As summer finally begins to reveal itself and the temperature shoots up, running can become a struggle, often due to difficulties fuelling. After all, you’re hot, sweaty and dehydrated, your stomach may not be feeling in perfect condition and downing another gel or chew is not always the most appealing option.
But do you really need to consume as many carbohydrates in the heat? And do carbohydrates still work as effectively for runners in hot weather?
Researchers recently looked at nine studies that examined the effects of carbohydrate fuelling during endurance exercise in the heat – that is, in temperatures above 23ºC, up to a maximum of 40ºC – in an attempt to determine whether the usual fuelling advice is still true when the temperature goes up. They then published the results in the Journal of the International Society of Sports Nutrition.
When the temperature is cool, carbohydrate fuelling recommendations during exercise are around 30-75g of carbohydrate per hour for runs of 90 minutes to 2.5 hours.
This is because carbohydrates are one of the body’s most efficient sources of energy during running, providing a readily available fuel source and helping to slow the depletion of your glycogen stores. These stores can begin to deplete after around 60 minutes of exercise, depending on the intensity of the workout and environmental conditions. Ingesting carbohydrate while running aids post-run recovery, too.
But your body responds differently when working out in the heat. You burn through your glycogen stores more rapidly, regardless of how much carbohydrate you ingest. Your digestion also slows down, as blood diverts to the skin to help reduce your core temperature, which means that your digestive process may not work to optimal levels and carbohydrate absorption can be compromised.
Of the nine studies, five showed that carbohydrates improved performance in the heat – in fact, some found that it improved performance by up to 19%.
However, four studies found that carbohydrates had no significant effect. One study even showed a drop in performance, although it should be noted this was when 140g of carbohydrate per hour was consumed and no reason was given for this drop – it could well be linked to negative effects on the gut.
The practical advice for runners is to not simply take on more carbohydrates during hot runs. Aim for your usual intake, as this will likely benefit your performance rather than hinder it. However, be prepared for the effects to be inconsistent and perhaps not as reliable as you might usually expect.
The studies also showed that starting fuelling earlier than usual might help – the usual advice in cool weather is to fuel after 60 minutes, but you may need to think about starting earlier when running in the heat.
Gut problems may also be more of an issue in warmer conditions, so ensure that you practise fuelling with your gel of choice, building to your target number of grams per hour before race day so that your stomach is trained as much as possible and used to your nutrition brand of choice.
Perhaps the most important takeaway from the research, though, is to prioritise hydration, as carbohydrates should not be seen as a replacement for fluid intake in the heat.
