Exercise Physiology

Step 2: Sports Dietetics

Ex. Phys. Crash Course

1. Energy Storage2. Energy metabolism, substrate utilization, and

oxygen transport3. Enzymes and hormones – the good ones and

the bad ones!4. Physiological response to training5. Fatigue

= practical application

1. Energy Storage

Energy = ability to do work

Examples of WORK: exercise, muscle contraction, synthesis, nutrient transport, repair, etc.

Splitting ATP bonds

ATP is cellular currency. Compare it to a bank account with lots of withdrawals (energy expenditure) and deposits (substrates).

Generally, about 10 seconds worth of ATP stored in muscle so other energy systems must kick-in very quickly during exercise

Energy Sources

SubstratesPhosphocreatine (PCr)Carbohydrates Fat (Protein)

when oxidized……re-phosphorylate ADP

ADP + P → ATP

Where is fuel stored?

Adapted from Mcardle, Katch, and Katch. Sports and Exercise Nutrition. Lippincott, Williams & Wilkins, 2005

Muscle:ATP, PCr, Glycogen, IMTG, Carbons from AAs

Mitochondria

Liver: Glycogen → Glucose AAs

Blood: Glucose FFA Deaminated AAsAdipose Tissue:

TG FA

Where is fuel stored?

Adapted from Mcardle, Katch, and Katch. Sports and Exercise Nutrition. Lippincott, Williams & Wilkins, 2005

Muscle:ATP, PCr, Glycogen, IMTG, Carbons from AAs

Mitochondria

Liver: Glycogen → Glucose AAs

Blood: Glucose FFA Deaminated AAsAdipose Tissue:

TG FA

How much fuel is stored? Carbohydrate

Liver glycogen ≈ 400 kcals Muscle glycogen ≈ 1,400 kcals

100g CHO in liver, 375g CHO in muscle… consider how rapidly this can be used up during exercise at an oxidation rate of 1g/min!

Fat Intramuscular Triglycerides (IMTG) ≈ 3,000kcals Adipose tissue ≈ 80,000kcals

This is true even in very lean individuals.

2. Energy Metabolism, Substrate Utilization, & Oxygen Transport

Two types of energy metabolism

1. Anaerobic Occurs when oxygen is not available

Phosphocreatine System Anaerobic Glycolysis (aka: Lactate Glycolysis)

2. Aerobic Requires oxygen

Aerobic Glycolysis Kreb’s Cycle (aka: TCA Cycle, Citric Acid Cycle) Electron Transport Chain (ETC) Beta-oxidation (fat)

ATP Production from…

Phosphocreatine Carbohydrate Fat10 seconds 1-3 minutes w/o Oxygen Long, requires Oxygen

FAST ATP > 60 minutes w/ Oxygen SLOW ATP

Short, quick, powerful Higher intensity Low intensity

Runs out of gas fast Gas is expensive & limiting Gas is cheap, never runs out, speed cap

ATP Production from…

Phosphocreatine Carbohydrate Fat10 seconds 1-3 minutes w/o Oxygen Long, requires Oxygen

FAST ATP > 60 minutes w/ Oxygen SLOW ATP

Short, quick, powerful Higher intensity Low intensity

Runs out of gas fast Gas is expensive & limiting Gas is cheap, never runs out, speed cap

Practice Question

What substrate(s) is/are able to be used foranaerobic energy production?

A. PhosphocreatineB. GlucoseC. Phosphocreatine and glucoseD. Phosphocreatine, glucose, and BCAA’s

SEE YELLOW PAGE AT END OF MANUAL FOR CORRECT ANSWERS.

Practice Question

What substrate(s) is/are able to be used in aerobic energy production?

A. Fatty acidsB. Fatty acids and amino acidsC. Glucose and amino acidsD. Fatty acids, glucose, and amino acids

SEE YELLOW PAGE AT END OF MANUAL FOR CORRECT ANSWERS.

Rate of ATP Synthesis by Different Energy Systems

µmol per minute per gram of muscle

Phosphocreatine Breakdown ≈ 450

Anaerobic Glycolysis ≈ 200

Fat Oxidation ≈ 20

References:

Miller, W. The Biochemistry of Exercise and Metabolic Adaptation. Brown and Benchmark, 1992

Maughan, R and Burke, L. Sports Nutrition: Handbook of Sports Medicine and Science. Blackwell Science, 2002.

Substrate utilization…

…exists on a continuum.

Energy systems work simultaneously.

All vehicles on the road at the same time (even though one may be leading the way!)

