Lecture 2 - Fuel Sources for Muscle and Exercise Metabolism

Question 1

role of ATP for energy

Answer 1

ATP is the only source of eergy that can be used directly for muscle contraction and all other energy-requiring processes in the cell
- the resting concentration of ATP in skeletal muscle is 4-5 mmol/kg wet weight of muscle

Question 2

why is it so important to maintain ATP concentration

Answer 2

• complete depletion of ATP would be fatal to the cell

Question 3

how is ATP concentration maintained?

Answer 3

it is maintained through a variety of processes:
- phosphocreatine hydrolysis
- anaerobic metabolism of glycogen or glucose (Glycolysis)
- aerobic metabolism (oxidation) of acetyl co-A derived principally from the breakdown of carbs or fat
- the carbon skeleton of amino acids can be used as fuel for oxidative metabolism but is not a major fuel for energy production during exercise

Question 4

anaerobic vs aerobic metabolism pathways

Answer 4

Anaerobic
- glycolysis
Aerobic
- beta-oxidation
- glycolysis
- deamination (protein)
- citric acid cycle
- Electron transport chain

Question 5

explain oxidative phosphorylation

Answer 5

See slide 6

Question 6

explain how the main pathways interact and work with each other

Answer 6

See slide 5

Question 7

glucose oxidation equation

Answer 7

glucose + 6O2 + 38 ADP + 38Pi - > 6CO2 + 6H2O + 38 ATP

Question 8

when are carbohydrate stores depleted? At what rate?

Answer 8

• carb stores are rapidley depleted during exercise (muscle glycogen) or during fasting (liver glycogen)
• muscle glycogen stores are normally depleted after 1-2hrs of hard exercise
• in very high intensity exercise, the muscle glycogen content falls rapidly but not completely depleted at the point of fatigue

Question 9

relate glycogen stores to performance

Answer 9

consuming carbs (glucose) can allow athletes to maintain intensity for longer, this can also be extended by consuming carbs mid workout

Question 10

how do we use fat as fuel for exercise?

Answer 10

fat is primarily stored in the body as triglycerides (glycerol +3 fatty acids) within white adipose tissue
- triglyceride store are also in liver, muscles, and as lipoproteins in blood.
muscles cannot oxidize triglycerides directly, so they must be broken down by lipolysis (this action is activated during exercise by the actions of catecholamines, glucagon, and cortisol)

Question 11

how can amino acids be used as fuel for exercise?

Answer 11

• after removal of the amino (NH2) group, the carbon skeletons of AAs can be used for oxidation (only 6 of the 20 AAs in protein are oxidized in significant amounts)
• AA oxidation has been estimated to contribute up to only 15% of energy expenditure in resting conditions
• during exercise, this relative contribution likely decreases to less than 5% because of an increasing importance of carbs and fat as fuel

Question 12

list the 6 AAs that are oxidized and used as fuel

Answer 12

• asparagine
• aspartate
• glutamate
• isoleucine
• leucine
• valine

Question 13

factors influencing fuel selection for exercise

Answer 13

• exercise intensity and substrate availability
• exercise duration
• training status
• diet
• hormones
• prev exercise
• sex
• environmental conditions

Question 14

how does exercise intensity affect which metabolic substrate is being used

Answer 14

• fax uptake and oxidation by muscle is slow and can only supply ATP at a rate to maintain exercise at only an intensity of about 60% of VO2 max
• at intensities above 85% of Vo2 max, the oxidation of carbs will be prominant fuel, with fat contributing 30% or less
• both the oxidative pathway of carb utilization and the anaerobic pathway of glycolysis can supply at a much faster rate than fat oxidation can

Question 15

what is fatigue

Answer 15

the inability to maintain a given or expected force or power output, which is an inevitable feature of maximal exercise

Question 16

fatigue in high intensity exercise

Answer 16

• typically, the loss of power output or force production is likely to be in the region of 40% to 60% of the maximum observed during 30 seconds of all-out exercise

Question 17

what contributes to fatigue in high intensity exercise

Answer 17

• during maximal short-duration exercise, it will be caused primarily by a gradual decline in anaerobic ATP production or an increase in ADP and Pi accumulation
• in HIE lasting 1-5 mins, lactic acid accumulation may contribute to the fatigue process

Question 18

fatigue in prolonged exercise

Answer 18

• at marathon-running pace, muscle glycogen stores alone can fuel about 80mins of exercise before depletion
• oxidation of blood glucose (derived from liver glycogen stores) enables ATP production to be maintained and exercise to continue
• when muscle and liver glycogen becomes depleted, ATP production becomes compromised because of the inability of fat oxidation to increase sufficiently to offset the deficit