Chapter Four: Exercise Metabolism

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Energy Requirements at Rest
At rest, the body is in homeostasis, and therefore the body's energy requirement is also constant

Almost 100% of ATP at rest is produced by aerobic metabolism
- Blood lactate levels are low ( <1.0 mmol/L)

Resting O2 consumption for a 70 kg adult:
- 0.25 L/min
- 3.5 ml/kg/min

Measurement of O2 consumption at rest provides an estimate of the body's baseline energy requirement
- At rest, the total energy requirement is low
Rest-to-Exercise Transitions
ATP production increases immediately

Oxygen uptake increases rapidly:
- In transition from rest to light/moderate exercise, O2 consumption 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
Jumping on a Treadmill Example
At onset of exercise, ATP production increases immediately to meet the requirement

However, O2 consumption does not increase immediately, but reaches a steady state in consumption 1-4 minutes after the onset of exercise

The fact that O2 consumption does not increase instantly to a steady state value means that anaerobic energy sources contribute to the overall production of ATP at the beginning of exercise:
-ATP-PC system: highest contribution to ATP generation in first minute and then falls off (PC depletion)
- Glycolysis: during the first minute of exercise, already functioning but then increases during the second minute

The effectiveness of anaerobic systems in the first minutes of exercise is such that ATP levels in muscle are virtually unchanged, even though ATP is being used at a much higher rate

As the steady state of O2 consumption is reached however, the body's ATP requirement is met via aerobic metabolism

From rest to work transitions, energy is not provided by a single bioenergetic pathway
- Energy is produced by a mixture of metabolic systems operating with considerable overlap
Oxygen Deficit
Lag in oxygen uptake at the beginning of exercise; the difference between oxygen uptake in the first few minutes of exercise and an equal time period after steady state has been obtained

Causes oxygen debt during recovery phase
Comparison of Trained and Untrained Subjects in the Oxygen Defecit
Trained subjects have a lower oxygen deficit
- Better-developed aerobic bioenergetic capacity to allow the aerobic energy systems kick in faster
- Due to cardiovascular and muscular adaptations

Results in less production of lactate and H+ and spare PC
Recovery from Exercise
Oxygen uptake remains elevated above rest into recovery

Oxygen debt: refers to the repayment for O2 deficit that occurred at the onset of exercise
- Term used by AV Hill (father of exercise physiology)

Excess post-ecercise oxygen consumption (EPOC):
- Terminology reflects that only about 20% of elevated O2 consumption after exercise is used to "repay" the oxygen deficit

Many scientists use "oxygen debt" and EPOC interchangeably
Oxygen Debt
AKA EPOC; refers to the O2 consumption above rest following exercise

"Rapid" portion of O2 debt (2-3 minutes post-exercise):
- Resynthesis of stored PC
- Replenishing muscle and blood O2 stores

"Slow" portion of O2 debt (can last for greater than 30 minutes):
- Elevated heart rate and breathing = increase in energy needed
- Elevated body temperature = increased metabolic rate
- Elevated epinephrine and norepinephrine = increased metabolic rate
- Conversion of lactate to glucose (gluconeogenesis)
EPOC is Greater Following Higher Intensity Exercise
(Initial oxygen deficit is higher)
EPOC is greater following high intensity exercise than low intensity exercise because:
1) Heat production and body temperature are higher
2) Greater depletion of PC
- Additional O2 required for resynthesis
3) Greater blood concentrations of lactate
- More O2 is needed for lactic conversion to glucose in gluconeogenesis, even if only 20% of the lactate is involved
4) Higher levels of blood epinephrine and norepinephrine
Removal of Lactic Acid Following Exervise
Classical theory: majority of lactic acid converted to glucose in liver (used for gluconeogenesis)

Recent evidence:
- 70% of lactic acid is oxidized: used as substrate by heart and skeletal muscle
- 20% converted to glucose (gluconeogenesis)
- 10% converted to amino acids

Lactic acid is removed more rapidly with light exercise in recovery
- Optimal intensity is about 30-40% of VO2max
General Metabolic Response to Exercise
Short term, high intensity exercise less than 10 secs relays on the anaerobic metabolic pathways

Events longer than 10 to 20 seconds and less than 10 minutes generally produces the needed ATP for muscular contraction via a combination of anaerobic and aerobic pathways

Marathon primarily uses aerobic production
Metabolic Responses to Short-Term, Intense Exercise
Energy to perform short-term high intensity exercises comes primarily from anaerobic metabolic pathways

