Fat & Carbohydrate Exercise Physiology

Question 1

Fuel use based on exercise intensity: high vs. low intensity

Answer 1

• Fat and carbohydrate (CHO) generate ATP during aerobic exercise. Depending on the level of fitness of an individual, fat can sustain the biggest part of ATP production.
• At low exercise intensities fat is the preferred substrate although there is always some glucose oxidation. Used when VO2 max is up to 75%
• CHO metabolism is of great importance during exercise, especially during high exercise intensity where it is the predominant energy substrate for skeletal muscle. Used when VO2 max >75%.

Question 2

Mechanism of metabolism used in muscles during very high intensity exercise

Answer 2

• When exercise intensities are very high/close to maximal (above 100% of VO2 max) ATP cannot be generated by the aerobic mechanism
• ATP generated through the anaerobic mechanism also called substrate level phosphorylation
• ATP synthesis derived from CHO is much faster and therefore the choice of substrate by skeletal muscle fibers during faster twitch, high exercise intensities.

Question 3

Metabolic adaptations caused by training

Answer 3

• After training = ↓ work intensity, ↓ glycolytic flux, ↓ glycogenolysis, ↓ energy requirement from glucose
• In trained individuals, higher exercise intensities cause lower blood lactate accumulation due to increased lactate clearance capacity through an increase in mitochondrial lactate dehydrogenase (mLDH)
• Mitochondrial density and oxidative enzymes increase with training and this increases the capacity for fat utilization.
  
  • Endurance training can double the mitochondrial capacity to oxidize FFA and pyruvate.

Question 4

Importance of glycogen for exercise

Answer 4

• Muscle glycogen is crucial for ATP synthesis during exercise.
• Proper glycogen storages are of great importance for athletic performance.
• Multiple studies show that fatigue and decrease in performance is often associated with glycogen depletion and that athletes who have low carbohydrate diets or low glycogen storages

Question 5

Role of lactate during exercise

Answer 5

• Lactic acid is the byproduct of glycolysis.
• During competitive exercise ATP generation demands increase and glucose becomes the predominant muscle substrate for ATP generation.
  
  • The higher the glucose flux into the cell the higher the lactate production.
• Lactate production is necessary for the continuation of glycolysis
• Lactate is an important carbohydrate source and a main gluconeogenic precursor during exercise as it is shuttled to other cells, tissues and organs where about 75-80% is oxidized to pyruvate for energy purposes.
• Lactate accumulation inhibits lipolysis at high exercise intensities → similar to ↑ epi.

Question 6

Fitness testing parameters

Answer 6

• VO2 max is meaningless. Average athletes can have higher than pros.
• Muscle lactate metabolism is much more useful and can subdivide beginners and pros from amount of lactate produced with increasing exercise workload.