases the H concentration, or
increases the acidity (lowers the pH)
28
Anaerobic metabolism
The combined actions of the ATP-PCr and
glycolytic systems allow muscles to resynthesize
ATP when there is insufficient oxygen in the
cells in the absence of oxygen thus, these two
energy systems are the major energy contributors
during the early minutes of high-intensity
exercise before respiration and cardiac output
reach the new steady state.
29
Oxidative System
w Relies on oxygen for ATP production
w Produces ATP in the mitochondria of cells
w Can yield much more energy (ATP) than anaerobic
systems
w Is the primary method of energy production
30
Oxidative Production of ATP from CHO
1. Aerobic glycolysis cytoplasm
2. Krebs cycle mitochondria
3. Electron transport chain mitochondria
31
Aerobic Catabolism of CHO
32
KREBS CYCLE
33
Oxidation of Carbohydrate
1. Pyruvic acid from glycolysis is converted to
acetyl coenzyme A (acetyl CoA) by pyruvate
dehydrogenase.
2. Acetyl CoA enters the Krebs cycle in the
cycle, ATP, carbon dioxide, and hydrogen are
produced.
3. Hydrogen produced in the cycle combines with
coenzymes (NAD or FAD) that carry it to
the electron transport chain.
4. Hydrogens pass down the electron transport
chain producing ATP and combine with oxygen to
form water.
5. One molecule of glucose from glycogen can
generate up to 39 molecules of ATP.
34
OXIDATIVE PHOSPHORYLATION
35
Thought Question
In exercise physiology, two of the most
important considerations are oxygen uptake (VO2)
and carbon dioxide production (VCO2). Where is
the oxygen used in the body, and where does the
expired carbon dioxide come from?
36
ATP Production From the Oxidation of Muscle
Glycogen
37
Oxidation of Fat
w Lypolysis breakdown of triglycerides into
glycerol and free fatty acids (FFAs) in
adipocytes, hepatocytes, or muscle.
w FFAs travel from adipocytes via blood to muscle
fibers and are broken down by enzymes in the
ß-oxidation pathway in the mitochondria to acetyl
CoA.
w Acetyl CoA enters the Krebs cycle and hydrogens
produced enter the electron transport chain.
38
Triglyceride Molecule (1 Glycerol and 3 Fatty
Acids)
Brooks et al., Exercise Physiology, 2000
39
METABOLISM OF FAT
40
ATP Production From the Oxidation of Palmitic
Acid (C16H32O2)
41
Protein Metabolism
w Body uses little protein during rest and
exercise as fuel (contributes less than 5 to
ATP production) in normal fed condition becomes
important during starvation.
w Some amino acids that form proteins can be
converted into glucose.
w The nitrogen in amino acids (which cannot be
oxidized) makes the energy yield of protein
difficult to determine.
42
INTERACTION OF ENERGY SYSTEMS ILLUSTRATING THE
PREDOMINANT ENERGY SYSTEM
43
What Determines Oxidative Capacity?
w Muscle oxidative enzyme activity (mitochondrial
density)
w Fiber-type composition
w Capillary density in the muscles
w Oxygen delivery to the muscle capillaries
O2
Ventilation
Circulation
Oxidative phosphorylation
Taylor et al., Resp Physiol 69 1, 1987
44
OXIDATIVE ENZYME ACTIVITY AND OXIDATIVE CAPACITY
Succinate dehydrogenase (SDH) is a Krebs cycle
enzyme (i.e., located in the mitochondria)
45
Measuring Energy Costs of Exercise
Direct calorimetrymeasures the body's heat
production to calculate energy expenditure.
46
Direct Calorimetry in a Chamber
47
Indirect Calorimetry by Measuring Respiratory Gas
Exchange
48
Measuring Energy Costs of Exercise
49
CALCULATING OXYGEN CONSUMPTION
50
Respiratory Exchange Ratio (RER)

• The RER value at rest is usually 0.78 to 0.80,
  indicating the body is primarily metabolizing
  fat e.g., oxidation of palmitate
• C16H32O2 23O2 ? 16CO2 16H2O 129 ATP
• Thus, RER 16/23 0.70
• Oxidation of glucose
• C6H12O6 6O2 ? 6CO2 6H2O 38 ATP
• Thus, RER 6/6 1.00
• during intense exercise the RER may be 1.00 or
  above, indicating the body is primarily
  metabolizing carbohydrate.

51
Respiratory Exchange Ratio (RER)

• The RER value can be used to determine energy
  substrate used at rest and during exercise, with
  a value of 1.00 indicating CHO and 0.70 (0.71)
  indicating fat.
• fat contribution 100 x (1 - RQ)/0.29
• e.g., if RER 0.75, the fat contribution to
  metabolism would be 86