First 1-5 seconds of exercise: ATP through ATP-PC system
- Dominates until 20 seconds

Intense exercise longer than 5 seconds: gradual shift to ATP production via glycolysis
- Dominates after about 20 seconds

Events longer than 45 seconds (ex: 400 m dash): ATP production through ATP-PC, glycolysis, and aerobic systems
- 70% anaerobic/30% aerobic at 60 seconds
- 50% anaerobic/50% aerobic at 2 minutes
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, moderate-intensity exercise; 2 exceptions to this are exercising in a hot/humid environment, and exercising at high intensity >75% VO2max

Prolonged exercise in a hot/humid environment or at high intensity
- Upward drift in oxygen consumption over time
- Due to high body temperature and rising epinephrine and norepinephrine (increases metabolic rate, resulting in increased oxygen uptake over time )
Metabolic Responses to Incremental Exercise
Oxygen uptake increases linearly until maximal oxygen uptake (VO2max) is reached
- No further increase in VO2 with increasing work rate

VO2max: maximal capacity to transport and utilize oxygen during exercise
- "Physiological ceiling" for delivery of O2 to the muscle
- Affected by genetics and training

Physiological factors influencing VO2max:
- Maximum ability of cardiorespiratory system to deliver oxygen to the muscle
- Ability of muscles to use oxygen and produce ATP aerobically
Lactate Threshold
The point at which blood lactate rises systematically (exponentially) during incremental exercise (describes the blood lactate inflection point)
- Appears at about 50-60% VO2max in untrained subjects
- Appears at higher work rtes (65-85% VO2max) in trained subjects

AKA
- Anaerobic threshold
- Onset of blood lactate accumulation (OBLA): the exercise intensity (or oxygen consumption) at which a specific blood lactate level is reached (4 mmol/L)
Explanations for the Lactate Threshold
1) Low muscle oxygen (hypoxia)

2) Accelerated glycolysis:
- NADH produced faster than it is shuttled into the mitochondria
- Excess NADH in the cytoplasm coverts pyruvate to lactate

3) Recruitment of fast-twitch fibers:
- Lactate dehydrogenase isoenzyme in fast fibers promotes lactic acid formation (has greater affinity for binding to pyruvate); as intensity increases, more fast twitch fibers are recruited to generate more force
- Lactate dehydrogenase isoenzyme in slow fibers promotes formation pyruvate from lactate

4) Reduced rate of lactate removal from the blood
- At any given time during exercise, some muscles are producing lactate and releasing it into the blood, an some tissues (ex: liver, skeletal muscles, heart) are removing lactate
- blood lactate concentration = lactate entry into blood - blood lactate removal
Lactate Dehydrogenase
AKA LDH; catalyzes the conversion of pyruvate to lactate (and NAD+) and the conversion of lactate to pyruvate (and NADH)

Two isoforms (isozymes): M and H
H has highest affinity for lactate -> pyruvate
- Slow-twitch fibers
M has highest affinity for pyruvate -> lactate
- Fast-twitch fibers
Practical Uses of the Lactate Threshold
Prediction of performance
- Combined with VO2max

Planning training programs
- Marker of training intensity
- Choose a training HR based on LT

Frank Shorter and Steve Prefontaine
- Prefontaine: highest VO2max recorded
- Shorter: high LT
Cool Down
Lactate can be used to form glucose as an energy fuel during cool down
Cool down isn't totally necessary in healthy individuals
Does Lactate Cause Muscle Soreness
Lactate production is commonly believed to cause muscle soreness:
- Delayed-onset muscular soreness (DOMS)
- 24-48 hours after exercise

Physiological evidence does not support this claim
- Lactate removal is rapid (within 60 min) following exercise
- Power athletes should experience DOMs after every work out
- Muscle soreness is rare following routine workout

Microscopic injury to muscle fibers leads to inflammation, which causes soreness after work out
Estimation of Fuel Utilization
Respiratory exchange ratio (RER or R): the ratio of CO₂ produced to the O₂ consumed
- Commonly used to estimate the percent contribution of carbohydrate or fat to energy metabolism during exercise, since fat and carb differ in the amount of O2 used and CO2 produced during oxidation
- Role of protein contribution is ignored

R = VCO₂/VO₂

R for fat (palmitic acid)
C₁₆H₃₂O₂ + 23O₂ -> 16CO₂ +16 H₂O
R = 16CO₂/23O₂ = 0.70
- The oxidation of fat results in an R of 0.70

