Does carbohydrate feeding during exercise influence endurance performance and whole-body metabolic perturbations when exercise is commenced with low carbohydrate availability?

Free, Bonnie Grace (2019). Does carbohydrate feeding during exercise influence endurance performance and whole-body metabolic perturbations when exercise is commenced with low carbohydrate availability? University of Birmingham. M.Sc.

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Background - Training with low carbohydrate availability has been shown to stimulate enhanced adaptation of skeletal muscle. These metabolic adaptations, such as upregulated markers of lipid metabolism, theoretically should convey an ergogenic benefit to endurance performance. However, the caveat to this finding is a reduction in attainable training intensity. One way to combat this limitation appears to be to consume carbohydrates during the exercise, however, traditionally this has been shown to blunt the beneficial metabolic response elicited from training low. It is possible that delaying the feeding of carbohydrate until after the start of exercise might minimise the metabolic disturbance whilst also conferring an ergogenic advantage. Therefore, the purpose of this study was to investigate the potential of a delayed carbohydrate feeding strategy in the glycogen depleted state as a viable method of maintaining a favourable metabolic response to exercise whilst also improving endurance performance.

Methods – Eight endurance trained cyclists underwent a 2-day endurance cycling protocol in three different nutritional conditions; placebo, high carbohydrate availability with carbohydrate feeding or low carbohydrate availability with carbohydrate feeding during exercise. The protocol consisted of a glycogen depletion on day 1, followed by a 6-hour refeeding protocol in which participants either consumed carbohydrate ( or a placebo dependent on the condition of the trial. On day 2 participants performed 1-hour of steady state cycling (50%Wmax) with gas measurements and blood samples taken at regular intervals. During the steady state bout, either carbohydrate or a placebo was consumed at 15-minute intervals dependent on condition. However, carbohydrate was fed after a delay, at 30-minutes into the exercise, in the high carbohydrate and the glycogen depleted conditions. A total of 75g of carbohydrate was consumed throughout these trials, at 15g intervals. Immediately on completion of the steady state, subjects completed a time-trial (~40 minutes). Gas exchange measurements and blood samples were analysed to indicate the metabolic responses to each trial condition, whilst time-trial time was used as the performance measure. All data was analysed using SPSS software to identify any significant findings.

Results – There was an initial upregulation in both plasma nonesterified fatty-acid concentration and lipid oxidation in the glycogen depleted state preceding the feeding of any carbohydrate (1.41±0.34gmmol.L-1 and 0.68±0.15g.min-1 respectively) compared to the glycogen replete condition (0.97±0.57mmol.L-1 and 0.51±0.30g.min-1 respectively). However, when carbohydrate was fed 30-minutes after the onset of exercise, plasma non-esterified fatty acid concentration and lipid oxidation were significantly suppressed in the low glycogen condition (0.61± 0.25mmol.L-1 and 0.76± 0.13g.min-1 respectively, P < 0.05), to a similar level as in the normal glycogen condition (0.58±0.55mmol.L-1 and 0.62±0.29g.min-1) . Furthermore, there was no observed ergogenic benefit to endurance cycling performance in a time-trial when carbohydrates were ingested in the glycogen depleted state (44.05±7.68 minutes), compared to placebo (47.74±9.73 minutes) nor was there a decrement as compared to a condition of high carbohydrate availability (42.15±8.58 minutes, P = 0.22).

Conclusions – The ingestion of carbohydrates during exercise in the glycogen depleted state did not convey a metabolic advantage compared to the replete glycogen condition or any performance advantage in a cycling time-trial as compared to a placebo. However, there appears to be no negative impact of this strategy in comparison to performance in the fully replete state. Therefore, further research must be undertaken in order to explore the potential of this training strategy as a tool for improving both the metabolic response to exercise and benefits to performance.

Type of Work: Thesis (Masters by Research > M.Sc.)
Award Type: Masters by Research > M.Sc.
Licence: All rights reserved
College/Faculty: Colleges (2008 onwards) > College of Life & Environmental Sciences
School or Department: School of Sport, Exercise and Rehabilitation Sciences
Funders: None/not applicable
Subjects: Q Science > QP Physiology


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