Week 3 (2) lecture - CHO intake before and during exercise

Question 1

What are the pre-exercise recommendations?

Answer 1

• Before exercise (> 60min), 1- 4 g.kg-1 BM should be consumed 1-4hr before exercise
• E.g., 100kg male should consume 100g – 400g before exercise (not very refined)
• Wide ranging, individualistic suggestions

Question 2

Muscle glycogen storage of the gastrocnemius

Answer 2

muscle glycogen peaks after ~4hours after a mixed meal

Question 3

Muscle glycogen (Coyle et al 1985):

Answer 3

• 7 subjects, 105min of cycling @70% vo2 max. CHO meal 4h before or 16h fast
• Consume CHO – high in muscle glycogen – can use more during exercise
• Blood glucose is lower after feeding – hypoglycaemic response (when you eat you increase insulin which serves to reduce blood glucose)
• No fuel – plasma free fatty acids are mobilised
• Higher respiratory exchange ratio in fed state which indicates more carbohydrate oxidation

Question 4

Effectiveness of pre-exercise feeding (Chryssanthopoulos et al 1989):

Answer 4

• Feeding during exercise causes a rise in blood glucose during exercise

Question 5

The effect of breakfast (Mears et al 2018)

Answer 5

• Different breakfasts were given to ppts before the exercise, findings showed that placebo breakfast and CHO performed better than a water based breakfast
• CHO wasn’t needed for short duration exercise
• The perception of eating may impact performance for short duration exercise

Question 6

Aims of exogenous CHO

Answer 6

• To provide an additional source of fuel
• To maximise oxidation rates
• To prevent gastrointestinal distress
• To maximise performance

Question 7

Carbohydrate utilisation: Prolonged exercise (Coyle et al 1986):

Answer 7

• Looked at time to exhaustion at ~70% vo2 max with or without CHOs during exercise
• NO CHOs – as fatigue was reached reductions in plasma glucose (as cant supply from liver effectively and no exogenous sources- deplete liver glycogen), muscle glycogen, reduced CHO oxidation
• When fed CHOs during exercise, plasma glucose was maintained, muscle glycogen dropped by similar amounts, CHO oxidation was maintained. Utilised same amount of muscle glycogen but had an enhanced delivery of plasma glucose. End response = 1hour more exercise when compared to the placebo group
• CHOs enabled exercise to continue for longer (through maintained plasma glucose and CHO oxidation)

Question 8

CHO feeding during exercise (Jeukendrup and Jentjens 2000):

Answer 8

• If we are efficient we should see a 1-1 ratio between carbohydrate ingestion rate (g/min) and exogenous CHO oxidation (g/min) – but we don’t see this
• Rarely see an exogenous CHO oxidation of 1 (when ingested 1.0 CHO)
• By ingesting more we don’t see further benefit
• At around 1.2 we stop increasing exogenous carbohydrate oxidation.

Question 9

What limits exogenous CHO oxidation?

Answer 9

• Intestinal CHO absorption appears to be the rate limiting step - controlled by what we can transport through the intestines

Question 10

Glucose absorption:

Answer 10

Glucose uses transporter SGLT1 to get into the cell and then GLUT2 to get into the blood.
- This is a sodium dependent transporter
- SGLT1 is saturated at 1.1 – 1.2 g/min
To bypass this we can add fructose to the CHO mix
Make drinks fructose and glucose (to use a multiple transporter system)

Question 11

Fructose absorption:

Answer 11

Fructose uses GLUT5 to cross the 1st part of the cell membrane – helps provide more CHO from intestine to blood
Fructose was originally seen at a ratio of 2-1 (ingesting 90g Cho per hour, glucose would be about 60g and fructose 30g). the ratio is now getting closer to ~2-1.6
- We can ingest more fructose and not get the same gastrointestinal problems we’d see with the same amount of glucose

Question 12

Exogenous CHO oxidation with glucose + fructose

Answer 12

• The addition of high glucose doesn’t increase oxidation rates (g/min)
• By combining fructose and glucose, oxidation rate is much higher (goes above the 1-1.2g/min limit that is typically seen)
• By adding fructose we can increase the amount of available CHO that can go through exogenous oxidation

Question 14

CHO rinsing

Answer 14

Rinsing 10-15g of CHO for 10-12 seconds may change senses that anticipate CHO ingestion so you can use more of the fuel sources
Is it Practical? Causes ventilation rate to increase (breathing harder to get air in) when swirling water for 15 seconds

Question 15

The effect of CHO mouth rinse:

Answer 15

• CHO or PLA rinse every 12.5%
• ~1hr/ ~40km TT performance with or without CHO-E (7.6%)
• Used if you want to train in a fasted state (get the benefits from training in a fasted state)
• Used to Avoid GI problems

Question 16

How many grams of carbohydrate are in 500mL of a 6% drink (standard)?

