# Heat and exercise: a 2026 study finds your muscles use less sports drink fuel in extreme heat > University of Birmingham cyclists burned 27% less ingested carbohydrate at 40°C in a Journal of Physiology trial. What to do about fuel, fluids and heat acclimation. Published 2026-11-12 · Updated 2026-11-12 · revns editorial **When you exercise in extreme heat, your muscles may burn less of the carbohydrate you drink during the session**, even if your gut absorbs it normally. That is the headline from a **November 2026** paper in *The Journal of Physiology* led by researchers at the **University of Birmingham**. In **17 trained cyclists**, [exogenous carbohydrate oxidation](/dictionary/exogenous-carbohydrate-oxidation) during a **90-minute** ride at **40°C (104°F)** was about **27% lower** than the same ride at **20°C (68°F)**. **Nine days of heat acclimation** improved comfort and heart rate but **did not** bring drink-fuel use back to cool conditions. **TL;DR** | Finding | Numbers (lab) | Plain meaning | | --- | --- | --- | | Who | 17 trained cyclists (10 men, 7 women); ~24 years; V̇O₂ peak ~52 mL/kg/min | Fit club riders and triathletes, not sedentary adults | | Session | 90 min cycling at 45% max power | Moderate, not an all-out race | | Drink fuel | 90 g glucose per hour | Aggressive race fueling rate | | Hot vs cool oxidation | ~0.37 vs ~0.51 g/min ingested carb burned | ~27% less drink carb used in heat | | After heat acclimation | ~0.38 g/min | No meaningful recovery vs first hot trial | | Total carb burn | ~1.8 g/min all trials | Body still used carbohydrate; more came from stores in heat | | Practical shift | Do not slash carbs because it feels hard | Prioritize **fluids**, **electrolytes**, **pacing** and **cooling** | ## What did the researchers actually test? The peer-reviewed paper is *Heat acclimation does not alleviate the reduction in exogenous glucose oxidation that occurs when exercising in hot conditions* (Podlogar et al., *The Journal of Physiology*, DOI [10.1113/jp289304](https://doi.org/10.1113/jp289304)). The team used **stable isotope tracers** in the glucose drinks, **indirect calorimetry** every 15 minutes, **blood samples**, and **muscle biopsies** from the vastus lateralis before and after exercise. Each participant completed **three main rides** after 48 hours of standardised diet and training: 1. **COOL:** 90 minutes at 20°C and 20% relative humidity. 2. **HOT:** 90 minutes at 40°C and 20% relative humidity. 3. **POST:** another hot ride after **nine days of heat acclimation** (95 minutes of cycling per day in 40°C, with one rest day allowed, targeting a core temperature at or above 38.5°C during acclimation sessions). COOL and the first HOT trial were **randomised**. Everyone drank the same glucose beverage rate: **90 g per hour**, labelled so the authors could measure how much came from the bottle versus stored glycogen. ## How much less carbohydrate from drinks was used in the heat? **Exogenous carbohydrate oxidation** is the part of your energy that comes from carbohydrate you take in during exercise, not from glycogen already in the muscle. In the conference abstract presented at The Physiological Society, which matches the paper's main outcomes, mean rates were: - **COOL:** 0.51 g/min (95% confidence interval 0.45 to 0.57) - **HOT:** 0.37 g/min (0.31 to 0.43) - **POST (after acclimation):** 0.38 g/min (0.31 to 0.44) That is roughly **one quarter to one third less** of the ingested glucose oxidised in heat. **Total carbohydrate oxidation** (~1.8 g/min) did **not** differ across trials, so the body still covered energy demand, but **more of it came from internal stores** when heat blunted drink-fuel use. Net muscle glycogen use was **higher in the first hot trial** than after acclimation, which suggests acclimation may spare glycogen somewhat, but **it did not** restore how much sports drink carbohydrate was burned. ## Was the problem the gut? Press coverage of the Birmingham work emphasises a shift in mechanism. Earlier thinking sometimes blamed **gut absorption** in the heat. In this design, tracer data indicated carbohydrate was **still absorbed**, but **less of the glucose in circulation was taken up and oxidised by muscle** during hot exercise. That matters for how you troubleshoot a bonk in summer: **switching gel brands** may not fix a heat-driven drop in utilisation. **Lowering core temperature**, **hydration**, and **pacing** move higher on the list. ## Did heat acclimation help? **Yes for heat stress, no for drink-fuel oxidation.** After nine days: - **Rectal temperature** during exercise was slightly **lower** than in the first hot trial (about 37.9°C vs 38.2°C during work), though still above cool conditions. - **Heart rate** in the heat fell from about **155** to **146** beats per minute but stayed **above** the cool trial (~130). - Participants reported **less discomfort**. Those are real adaptations for anyone racing in hot climates. They just **did not** raise exogenous oxidation back toward cool values. HOT and POST drink-fuel rates were **statistically the same**. So keep **heat acclimation** on your calendar before a hot goal event, but **do not assume** it makes every gram of maltodextrin count the same as in spring weather. ## How does this fit with other heat and carb research? This trial is not the only line of evidence that heat reshapes fuel use: - A **2025** study in the *Journal of Applied Physiology* had **10 trained runners** complete 100 minutes at ~65% V̇O₂ peak in **19°C versus 34°C** while replacing ~90% of sweat losses with water and taking **60 g/h** of labelled glucose. Even