Effect of carbohydrate dose on liver and muscle glycogen metabolism during exercise
Carbohydrate (CHO) ingestion during prolonged exercise is common practice amongst athletes and improves endurance performance. Glycogen is a limited energy resource during prolonged exercise and as such sparing glycogen has been proposed as a mechanism behind the ergogenic effect of CHO. However, evidence to support such an effect is equivocal and it appears that the dose of CHO may play an important role in this regard. The aim of this thesis was to investigate a dose response to prolonged exercise CHO ingestion on exogenous CHO, liver and muscle glycogen utilisation and exercise performance.
Two multiple condition studies were conducted consisting of either a 2 or 3 hour continuous period of fixed workload cycling followed by a 30 minute time trial. During the fixed workload period, a placebo solution or various CHO solutions were ingested which had been artificially labelled with a 13Carbon (13C) stable mass isotope tracer. By analysing the appearance of the 13C tracer in expired air and plasma glucose, rates of exogenous CHO, plasma glucose and liver and muscle glycogen were calculated. Study 1 investigated 4 CHO doses (60 to 112.5 g.h-1) using both single and multiple transportable CHO designed to saturate and over saturate the intestinal transport proteins SGLT1 and GLUT5 during 2 hours of continuous cycling at 77% V̇ O2max. Study 2 examined a smaller dose range (80 to 100 g.h-1) in order to provide evidence to the effectiveness of a 90 g.h-1 dose during 3 hours of continuous cycling at 60% VO2max. ̇
In Study 1 the ingestion of multiple transportable CHO at 90 g.h-1 improved performance by 9-15% and spared muscle glycogen relative to the ingestion of glucose only. Fat oxidation was spared relative to placebo ingestion with CHO. It was also seen that increasing CHO dose with ingestion of both single CHO and multiple transportable CHO at rates exceeding intestinal saturation did not further increase exogenous CHO oxidation, and had potentially negative consequences on time trial performance and endogenous substrate (fat and glycogen) oxidation. Study 2 showed that multiple transportable CHO ingestion at 90 g.h-1 produced a small reduction (17.8%) in muscle glycogen oxidation rate and an improvement (3.6 - 7.1%) in mean power output in the time trial, relative to 80 and 100 g.h-1 .
This thesis provides novel evidence that with the right dose of CHO ingestion during prolonged exercise, muscle glycogen was spared, which coincided with improved subsequent time trial performance. Further, the effect of ‘over-dosing’ exogenous CHO intake was potentially detrimental to endogenous substrate oxidation and time trial performance.
History
Qualification name
- PhD
Supervisor
King, Roderick ; O'Hara, JohnAwarding Institution
Leeds Beckett UniversityCompletion Date
2015-08-01Qualification level
- Doctoral
Language
- eng