Most endurance athletes develop a fuelling strategy over months of training, and it works. Until summer arrives and the rules shift. In the heat, the relationship between exertion, digestion, and fuel delivery changes in ways that can unravel even a well-rehearsed nutrition plan. GI distress becomes more likely. Appetite disappears. Solid food that sits perfectly on a cool autumn ride suddenly feels impossible mid-race in July.
Understanding exactly why heat disrupts fuelling, and how to adapt around it, is one of the most practical things you can do to protect your performance across the summer season.
Why Heat Disrupts Digestion
During intense exercise, blood flow is directed preferentially to working muscles and the skin for cooling. The gut is deprioritised. Blood flow to the gastrointestinal tract can drop by 60–70% during hard efforts in the heat, significantly impairing the gut's ability to absorb nutrients, motility slows, and the epithelial lining becomes more permeable — a condition sometimes described as 'leaky gut', which contributes to nausea and GI distress.
Blood flow to the gut can drop by up to 70% in the heat. What worked in March may not work in July.
The practical implication: foods and fuels that are well-tolerated in cooler conditions may cause genuine problems when temperatures rise. Solid foods, high-fibre products, and anything requiring significant digestion are at greatest risk. The gut becomes more dependent on simple, rapidly absorbed sources of carbohydrate and less forgiving of everything else.
The Case for Dual-Carb Gels and Drinks
This is precisely where the science behind dual-carbohydrate formulations, combining maltodextrin (a glucose polymer) and fructose, becomes especially valuable in hot conditions. These two carbohydrate sources use different intestinal transporters: SGLT1 for glucose and GLUT5 for fructose. Using both simultaneously maximises absorption capacity at the intestinal level, allowing higher carbohydrate delivery rates without the accumulation that drives GI symptoms.
In heat-stressed conditions where gut blood flow is already compromised, using a formulation that places less demand on any single transport pathway is not just an efficiency consideration, it is a gut-protection strategy. Athletes who fuel with dual-carb products in summer typically report meaningfully better GI tolerance than those relying on glucose-only or maltodextrin-only sources.
Adjusting Your Fuelling Strategy for Hot Conditions
Reduce solid food reliance. If your race plan involves real food, bars, or rice cakes, consider replacing some or all of this with gels and liquid nutrition as temperatures rise. The less mechanical digestion required, the less the heat-stressed gut has to do.
Increase fuelling frequency, reduce portion size. Rather than taking a large gel every 45 minutes, consider smaller amounts every 20–25 minutes. This prevents large boluses arriving in the gut at once, reducing the absorption demand at any single point.
Front-load your carbohydrates. The first 60–90 minutes of a long effort, before core temperature has risen significantly, is when your gut is most capable of absorbing carbohydrate efficiently. Use this window aggressively. Taking on carbohydrates proactively in this phase reduces the demand placed on a heat-stressed digestive system later in the race.
Prioritise palatability. In hot conditions, appetite often diminishes significantly. If your fuel is unpalatable, you simply won't take it, regardless of your best intentions. Testing your fuelling options in heat training so that you know exactly what works for your taste preferences is non-negotiable pre-race preparation.
The athletes who perform best in summer heat are not those who simply push harder through the discomfort. They are the ones who adapted their fuelling protocol before the race to match the conditions and had the right products to support it.
