Extreme heat stops athletes from fully using carbohydrates for fuel

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 or more per hour during a race. This step change has contributed to the first competitive sub 2hr 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 scale of how much heat stops 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 sub-maximal cycle ride on a static bike to see how much glucose that they consumed was able to be used as fuel under different heat conditions. 17 participants took part in the study, which included two tests performed under very hot temperatures of 40C and 20% humidity, and one at a controlled 20C. 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. Analysis of the findings suggest that heat is creating a barrier that stops the ability of all carbohydrates in the blood from being transferred into or used by the muscle and other body systems for fuel.

Endurance athletes are taking on significantly higher amounts of carbohydrate during sport by rule. As some of the top events around the world are being held in increasingly oppressive heat, understanding what heat does to our bodies and how it uses carbohydrates is crucial – and could make the difference between podium places among the very best competitors.

The headline findings are significant. With nearly 30% of glucose not being able to be used to fuel aerobic performance, extreme heat could be having a significant effect on endurance athletes' ability to hit their best speeds."

Professor Gareth Wallis, University of Birmingham and senior author of the study

What is reducing the body's use of carbs in hot conditions?

The team of sports scientists analysed data on carbohydrate use taken during the test. They found that this reduction in the ability to burn carbs as fuel was not due to a reduced ability to process carbs in the gut, as was thought previously.

Instead, the team discovered that there is a mechanism of action stopping a typical amount of glucose that would be taken up from circulation by muscles and being used as energy. A combination of blood and muscle biopsy analysis, and the use of stable-isotope tracers, enabled the researchers to measure how carbohydrates consumed during the trial were absorbed and used during exercise.

While the team thought that heat acclimation might have a benefit in making more ingested glucose available under hot conditions, there was no observed difference. After heat acclimation, participants showed expected signs of adapting to the heat. Their core temperature was lower both at rest and during exercise in the heat, their blood plasma volume expanded, and they had lower heart rates and found the exercise less hard and less uncomfortable.

Professor Gareth Wallis continued: "We have observed for the first time that heat is creating a barrier that is affecting how the body removes glucose from blood and uses it as energy. While we can't yet say exactly what is stopping the full transfer of glucose into systems to fuel exercise performance, we have shown that the issue exists at the point of transfer out of the blood supply and subsequent use rather than from the gut into the blood stream.

"We have also been able to show that heat acclimation, despite improving people's ability to cope with the heat, did not rescue the reduction in capacity to use ingested carbohydrates. The same mechanisms that are creating a barrier affecting the transfer and use of glucose from the blood supply were not alleviated with acclimation - this was contrary to what we and most would expect."

The findings taken from hot conditions of 40C and 20% humidity suggest that athletes should not reduce their carbohydrate intake during exercise as they look to prioritise consuming enough liquid for cooling core body temperature during extreme heat conditions. Future studies will identify the precise barrier that stops the muscles from being able to use the full glucose load.

Dr Tim Podlogar from Exeter University and lead author of the study said: "The key message for athletes is simple: do not cut back on carbs in the heat. Your gut is still absorbing them, so they are getting into your body, and eating less would only leave you with even less fuel. Aim for a similar carbohydrate intake as you would in cooler conditions and make sure you are drinking enough to allow the body to produce sweat and cool you down. Getting both in can be challenging when it is hot, so practise your race fuelling and drinking in training, ideally in warm conditions, to get your gut used to it. And use the usual tricks to stay cool, like cold drinks, ice slushies, ice socks, and pouring water over yourself."

The study has been led by the team involved in a personalised carbohydrate test for performance athletes. Launched by Professor Gareth Wallis, fuelsync® shows exactly how much carbohydrate taken during exercise is converted into usable energy. Validated in a peer reviewed paper published in Performance Nutrition, the fuelsync® test demonstrates how athletes can have an optimum amount of glucose intake that is up to 25% more or less than current academic-led guidelines.

The study received financial support from the U.S. Army Medical Research and Development Command.