Fuel utilization depends on EXERCISE DURATION

1 MIN 3 MIN 1 HOUR 2 HOURS

ANAEROBICAEROBIC

Exercise at Constant Intensity

% CONTRIBUTION

Crossover Concept

Fuel utilization depends on EXERCISE INTENSITY

%CONTRIBUTION

REST LOW TO MODERATE HARD EXERCISE EXERCISE

FAT

CARBS

PROTEIN

Phosphocreatine System

PCr Cr + P

P + ADP ATP

Reactions “work” in both directions

Anaerobic GlycolysisGLYCOLYSIS

KREBS CYCLE

Glycogen

Cytoplasm

Glucose is starting point and when oxygen is not present, pyruvate is endpoint.

Muscle glycogen enters Glycolysis at G-6-P, skipping first step.

NADH + H

Pyruvate

Acetyl-CoA

Mitochondria(Krebs Cycle = TCA Cycle = Citric Acid Cycle)

Glucose

Glucose-6-Phosphate

ATP

×

Anaerobic GlycolysisGLYCOLYSIS

KREBS CYCLE

Glycogen

Cytoplasm

Glucose is starting point and when oxygen is not present, pyruvate is endpoint.

Muscle glycogen enters Glycolysis at G-6-P, skipping first step.

When H atoms are produced more rapidly than NADH and ETC can process, lactate is formed.

NADH + H

Pyruvate

Acetyl-CoA

Mitochondria(Krebs Cycle = TCA Cycle = Citric Acid Cycle)

Glucose

Glucose-6-Phosphate

ATP

× Lactate

Lactate

When hydrogen ions are produced too fast to combine with NADH and be oxidized by ETC a back-up of H+ occurs

These H+ combine with pyruvate to form LACTATE

Lactate should be considered a BY-PRODUCT rather than a WASTE PRODUCT. For a period of time, lactate can be recycled through glycolysis to keep producing ATP (CORI CYCLE).

Remember: glycolysis “works” both ways!

This continues until lactate produced in the blood from working muscles exceeds lactate clearance from liver (LACTATE THRESHOLD).

At Lactate Threshold, H+ builds up and can contribute to fatigue

?

Lactate Threshold

Intensity of exercise when lactate production exceeds clearance

Training regimens often involve training at a level > lactate threshold.

Typical LT:

70-80% VO2max for trained people

50-60% VO2max for untrained people.

Practice Question

Which of the following is true regarding lactateaccumulation in skeletal muscle?

A. It is formed as a waste product of aerobic glycolysis.B. It is a by-product of the electron transport chain.C. It is a gluconeogenic precursor.D. It is a co-factor in the carnitine shuttle.

SEE YELLOW PAGE AT END OF MANUAL FOR CORRECT ANSWERS.

Aerobic System: CarbsGLYCOLYSIS

KREBS CYCLE

Glycogen

Cytoplasm

NADH + H

Pyruvate

Acetyl-CoA

Mitochondria

Glucose

Glucose-6-Phosphate

ATP

Oxygen

H

H

H

HH

H

HATP

With oxygen present, pyruvate is converted to Acetyl-CoA and into Krebs Cycle.

Aerobic System: CarbsGLYCOLYSIS

KREBS CYCLE

Glycogen

Cytoplasm

NADH + H

Pyruvate

Acetyl-CoA

Mitochondria

Glucose

Glucose-6-Phosphate

ATP

Oxygen

H

H

H

HH

H

HATP

Hydrogen generated from glycolysis and Krebs Cycle enter Electron Transport Chain.

ELECTRON TRANSPORT

CHAIN

Practice Question

In the presence of oxygen, how many molecules of ATP are produced from one glucose molecule?

A. 1B. 2C. 38D. 56

SEE YELLOW PAGE AT END OF MANUAL FOR CORRECT ANSWERS.

Aerobic System: FatGLYCOLYSIS

KREBS CYCLE

Cytoplasm

Pyruvate

Acetyl-CoA

Mitochondria

Glucose

Oxygen

ATP

EFAFA

FA

glycerol

Triglyceride

Beta-oxidation: FA, which are 16-24 carbons long, are broken down to Acetyl-CoA (2 carbons), fatty acid activation are slow

Aerobic System: FatGLYCOLYSIS

KREBS CYCLE

Cytoplasm

Pyruvate

Acetyl-CoA

Mitochondria

Glucose

Oxygen

EFAFA

FA

glycerol

Triglyceride

Breakdown of TG:

Yields lots of carbons, lots of hydrogens, and lots of ATP…460!

H

H

H

Electron Transport Chain

Series of oxidation-reduction reactions Oxidation: when oxygen, hydrogen, or electrons are

TRANSFERRED Reduction: when electrons are ACCEPTED

End point = oxygen accepts electrons and is reduced to form water

Many cytochromes (iron-containing electron carriers are involved)

No wonder iron deficiency causes decreased aerobic performance!