52
Caloric Equivalence of the Respiratory Exchange
Ratio (RER) and kcal From Carbohydrates and Fats
53
Measurements of Energy Expenditure
Carbon-13Infused and selectively traced to
determine its movement and distribution
Doubly labeled water2H218O is ingested and the
rates at which 2H and 18O diffuse throughout the
bodys water and bicarbonate stores and leave the
body are monitored and used to calculate how much
energy is expended
54
Metabolic Rate
w Rate at which the body expends energy (or uses
ATP)
w Measured as whole-body oxygen consumption and
its caloric equivalent
w Basal or resting metabolic rate (BMR) is the
minimum energy required for essential
physiological function (varies between 1,200 and
2,400 kcal/24 hr), e.g., protein synthesis, ion
pumping, etc.
w The minimum energy required for normal daily
activity (including resting metabolism) is about
1,800 to 3,000 kcal/24 hr
55
Factors Affecting BMR/RMR
w The more fat-free mass, the higher the BMR
w The more body surface area, the higher the BMR
w BMR gradually decreases with increasing age
w BMR increases with increasing body temperature
w The more stress, the higher the BMR
w The higher the levels of thyroid hormone and
epinephrine, the higher the BMR
56
Caloric Equivalents
w Food energy equivalents CHO 4.1
kcal/g Fat 9.4 kcal/g Protein 4.1 kcal/g
w Energy per liter of oxygen consumed CHO 5.0
kcal/L Fat 4.7 kcal/L Protein 4.5 kcal/L
57
Thought Question
What are the primary advantages and
disadvantages of fat as a substrate during
exercise? What are the primary advantages and
disadvantages of carbohydrate as a substrate
during exercise?
58
O2 Uptake vs Time and Power Output (1986)
59
O2 Uptake vs Time and Power Output (1996)
60
w Upper limit of a person's ability to increase
oxygen uptake.
w Good (best) indicator of cardio-respiratory
endurance and aerobic fitness.
w Can differ according to sex, body size, and
age, and is greatly influenced by the level of
aerobic training.
w Either expressed in absolute terms, i.e.,
liters of O2 consumed per minute, or expressed
relative to body weight in ml of O2 consumed per
kg body weight per min (ml kg-1 min-1)
think about the difference in VO2max between a
150 pound marathon runner and a 300 pound
offensive lineman.
61
EXERCISE INTENSITY AND OXYGEN UPTAKE
62
VO2max in the Lay Press
Units of measure?
USA Today, August 27, 2004
63
Estimating Anaerobic Effort
There is not yet available a method that
definitively measures anaerobic capacity, however
there are ways to estimate it
w Examine excess postexercise oxygen consumption
(EPOC) the mismatch between O2 consumption and
energy requirements during recovery from exercise
w Estimate lactate production in muscles through
blood analysis estimate lactate threshold (LT)
w Use the maximal accumulated oxygen deficit
test, the critical power test, or the Wingate
anaerobic test, which also show good promise for
estimating the metabolic potential of anaerobic
capacity
64
OXYGEN DEFICIT AND EPOC
65
OXYGEN DEFICIT AND EPOC
Powers and Howley, Exercise Physiology, 2004
66
Factors Responsible for EPOC
w Restoring depleted ATP supplies
w Metabolizing lactate produced during the
exercise
w Replenishing O2 supplies depleted from
hemoglobin and myoglobin
w Removing CO2 that has accumulated in body
tissues
w Increased metabolic and respiratory rates due
to increased body temperature and norepinephrine
and epinephrine levels
67
Lactate Threshold
w The point at which blood lactate begins to
accumulate above resting levels during exercise
of increasing intensity, where lactate appearance
in the blood exceeds lactate clearance

• Sudden increase in blood lactate with increasing
  effort can be the result of an increase in the
  production of lactate or a decrease in the
  removal of lactate from the blood, e.g.,
• An increased recruitment of fast-twitch fibers
• A decreased blood flow to the liver, i.e.,
  decreased removal by the liver

w Can indicate potential for endurance exercise
the higher the threshold, the greater the
exercise tolerance since lactate accumulation
contributes to fatigue
68
The Lactate Threshold
69
Lactate Threshold and Performance
70
Thought Question
71
Determining Endurance Performance Success
w High lactate threshold
w High economy of effort
w High percentage of slow-twitch muscle fibers
(high oxidative capacity, i.e., ATP synthesis
rate, coupled with a relatively slow rate of ATP
hydrolysis, i.e., low myosin ATPase activity)
72
Running Economy
73
Swimming Economy of Swimmers and Triathletes
74
Factors Influencing Energy Costs
w Type of activity
w Size, weight, and body composition
w Intensity of the activity
w Activity level
w Duration of the activity
w Age
w Efficiency of movement
w Sex
75
Fatigue and Its Causes
w Phosphocreatine (PCr) depletion (especially in
events lasting less than 30 seconds)
w Accumulation of lactate and H (especially in
events shorter than 30 minutes)

• Excitation-contraction uncoupling, i.e., reduced
  calcium release from SR during excitation of the
  muscle fiber occurs during any intense muscular
  exertion.
• Decreased central neural drive (events lasting
  hours)

w Glycogen depletion (especially in activities
lasting longer than 1 hour but no longer than 3
hours)
76
Recovery from fatigue
Human TA muscle
2-min static max (S)
15-20 min intermittent max (L)
Time (min)
Baker et al. J Appl Physiol 74 2294, 1993
77
Mechanisms of fatigue
Human TA Muscle
Baker et al. J Appl Physiol 74 2294, 1993.
78
Glycogen Depletion during Prolonged Exercise
79
MUSCLE FIBERS STAINED TO SHOW GLYCOGEN
Myosin ATPase
Glycogen
At low to moderate exercise intensities, ST
fibers are the first to lose glycogen because
they are the primary fibers being recruited.
80
GLYCOGEN USE DURING RUNNING
81
Glycogen depletion at different intensities to
exhaustion
Powers and Howley, Exercise Physiology, 2004
82
Metabolic By-Products and Fatigue
w Short duration, high intensity activities
depend on anaerobic glycolysis and produce
lactate and H.

• Cells buffer H with bicarbonate (HCO3-) to keep
  cell pH between 6.4 and 7.1.
• H HCO3- ? H2CO3 ? H2O CO2
• Carbonic Anhydrase
• Metabolism

w Intercellular pH lower than 6.9, however, slows
all enzyme reactions (including myosin ATPase).
w When pH reaches 6.4, H levels stop any
further glycolysis and result in exhaustion.
83
CHANGES IN MUSCLE pH
84
Thought Question
If you wanted to perform an experiment to
determine whether or not there was fatigue in the
central nervous system during a bout of exercise,
what could you measure to make the determination?