R for carbohydrate (glucose)
C₆H₁₂O₆ + 6O₂ -> 6CO₂ + 6H₂O
R = 6CO₂/6CO₂ = 1.00
- The oxidation of carb results in an R of 1.00

Fat oxidation requires more O2 than carbohydrate because carb contains more O2 than fat does
- The caloric equivalent of 1 L of O2 is approximately 4.70 kcal when fat alone is used, and 5.00 kcal when carb alone is used
- About 6% more energy per liter O2 is obtained when using carb, compared to fat, as the sole fuel for exercise

For R to be used s an estimate of substrate utilization during exercise, the subject must have reached a steady state

R = 0.85 when % fat and % carb are equal
Factors Governing Fuel Selection
Intensity and duration of exercise
-During low intensity, prolonged exercise, there is a progressive increase in the amount of fat oxidized by the working muscles
- Endurance gained subjects use more fat and less carb than less fit subjects during prolonged exercise at the same intensity
Exercise Intensity and Fuel Selection
Low intensity exercise (<30% VO2max)
- Fats are primary fuel

High intensity exercise (>70% VO2max)
- Carbs are primary fuel

R increases as exercise intensity increases

Proteins only contribute only a small percentage (about 2%) during exercise of less than one hour
- During exercise greater than 3-5 hours, contribute 5-10% during final minutes

"Crossover concept"
Crossover Concept
Describes the shift from fat to carb metabolism as exercise intensity increases

Due to:
1) Recruitment of fast muscle fibers:
- As exercise intensity increases, more fast muscle fibers are recruited
- Have an abundance of glycolytic enzymes, but not many lipolytic enzymes

2) Increasing blood levels of epinephrine
- High levels of epinephrine increase phosphorylase activity (breaks down more glycogen to glucose)
Crossover Point
Work rate (exercise intensity) at which the energy derived from carbs exceeds that of fat
- As intensity increases beyond the crossover point, a progressive shift occurs from fat to carb metabolism
McArdle's Syndrome
Genetic error in muscle glycogen metabolism:
Cannot synthesize the enzyme phosphorylase due to a gene mutation

Results in inability to break down muscle glycogen
- Use more fat as a fuel during submax exercise

Also prevents lactate production
- Blood lactate levels do not rise during high-intensity exercise

Patients complain of exercise intolerance and muscle pain
Is Low-Intensity Exercise Best for Burning Fat?
At low exercise intensities (about 20% VO2max):
- High percentage of energy expenditure (about 60%) derived from fat
- However, total energy expended is low
- Total fat oxidation is also low

At higher exercise intensities (about 50% of VO2max):
- Lower percentage of energy (about 40%) from fat
- Total energy expended is higher
- Total fat oxidation is also higher

Total energy expenditure is key to losing weight! So higher exercise intensities result in higher weight loss even though low intensity burns the most fat
Exercise Duration and Fuel Selection
Prolonged (greater than 30 min), low-moderate intensity (40-59% VO2max) exercise:
- Shift from carbohydrate metabolism toward fat metabolism
- R value decreases

Due to an increased rate of lipolysis (fat break down)
- Triglycerides -> glycerol + FFA by enzymes called lipases
- Increase in lipolysis results in an increase in blood and muscle levels of FFA and promotes fat metabolism
- Lipolysis is generally a slow process, and an increase in fat metabolism occurs only after several minutes of exercise

FFA is inhibited from moving into the blood by insulin and high blood levels of lactate
- Normally, blood insulin levels decline during prolonged exercise
- Elevation in blood insulin following eating a high-carb meal before exercise, blood sugar rises and more insulin is released from pancreas (resulting in diminished lipolysis)

Stimulated by increased blood levels of epinephrine
At Rest
Most energy comes form burning fat (60%)
Interaction of Fat and Carb Metabolism During Exercise
"Fats burn in the flame of carbohydrates"
- A reduction in kreb's cycle intermediates (due to glycogen depletion) results in a diminished rate of ATP production from fat metabolism because fat can be metabolized only via kreb's cycle oxidation

Glycogen is depleted during prolonged high-intensity exercise:
- Reduced rate of glycolysis and production of pyruvate
- Reduced Kreb's cycle intermediates
- Reduced fat oxidation (fats are metabolized by the Kreb's cycle)

