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Endurance sport has seen a renaissance in the understanding of carbohydrates for fuel in recent years. Advances in sports science have led to much higher levels of carbohydrates either eaten or drunk during exercise, with some top sports stars consuming 120g (4.2 ounces) or more per hour during a race. This step change has contributed to the first competitive sub-two-hour marathon and record-breaking triathlon and cycling race times.

While scientists have known that high temperatures stop athletes from fully using these high levels of carbohydrates as fuel during intense exercise, the mechanisms that are stopping this from happening have been poorly understood.

But now, a new study from the University of Birmingham, which worked with trained cyclists, has investigated the extent to which heat prevents ingested carbohydrates from being available to fuel exercise.

In the paper published in the Journal of Physiology, amateur cyclists were tested during a 90-minute submaximal cycle ride on a stationary bike to see how much glucose they consumed could be used as fuel under different heat conditions.

Seventeen participants took part in the study, which included two tests performed under very hot temperatures of 40°C (104°F) and 20% humidity, and one at a controlled 20°C (68°F). Prior to the second "hot" test, participants took part in a nine-day period of heat acclimation, exercising in similar conditions.

Across the study, participants on average were able to use 27% less ingested carbohydrate in hot conditions, regardless of whether they had done heat acclimation.

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