Project Description

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Glucose production during exercise in athletes on a very low or high carbohydrate diet

Written by: TalkFEED Media 31 May 2015 Researcher: Christopher Webster


The value of a high carbohydrate low fat (HCLF) diet for endurance athletes, like competitive cyclists, has been under contention since the merits of a low carbohydrate high fat (LCHF) diet, for both athletes and non-athletes have been observed.

Traditionally, athletes favour high carbohydrate diets, as carbohydrate is stored and the energy easily accessed by the muscles and the brain while training and competing. However, these stores do not last as long as required in endurance sports.


It is hypothesised that LCHF eating will increase athletes’ reliance on fat, and thereby improve the body’s ability to use fat at higher intensities.


The aim of this study was to determine how the amount of carbohydrate in the diet affects fuel/energy utilisation and glucose production during exercise in male cyclists between 18 and 40 years of age who ate either HCLF or LCHF and did not smoke or have any illnesses or conditions that would affect physical performance. The exact effects of these drastically different diets were studied with great care. Separate studies of performance in sport are expected to follow.

Test subject

The test subject was a healthy male cyclist in his twenties following a HCLF and mostly vegetarian diet.


The subject was required to log his (HCLF) diet for three days, after which his peak power output was tested at the Exercise Science and Sports Medicine research unit at the University of Cape Town. A blood sample was taken and the subject was given deuterium oxide (D20) to drink the night before glucose utilisation testing as this would assist analysis of glucose production in the liver.

On the day of testing, a thigh muscle biopsy was performed on the subject, after which he was required to peddle a stationary bicycle for two hours against increasing resistance, receiving deuterium glucose tracer intravenously. Afterwards, another muscle biopsy was performed on the subject’s other thigh. The extracted muscle tissue was then placed in a centrifuge, which separated the plasma from the cells, so that the plasma metabolites, hormone and tracer could be analysed to determine the amount of stored carbohydrate that was being used during strenuous cycling.


To be announced.

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