Is fat gluconeogenic?

NO…by-products of FA oxidation can only enter metabolism at Acetyl-Co A

Pyruvate(end of glycolysis)

Acetyl-Co A(beginning of Krebs cycle)

How does protein enter energy metabolism?

The process by which amino acids are used for energy is DEAMINATION (amine group removed in liver) or TRANSAMINATION (amine group discarded in muscle) to leave carbons that can enter energy metabolism somewhere in Krebs Cycle, at pyruvate, or conversion back to glucose (glucogenic).

Removing the amine group (nitrogen) involves kidneys and increased urine production…depending on protein for fuel increases risk of dehydration.

KREBS CYCLE

GLYCOLYSIS

Acetyl-CoAAmino Acids

Notes on Carbohydrate Use At the beginning of exercise, carbs are used at a high rate since fat can’t

keep up with early ATP production. Carbs are very valuable…they make ATP fast and facilitate higher intensity exercise

Later in exercise, carbs are needed (at pyruvate) to foster fat movement through Krebs Cycle which is why carbs are called the “rate limiting” fuel.

In moderate endurance exercise, performance directly correlates with initial muscle glycogen concentration.

Glycogen depleted subjects cycled 57 min at 70% VO2max Glycogen loaded subjects cycled 114 min at 70% VO2max

Classic study by Bergstrom et al Acta Physiol Scand 1967.

Many athletes train hard all week long, aren’t eating enough carbs in general or during recovery, and then compete on the weekends. Major education point!

Notes on Carbohydrate Use Why does carb loading work to enhance performance?

Utilize less fat (means higher intensity is possible) Utilize almost no protein (unless exercise is very long) NOTE: ADA/ ACSM Position Statement on Nutrition for

Athletic Performance states that carb loading is effective for exercise LONGER than 90 minutes.

Consider that glycogen depletion has now been shown to affect even high intensity, shorter exercise In sports who don’t typically consider “carb loading”, there may be decreased physical performance, increased risk of injury, poor concentration, and decreased coordination when glycogen stores are too low.

Notes on Carbohydrate Use The amount of carbs used during exercise

varies based on: Diet Liver and muscle glycogen stores (i.e.: fasting in am = low

liver glycogen = less glucose/more fat used) Body and environmental temperature Carbs consumed during exercise

Why take in carbs during exercise? Keeps blood sugar high and favors carbohydrate use (faster

ATP production) Blunts FA oxidation

Notes on Fat Use Fat contributes 30-80% of energy for exercise depending on:

exercise intensity duration of exercise fitness level

Intramuscular TG contribute 15-35% of energy for exercise

Fat becomes more important when exercise is long or carbohydrate stores run low.

As blood flow increases w/ exercise, adipose tissue releases more FFA (but there is a lag time)

As exercise intensity increases, FFA energy production from adipose stays about the same while IMTG and glycogen breakdown increase

Notes on Fat Use Once FFA enter muscle cell, they either enter the carnitine shuttle for

energy metabolism (MCT and short chain TG do not need shuttle; they go straight to mitochondria). Others are stored as TG.

When TG are oxidized from adipose tissue, FA bind to albumin in blood stream and enter mitochondria for Krebs and ETC. Glycerol can enter glycolysis and produce ATP.

Carbs are needed to provide Krebs intermediates for fat breakdown. When carbs run out, ketones are formed from incomplete fat breakdown.

Fat in recovery period from exercise less likely to be stored and more likely to be oxidized (part of why regular exercise helps with weight loss)

High fat diet (fat-loading) may increase IMTG and fat enzyme function, theoretically leading to greater oxidation during exercise. Many find it intolerable and most studies show no performance benefits.

Keep client’s goals in mind

Exercising for fat-burning is different than exercising for optimal performance.

Optimal intensity for fat burning is 55 to 72% VO2 max

but training/performance may be optimized at higher level

Burn more fat in fasted state Ability to burn more carbs in fed state

help exercisers think of total calories expended

Practice Question

Which of the following is not a BCAA?

A. LysineB. LeucineC. ValineD. Isoleucine

SEE YELLOW PAGE AT END OF MANUAL FOR CORRECT ANSWERS.

Notes on protein use

Depending on novelty of exercise, protein breakdown increases modestly with exercise but protein synthesis increases significantly with BOTH endurance and resistance exercise

Some discrepancy over amount of protein that is used for fuel. Generally 2-6% of energy production.

Greater use in ultra-endurance. BCAA (leucine, isoleucine, and valine) are oxidized by skeletal

muscle rather than liver. Aspartate, glutamine, alanine, asparagine, and lysine can be oxidized in muscle also (with preference given to BCAA).

