Detail the distinct mechanisms through which ingestion of carbohydrate during exercise might improve performance and discuss how this might be mediated by an athlete’s feeding strategy.

Question 1

CHO ingestion during exercise can…

- reference

Answer 1

> improve performance in running and cycling
delay onset of fatigue in running and cycling
maintain skilled performance late in exercise.
When comparing CHO vs placebo.
- Jeukendrup 2004

Question 2

Why is the ergogenic benefits of CHO supplementation during exercise greater for prolonged exercise (>2 hours) than short duration?

Answer 2

Main fatigue mechanism during prolonged exercise is muscle glycogen depletion and a reduced blood glucose conc.

Question 3

What are the potential mechanisms for the ergogenic effect of CHO ingestion during exercise?

Answer 3

• Liver glycogen sparing
• Muscle glycogen sparing
• Maintenance of euglycaemia and CHO oxidation rates

Question 4

Describe the effect of CHO ingestion during exercise on hepatic glucose output.
- reference

Answer 4

Ingestion of large CHO doses during exercise reduces and could even block hepatic glucose output, allowing for greater CHO availability from the liver toward the end of the exercise when intensity typically increases.
- Jeukendrup et al (1999)
120 mins continuous cycling, ingestion of either water, 4% or 22% CHO solution every 15 mins.
placebo<4%<22% for the rate of glucose appearance. W/ 22% this was all exogenous glucose, no hepatic glucose, therefore, liver glycogen sparing.

Question 5

Describe the effect of CHO ingestion during exercise on plasma glucose conc and CHO oxidation and how this benefits performance.
- reference

Answer 5

Exogenous CHO maintains blood glucose conc and in turn maintains the rate of CHO oxidation over prolonged exercise. - Coyle et al (1986)
Nybo (2004) - 3 hrs @60% vo2 max in well-trained athletes lowered blood glucose conc and voluntary force production post ex. When 200 g CHO ingested during (maintaining euglycaemia) attenuation of pre-post force production was avoided.

Question 6

Describe the effect of CHO ingestion during exercise on muscle glycogen.
- reference

Answer 6

More exogenous CHO = muscle glycogen oxidised at a slower rate during prolonged exercise.
Multiple Tsintzas studies (1995-2001) show ~20-28% less muscle glycogen loss during prolonged exercise when CHO was ingested during.

Question 7

How does timing of CHO ingestion during exercise impact performance?
- describe the study and give the reference

Answer 7

• Coggan et al (1989)
  TTE cycling at 70% vo2 max. At 135 min ppts ingested either water or 50% CHO solution.
  CHO group had sig longer TTE. Placebo group TTE was ~160 mins.

Question 8

Why is timing (frequency) of CHO ingestion less critical in cycling than running?

Answer 8

Large amounts of fluid in one go during running may lead to higher GI discomfort due to bouncing of body/stomach.

Question 9

What is the optimal ingestion rate of glucose during prolonged exercise?

Answer 9

~1 g/min

Murray (1991)

Question 10

If 2 g/min of glucose were ingested then how much of this would appear from a) the stomach, b) the GI tract, c) the liver?

Answer 10

a) 2 g/min
b) 1.2-1.7 g/min
c) 1 g/min

Question 11

What is the optimal ingestion rate of multiple transportable CHOs during prolonged exercise?
- reference

Answer 11

~1.5 g/min

- Jeukendrup (2014)

Question 12

Why is the optimal ingestion rate for GLU+FRU ~1.5 g/min whereas GLU is only 1 g/min?
- reference

Answer 12

Possible that multiple transporters used in intestine and then liver converts fructose to glucose. However, GLU+FRU doesn’t alter glucose appearance in circulation or in muscle.
Therefore, increased exogenous CHO oxidation reflects…
- oxidation of liver CHO sources and/or
- increased production and oxidation of lactate from fructose.
- Jeukendrup et al (2006)

Question 13

What are the benefits of GLU+FRU (SUC) vs GLU only?

Answer 13

Higher exogenous CHO oxidation rates (1.5 g/min vs 1 g/min).

Lower gut fullness.

Question 14

Why would FRU on its own be a poorer intra-exercise CHO option than GLU?

Answer 14

Lower GI (oxidised slower)

Question 15

Why use maltodextrins over glucose in a sports drink?

Answer 15

> neutral tasting
lower osmolality than glucose
can be oxidised as rapidly as glucose.

Question 16

Solid or liquid or gel CHO ingestion during exerise?

- Describe the study and give reference

Answer 16

Multiple studies from Pfeiffer have shown solid and gel CHO oxidation rates to be similar to liquid. Doses were 1.55 g/min (Glu+Fru) and no severe GI symptoms were reported in any trial.

Question 17

What are the issues surrounding the evidence of muscle glycogen sparing during prolonged exercise with CHO ingestion?

Answer 17

Many studies report no effect - Coyle et al (1986).
However, studies vary a lot in their activity type, duration, intensity and muscle fibre type measured. Hard to definitively detect muscle glycogen sparing in mixed muscle glycogen content measuring methods due to differences in fibre type glycogen sparing and how this may change over an activity’s duration and intensity.

Question 18

How may liquid CHO ingestion be better than solid or gel?

Answer 18

Liquid CHO ingestion aids in fluid intake which helps to compensate for sweat loss and facilitates gastric emptying.

Question 19

What is a common methodological flaw in studies looking at CHO form and how may this affect correct real-world recommendations?

Answer 19

Use fixed intensities. (poor ecological validity as races typically ramp up in intensity at the end).
Increased intensity re-directs blood flow from viscera to muscles and skin, therefore, reducing gastric emptying rates - de Oliveira (2014).
Therefore, CHO ingestion as fluid (helps facilitate gastric emptying) may be more vital during high-intensity race portions to avoid GI symptoms.