Carbohydrates as Ergogenic Aids
Importance of Carbohydrate
Metabolism
• Involved in resynthesis of ATP during highintensity exercsise
• Also provide substrate for Krebs/TCA cycle
Carbohydrate Depletion and
Fatigue
• Glycogen depletion or hypoglycemia often
associated with exercise fatigue
• Glycolysis provides pyruvate which feeds
into Krebs/TCA
• If glucose is insufficient to fuel glycolysis,
Krebs may be slowed as a result
Exercise Intensity Limited in the
Absence of CHO
• If lipid is the sole energy source, exercise
above 50-60 % VO2max cannot be
sustained
McCardles Disease as a Model
• McCardles patients do not have PHOS
– Cannot utilize glycogen as a fuel source
• Exercise capacity only 50 % of predicted
• Also greater ATP degradation
– Elevated IMP levels compared to normals
Intensity vs Fuel Source
• at higher intensity workloads, carbohydrate
will become the primary fuel source
• conversely, at lower workloads, fats will
predominate
The crossover concept of metabolism vs.
intensity
Duration vs Fuel Source
• for short duration or during the early stages
of prolonged exercise, carbohydrates will be
the primary fuel source
• for longer duration fat will become the
predominate source
Fuel shifts from CHO to fat during
prolonged exercise
Sources of Fuel During Exercise
• Carbohydrate
– Blood glucose
– Muscle glycogen
Fat
• Plasma FFA (from adipose tissue lipolysis)
• Intramuscular triglycerides
Blood lactate
• Gluconeogenesis via the Cori cycle
Muscle fuel sources in highly trained
endurance athletes
carbs
Fuel Sources in Highly Trained
Athletes
• Low Intensity
– Primary source plasma FFA (from fat stores)
• Middle Intensity
– Equal from plasma FFA and muscle glycogen
• High Intensity
– Muscle glycogen predominate source
Contributions of four energy sources over
prolonged time in endurance athletes
Prolonged Exercise in Trained
Endurance Athletes (70 % VO2max)
• Less than 1 hour glycogen is primary single
source
– FFA and muscle triglycerides comprise 50 % though
As exercise progresses
• Contribution from glycogen reduced
• Muscle triglycerides (intramuscular fat)
reduced also
• FFA and plasma glucose increase
• Must feed to maintain plasma glucose