Use more protein for fuel in a glycogen-depleted state or if in chronic negative energy balance

Dieters or restrictive eaters may be at risk of lean tissue loss!

Amino Acids

ESSENTIAL (not produced endogenously)

1. Isoleucine2. Leucine3. Lysine4. Methionine5. Phenylalanine6. Threonine 7. Tryptophan8. Valine

NON-ESSENTIAL (produced endogenously)

1. Alanine2. Arginine3. Asparagine4. Aspartate5. Cysteine6. Glutamate7. Glutamine8. Glycine9. Histidine10. Proline11. Serine12. Tyrosine

Some may become essential in certain scenarios…

Bottom line

1. Substrate and energy system used depends on: Exercise intensity Exercise duration Substrate availability Training status

2. Don’t run out of carbs!

Gender differences

For females:

Carb loading is less effective. They use a lower proportion of carbs and higher proportion of fat

than males in similar intensity exercise. With training, there is a shift toward using more fat

(so, greater glycogen-sparing effect)

Why? Not fully understood, but possibly:

Differences in catecholamine response Estrogen/progesterone may enhance lipolysis and limit glycolysis

Muscle Fiber Types 2 primary types

TYPE I: Slow-twitch Contract slowly; primarily aerobic pathways Dense in mitochondria and capillaries (oxygen) Fatigue slowly

TYPE II: Fast-twitch Fast contractions

Type IIa: both aerobic and anaerobic pathways Type IIb: anaerobic pathways

Fatigue quickly

Proportion of each type depends on genetics, exercise training, other factors?

Other oxygen-related exercise terminology

Respiratory Quotient (RQ) Ratio of CO2 produced : O2 consumed RQ for carbs = 1, fat = .7 An estimate RQ of .82 is used for general activity Using exercise testing to determine RQ at specific workloads can help tailor

during-exercise fuel plans

VO2max

Measure of cardiorespiratory fitness Greatest rate of oxygen consumption attained during exercise

(L/min) or (ml/kg body weight/min) Don’t have a metabolic cart in your office?

HRmax = 220 – age (Karvonen Formula) 70% of HRmax correlates with approximately 75-80% VO2max

MET (Metabolic Equivalents) Measure of workload 1 MET = resting oxygen consumption of average human (3.5 mL/kg/min) Well-trained athletes may be able to work at a level of 15-17+ METS

3. Enzymes & Hormones

Enzymes

Definition: protein structure that catalyzes and accelerates chemical reactions without being consumed or changed in the process

Enzymes you should knowATPase

Converts ATP to ADP + P (and the reverse reaction); phosphorylates

Creatine phosphokinase Converts PCr to Cr + P (and the reverse reaction)

Glycogen Phosphorylase Glycogenolysis: breaks down glycogen → glucose

Glycogen synthase Glycogenesis: synthesizes glycogen from glucose

Hormone sensitive lipase (HSL) Breaks down muscle & adipose tissue TG → FFA

Cyclic AMP activates HSL

Lipoprotein lipase Breaks down circulating TG → FFA

Lactate dehydrogenase Combines pyruvate with 2 hydrogen atoms to form lactate

Hormones

Definition: internally secreted compounds formed in endocrine glands that affect the functions of specifically receptive organs or tissues when transported to them by the body fluids

Hormones you should know

Promote glycogen/carb breakdown: epinephrine, glucagon, norepinephrine

Promote glycogen/carb storage: insulin

Promote fat breakdown: epinephrine, glucagon, growth hormone, norepinephrine

Promote fat storage: insulin

Promote protein breakdown (catabolic): glucagon

Prevents protein breakdown: insulin

Promote protein building (anabolic): growth hormone

Hormones you should know

Estrogen, Progesterone, Follicle Stimulating Hormone

Testosterone

4. Response to Training

Response to training

glycogen storage capacity lactate threshold oxidation and transport of FA and IMTG increases number of capillaries plasma volume Krebs Cycle and ETC activity transamination enzymes to enhance use of protein for energy

effects of glycogen depletion

So, does a well-trained person require as much carbohydrate during exercise someone of lower fitness level?

ex: fast marathoner vs. 5 hour marathoner

5. Fatigue

Fatigue

Why do athletes “hit the wall” or “bonk”? Run out of substrate (carbs)

No glucose for brain (CF), carbs are needed to use fat When only fat is left, energy production is slow and intensity

decreases Build-up of lactate (or other metabolites)

Changes acidity of muscle environment → inhibition of some important energy enzymes, inhibits fat breakdown (aerobic metabolism) and the muscle contractile process itself

Central fatigue Disturbances at neural or contractile level of muscle

Calcium channels? Changes in pH