When carb stores are depleted (low levels of muscle and liver glycogen and low blood sugar), the rate at which fat is metabolized is also reduced
Carbohydrate Feeding via Sports Drinks Improves Endurance Performance
The depletion of muscle and blood carb stores contributes to fatigue
Ingestion of carbs can improve endurance performance
- During sub maximal (<70% VO2max), long-duration (>90 minutes) exercise
- 30-60 g of carb per hour are required to enhance performance

May also improve performance in shorter higher intensity events
Gatorade
Invented by Dr. Robert Cade; introduced in 1967 orange bowl
Harvard Fatigue Laboratory
DB Dill's dog Joe
Original study that carb drinks improve performance was conducted here when studying fatigue in soldiers during WWII

Made first treadmill

Couldn't go <3 hours on the treadmill
Ate carbs
Was then able to go >13 hours on the treadmill
Sources of Carb During Exercise
Carbohydrate is stored as glycogen in both the muscle and the liver
- Glycogen in skeletal muscle: provides a direct source of CHO for muscle energy
- Liver glycogen stores serve as a means of replacing blood glucose via glycogenolysis (controlled by phosphorylase)

Carbohydrate used as a substrate during exercise comes from both glycogen sites in muscle and from blood glucose

Muscle glycogen (stores are limited):
- Primary source of carbs during high-intensity exercise
- Supplies much of the carbs in the first hour of exercise and then contribution declines as duration increases
- Increased glycogen usage during high-intensity exercise can be explained by the increased rate of glycogenolysis due to recruitment of fast twitch fibers and elevated blood epinephrine levels

Blood glucose:
- Primary source of carbs during low intensity exercise
- Important during long-duration exercise as muscle glycogen levels decline; contribution increases as duration increases
- From liver glycogenolysis (conversion of glycogen to glucose); when blood glucose levels decline during prolonged exercise, liver glycogenolysis is stimulated and glucose is released into the blood, which is transported to the contracting muscle and used as fuel
Sources of Fat During Exercise
Most fat is stored in the form of triglycerides, but some is stored in muscle cells as well

Intramuscular triglycerides
- Primary source of fat during higher intensity exercise; contribution declines with increased exercise durations

Plasma FFA
- Primary source of fat during low-intensity exercise; contribution increases as duration increases
- Becomes more important as muscle triglyceride levels decline in long-duration exercise
- From adipose tissue lipolysis (triclyceride -> glycerol + FFA)
- FFA converted to acetyl-CoA and enters krebs cycle

At intermediate intensity, the contribution of fat as a fuel source is approximately equal between plasma FFA and muscle triglycerides
General Rule for Carb/Fat Use as Intensity Increases
As intensity increases, the % of fuel coming from fat decreases and the % of fuel coming from carbs increases

Carbs: as intensity increases, carbs contribute progressively more to energy production
- The contribution by muscle glycogen greatly increases with intensity (becomes greater than contribution of plasma glucose at moderate intensity level)
- Contribution by plasma glucose is greatest at low intensity (greater than the contribution of muscle glycogen), and increases a tiny bit as intensity increases

Fat: as intensity increases, fat contributes less and less to energy production
- Plasma FFA contributes most during low exercise intensity
- Tricglycerides contribute most during higher intensity exercise
- At moderate intensity, levels are equal

At moderate level of intensity (65%; contribution by fat and carb is equal)
General Effect of Exercise Duration on Muscle Fuel Source
As duration increases, carbs contribute less and fat contributes more (starts off roughly equal though)

Carbs: as duration increases, the contribution by carbs decreases
- Muscle glycogen decreases
- Blood glucose increases

Fats: as duration increases, the contribution by fat increases
- Muscle triglycerides decrease
- Plasma FFA increases
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 (about 2%) to total energy production during exercise
- May increase to 5-10% late in prolonged-duration exercise
- Enzymes that degrade proteins (proteases) are activated in long-term exercise
Lactate as a Fuel Source During Exercise
Can be used as a fuel source by skeletal muscle and the heart
- Converted to pyruvate, transformed to acetyl-CoA and enters the Krebs cycle

Can be converted to glucose in the liver (gluconeogenesis)
- Cori cycle

Lactate shuttle
- Lactate produced in one tissue and transported to another
Cori Cycle: Lactate as a Fuel Source
Lactate produced by skeletal muscle is transported to the liver

Liver converts lactate to glucose
- Gluconeogenesis

Glucose is transported back to muscle and used as fuel