Week 2 - Carbohydrate and fat Flashcards
Carbohydrate function
Provides ENERGY for the CNS (brain) and to perform physical activity
(fibre has health benefits)
Influence of exercise intensity on energy expenditure and contribution of fuel sources
- Rest
- 40%
- 55%
- 75%
Rest: energy expenditure provided by plasma FFA and plasma glucose, no contribution of muscle energy fuels
40%: increase in energy expenditure, increase in contributions of plasma FFA, IMTG and muscle glycogen.
55%: plasma FFA remains the same, slight increase in IMTG and blood glucose, increase in muscle glycogen
75%: large increase in energy expenditure, decline in fat sources (IMTG and plasma FFA) and an increase in carb sources (muscle glycogen and blood glucose)
How does exercise influence hepatic glucose output?
Exercise increases rate of hepatic (liver) glucose output as intensity increases.
Liver glucose output tries to match muscle glucose uptake to keep blood glucose steady.
What are the two ways that exercise increases CHO oxidation as intensity increases?
1) Increased liver glycogen breakdown (liver glucose output equal to muscle glucose uptake)
2) Increased muscle glycogen breakdown
What can explain fatigue during prolonged strenuous exercise?
Fatigue strongly correlated with CHO depletion.
- Muscle glycogen depletion
- Hypoglycaemia (low blood glucose) reflecting liver glycogen depletion
This means we cannot produce ATP at an adequate rate to meet ATP demand.
How does carbohydrate loading and carb intake during exercise affect fatigue?
Carb loading increases muscle glycogen and carb intake during exercise maintains blood glucose –> more able to match demands of ATP.
Describe relationship between repeated sprint ability and glycogen % drop.
Decrease in repeated sprint ability when glycogen % drop increases.
What is daily carbohydrate recommendations based on?
Body weight (g per kg) NOT % of energy intake
What are the (general) daily carbohydrate intake guidelines for light, moderate, high (1-3hrs) and very high exercise (4-6hrs)?
Light = 3-5g/kg/d
Moderate = 5-7g/kg/d
High = 6-10g/kg/d
Very High = 8-12g/kg/d
When preparing for events lasting <90min exercise, describe the approach we use.
General fueling up
- 7-12g/kg/d as per daily fuel needs
When preparing for events lasting >90min in sustained, intermittent exercise, describe the approach we use.
Carb-loading
- 10-12g/kg/d for 36-48h pre-event
What is the main goal on the day of an event?
Ensure liver glycogen stores are high ready for performance.
What are the carbohydrate targets for pre-event fuelling?
1-4g/kg body mass, consumed 1-4h before exercise
Breakfast before competition: toast, bagels, porridge, cereal, rice, bananas
After exercise, what is the priority? How does carbohydrate ingestion rate influence this?
- To replenish muscle glycogen.
- Higher carbohydrate ingestion rate increase muscle glycogen synthase rate.
What % of total energy intake is daily fat intake? What about saturated fat?
- 20-35% of total energy intake.
- Saturated fat limited to less than
10% of total energy intake.
What are the adaptations of high fat diet (fat adaptation)? Is this diet beneficial?
- Increased fat oxidation
- Decreased carbohydrate oxidation
- Spared muscle glycogen
Evidence suggests that fat adaptation impairs high-intensity exercise performance.
Ketogenic diet
Very low CHO intake and high fat intake.
- <50g carbohydrate/day, fat intakes >70-80% energy
- 15% or 1.5g/kg/d protein
- Ensure adequate sodium/potassium
Describe the adaptations of a ketogenic diet (Keto-adaptation). What is the logic/benefit of this type of diet?
- Takes at least 2-3 weeks.
- We get elevated blood levels of ketones (known as ketosis) and tissue adaptations to enhance their use as fuel.
- This increase use of fat as muscle fuels.
Fatigue is due to reduced CHO availability and inability to use alternative lipid sources. Keto-adaptation ensures stable fuel source for exercising muscle (and brain) in the face of low CHO availability.
What is the effect of a ketogenic diet on endurance performance?
Increased rates of fat oxidation. However, this results in reduced economy (increase oxygen demand for a given speed), reduced muscle glycogen and reduced muscle glycogen breakdown therefore low-carb and high-fat diets impair performance in elite endurance athletes.
What does the “Periodised approach” to carb intake refer to?
refers to daily carb intake being flexible to the demands of training and competition.
Why would a low-fat, low fibre nutritional strategy be recommended for a pre-exercise meal?
to reduce the risk of gastro-intestinal (GI) problems
Why should athletes try to avoid restricting fat intake below 20% of total energy intake for sustained periods of time?
It may reduce fat soluble vitamin and essential fatty acid uptake.
List adaptations that enhance metabolic flexibility.
- Increases in transport molecules that carry nutrients across membranes or to the site of their utilization within the muscle cell (GLUT4, CD36)
- Increases in enzymes that activate or regulate metabolic pathways (PFK - glycolysis).
- Enhancement of the ability to tolerate the side-products of metabolism (lactic acid).
- Increase in the size of muscle fuel stores.
Define energy availability.
the amount of energy available to the body to perform all other functions after the cost of exercise is subtracted.
(dietary intake - exercise energy expenditure normalized to FFM)