Answer 16

30g
- Some of the higher concertation drinks are now 16% CHO

Question 17

To ingest 60 g/h of a 10% glucose drink, what volume is required per h?

Answer 17

600ml
- 10% = 10g/ 100mL
- 60 g/h = 60/10 x 100mL = 600mL

Question 18

Can we train the gut?

Answer 18

• CHO intake improves exercise performance greater than 45min
• There appears to be a dose-response relationship. More CHO = better performance
• Limitation is gastro-intestinal absorption
• Therefore, could more CHO be delivered to the muscle, and result in increased oxidation and increase performance?
• There is evidence that we can train the gut: increase gastric emptying, increase SGLT1 appearance, increase uptake into the muscles, create stomach expansion which increases gastric emptying further

Question 19

Summary of CHO feeding during exercise

Answer 19

• CHO feeding during exercise maintains plasma glucose concentrations
• Exogenous CHO oxidation with single source (glucose) limited to 1-1.2g/min
• Can be increased to 90 g/h with multiple CHO sources

Question 20

Aims of recovery

Answer 20

• Ensure muscle and liver glycogen stores are restored
• The timing of this depends on when resynthesis is required

Question 21

Recovery of muscle glycogen (Piehl 1974): glycogen recovers vey quickly immediately after exercise

Answer 21

• Muscle glycogen is initially high, a period of work is performed to deplete it and then a series of meals is providing different amounts of carbohydrates is given.
• Found a rapid restoration in the first 5 hours after exercise due to increase in insulin
• Insulin increases glucose uptake into the muscle and liver

Question 22

Exercise and insulin

Answer 22

When we exercise or eat we increase insulin
Insulin moves GLUT4 proteins onto muscle cell membrane to help with glucose uptake into the muscle

Question 23

Muscle glycogen supercompensation

Answer 23

• After feeding they were able to take muscle glycogen above the content of the rested leg after exercise
• Depletion exercise (of muscle glycogen) and then immediate fuelling with high insulin concentrations, muscle glycogen concentrations were able to go above the concentrations found in the rested leg – supercompensation
• This is why tapering is performed

Question 24

Glycogen resynthesis – Timing of CHO (Ivy et al 1988):

Answer 24

• Muscle glycogen storage is much greater when feeding happened immediately after exercise
• Insulin response is greater as it’s increased by both exercise and feeding
• Rapid restoration requires immediate feeding

Question 25

Glycogen resynthesis- Amount of CHO (Jentiens and Jeukendrup 2003)

Answer 25

• General guidelines: 1-1.2 g/kg BM/h for 2-4hrs (the amount of CHO that should be consumed after exercise)

Question 26

Glycogen units:

Answer 26

• Diet = g/kg
• Exercise = g/min
• Recovery g/kg/hour

Question 27

Glycogen resynthesis – dietary CHO intake (Burke et al 2004)

Answer 27

• Tails off around 10-12g/kg-1/BM

Question 28

CHO and protein – muscle glycogen resynthesis (Van Loon et al 2000):

Answer 28

• 0.8 g/kg/h CHO (Carb trial), 0.8 g/kg/h + 0.4 g/kg/h protein (Carb+Pro) or 1.2 g/kg/h CHO (Carb+Carb)
• Highest spike in plasma insulin was with carb and protein
• Protein intake also helps
• If CHO is insufficient, protein can be added to provide the same insulin response and consequently aid muscle glycogen resynthesis

Question 29

Post-exercise recovery: CHO intake guidelines:

Answer 29

• General guidelines: 1-1.2 g/kg BM/h for 2-4hours
• This equates to 70-84 g CHO for a 70kg athlete