Macronutrient Metabolism in Exercise and Training

Chapter 5

Fuel for Exercise

The fuel mixture that powers exercise generally depends on:• The intensity of effort• The duration of effort • The exerciser’s fitness status• The exerciser’s nutritional status

Bioenergetics

• Cells need constant supply of ATP• Minimal amounts stored for cellular processes

– Intramuscular

• Muscular contraction-exercise– Constant, large supply

Bioenergetics

• Formation of ATP (3 metabolic pathways)– 1. Phosphocreatine (PC) breakdown

• ATP-PC system (phosphagen system)

– 2. Degradation of glucose and glycogen • Glycolysis (Glycolytic system)

– 3. Oxidative phosphorylation • Tricarboxylic cycle, Krebs cycle

Bioenergetics

• Anaerobic pathways (do not involve O2)

– 1. ATP-PC breakdown – 2. Anaerobic glycolysis

• Aerobic pathway– Requires O2

– 2. Aerobic glycolysis– 3. Oxidative phosphorylation

Bioenergetics

• ATP-PC ATP-PCr (30 sec)

• Activities (examples)– Sprints (< 30 sec)– High jumping– Weight lifting

Bioenergetics

• Anaerobic Glycolysis –– Lactic acid system (20 or 30 sec to 120 sec) Intermediate in duration

Examples:• Middle distance running • Swimming• Basketball

Bioenergetics

High intensity exercise – 2 minutes ATP-PCr (30 sec) Lactic acid systems (30-120 sec) Provide 50% of the energy

Aerobic sources• Provide 50% of the energy

Aerobic Energy

Longer duration Requires a steady energy supply Examples:

• Marathon running• Distance swimming or cycling• Jogging, hiking, or backpacking

Energy Continuum

100%

% C

apacity of E

nergy S

ystem

10 sec 30 sec 2 min 5 min +

Energy Transfer Systems and Exercise

Aerobic Energy System

Anaerobic Glycolysis

ATP - CP

Relative Aerobic & Anaerobic Contributions

Sources of Energy for ATP Synthesis

Sources of energy for ATP synthesis include:• Liver and muscle glycogen• Triacylglycerols within adipose tissue and

active muscle• Amino acids within skeletal muscle donate

carbon skeletons

Carbohydrate Use During Exercise

Intense anaerobic exercise Muscle glycogen Blood glucose

Sustained high levels - aerobic exercise. Glycogen stores Blood glucose –

Glycogen breakdown - released from liver

Macronutrient Contributions

Intense Exercise

Change in hormone release Epinephrine Norepinephrine Glucagon Decreased insulin release

Stimulation Glycogen phosphorylase - glycogenolysis

Intense Exercise

Early stages• Stored muscle glycogen

• Later stages• Glycogenolysis of liver stores

• Blood glucose

Moderate and Prolonged Exercise

As exercise continues• Glucose from the liver becomes major

contributor (as muscle glycogen falls; about 2-3 hrs)

• Fat use increases

CHO & Exercise

Glycogen Depletion

Blood glucose levels fall. Level of fatty acids in the blood increases. Proteins provide an increased contribution

to energy. Exercise capacity progressively

decreases.

Moderate and Prolonged Exercise

Fatigue

• “hitting the wall”– Fall in muscle/liver glycogen

• Results – CNS used blood glucose as energy (central

fatigue)– Increased fat metabolism – Fat provides energy at a slower rate

Trained Muscle

Trained muscle Increased ability to produce ATP (inc.

mitochondrial volume) Increased blood supply (capillary density) Increased glycogen storage

So, trained individuals Maintain a higher work rate Maintain that workrate for longer periods of

time

Dietary Influence

• Condition 1 (Low carb)– Normal caloric intake – 3 days– CHO 5%– Lipid 95%

• Condition 2 (normal)– Recommended % CHO, Fat, Pro

• Condition 3 (high carb)– CHO 82%

Dietary Influence

Influence of Diet

A carbohydrate-deficient diet Rapid depletion of muscle and liver glycogen.

Low carbohydrate levels Intensity decreases to level determined by

how well the body mobilizes and oxidizes fat. So, this diet reduces performance

Fat as an Energy Substrate

Fat supplies about 50% of the energy Light and moderate exercise.

Stored fat Important during the latter stages of

prolonged exercise (greater than 2 hours)

Caloric Equivalents

Sources of Fat During Exercise

Fatty acids released from adipocytes• Delivered to muscles as FFA bound to plasma

albumin

Circulating plasma triacylglycerol Very low-density lipoproteins Chylomicrons

Triacylglycerol Within active muscle itself

Lipolysis

Hormones activate lipase. • These hormones are secreted more during

exercise.• Epinephrine, Nor-epinephrine, GH

Mobilization of FFAs from adipose tissue Trained muscle has an increased ability to

utilize fat due to the higher volume of mitochondria

Exercise Training

Regular aerobic exercise:• Increases the ability to oxidize long-chain fatty acids• Improves the uptake of FFAs• Increases muscle capillary density • Increases size and number of muscle mitochondria• SO, at the same workload, after training

• Fat metabolism is up and CHO metabolism is down

• Same relative workload (% max), CHO and Fat utilization is the same

Exercise Training

Protein Use During Exercise

Carbohydrate-depleted state Causes significant protein catabolism.

Protein utilization rises Endurance Resistance-type exercise.

Protein Use During Exercise

Greater use as energy fuel than previously thought• Nutritional status • Intensity of exercise training or competition.• Duration of exercise• Certain AAs, the branched-chain amino acids

• Leucine, Iso-leucine and valine• Oxidized directly in skeletal muscle

Exercise and Protein Metabolism

Protein Use During Exercise

• Branched-chain amino acids • Oxidized in skeletal muscle not the liver.• Leucine, isoleucine, valine

• Make up 1/3 skeletal muscle• Important in protein synthesis

– Seem to be oxidized during longer term endurance exercise

Exercise and Protein Metabolism

Exercise and Protein Metabolism

Exercise and Protein Metabolism