when **hydration was maintained**, **exogenous carbohydrate oxidation was about 20% lower** in the heat, with a **greater share from endogenous carbohydrate** ([DOI 10.1152/japplphysiol.00873.2025](https://doi.org/10.1152/japplphysiol.00873.2025)). - A **2026 systematic review** in the *Journal of the International Society of Sports Nutrition* on carbohydrate supplementation during endurance exercise in hot ambient conditions (>23°C) concluded that **carbohydrate during exercise does not consistently improve performance** in heat across the nine randomised crossover trials it included, and called for more work on **gut symptoms** and mechanisms ([DOI 10.1080/15502783.2026.2669307](https://doi.org/10.1080/15502783.2026.2669307)). - A **2026 meta-analysis** in *Nutrients* on moderate-to-hot ambient exercise found a **small average benefit** of carbohydrate-electrolyte drinks on **aerobic endurance**, but trials compared drink versus placebo **within the same temperature**, not hot versus temperate ([DOI 10.3390/nu18193200](https://doi.org/10.3390/nu18193200)). Together, the picture is: **heat can blunt how much ingested carbohydrate becomes fuel**, while **total carbohydrate use** may stay stable because **glycogen** fills the gap. Performance still depends on pacing, fitness, hydration and how long you stay hot. ## What should a regular runner or cyclist do this week? This study was **cycling in a lab**, not your local park run. Transfer the **principles**, not every gram: 1. **Keep a carbohydrate plan in heat** unless your clinician has told you otherwise. Cutting carbs because a session feels sluggish can **accelerate glycogen depletion** when drink fuel is already under-used. 2. **Practice fueling in warmth** before race day. Gut training still helps tolerance; this paper argues the limit in extreme heat is **beyond the stomach**. 3. **Treat fluids and electrolytes as performance tools.** Use the [water intake calculator](/tools/water-intake-calculator) as a starting reference, then adjust to sweat rate. In long hot efforts, **sodium** matters; see [electrolytes](/dictionary/electrolytes). 4. **Cool the body, not only the mouth.** Shade, ice socks, pouring water on the head and neck, and starting slower in midday heat reduce the core temperature that accompanied lower drink-fuel oxidation in the lab. 5. **Adjust pace to temperature.** [Zone 2](/blog/zone-2-training-explained) and threshold sessions planned for mild weather may need **shorter intervals** or **earlier morning starts** when the forecast is hot. Use your [heart rate zones](/tools/heart-rate-zone-calculator) as feedback, not as a score to chase. 6. **Log what you tried.** A line in your tracker like “long ride 35°C, 60 g/h carb, 1 L fluid per hour, felt flat last 30 min” builds a personal dataset. [Meal and session logging](/features/meal-tracking) beside sleep and [recovery trends](/blog/what-is-hrv-heart-rate-variability-explained) beats guessing next time. **Diet-inclusive fuel examples** for practice sessions (adjust portions to your size and goal): - **Vegetarian:** ripe banana plus dates, or rice balls with jaggery and salt, plus coconut water. - **Vegan:** diluted fruit smoothie with a pinch of salt, or potatoes with salt and lemon. - **Eggetarian / non-vegetarian:** same carb sources; add eggs or lean protein **after** the session, not only before, aligned with your [daily protein target](/blog/how-much-protein-per-day). None of these replace an event's official nutrition policy or medical advice. ## What this study cannot tell you - **Race dynamics:** No pack, no surges, no descents; fixed power at 45% maximum for 90 minutes. - **Running:** Mechanisms may overlap with running data, but muscle recruitment and impact differ. - **Dehydration on purpose:** Participants followed standardised diet and hydration protocols; real-world dehydration could change results. - **Every temperature:** 40°C dry heat is extreme. Milder days may show smaller drops. - **Causation in the wild:** Lower oxidation **associates** with heat in controlled conditions; your bonk might still be pacing, sleep debt or illness. ## When to see a professional Talk to a sports dietitian or clinician who knows your sport if you **repeatedly** cannot tolerate any carbohydrate in heat, lose weight unintentionally during a training block, or have **diabetes**, **kidney disease** or a history of **eating disorders** and need individual fueling targets. **Stop and get urgent help** for collapse, confusion, seizure, very high body temperature with altered behaviour, or vomiting that prevents keeping fluids down during heat exposure. Heat stroke is a medical emergency. ## Related - [Zone 2 training explained](/blog/zone-2-training-explained) - [How much protein do you need a day?](/blog/how-much-protein-per-day) - [Water intake calculator](/tools/water-intake-calculator) - [Heart rate zone calculator](/tools/heart-rate-zone-calculator) - [Meal tracking without counting calories](/features/meal-tracking) This is general training guidance, not medical advice. ## Sources - [Podlogar T et al. Heat acclimation does not alleviate the reduction in exogenous glucose oxidation that occurs when exercising in hot conditions. The Journal of Physiology 2026](https://doi.org/10.1113/jp289304) - [Wallis GA et al. Heat acclimation does not reverse the reduction in exogenous carbohydrate oxidation that occurs when exercising in hot conditions. The Physiological Society abstract](https://www.physoc.org/abstracts/heat-acclimation-does-not-reverse-the-reduction-in-exogenous-carbohydrate-oxidation-that-occurs-when-exercising-in-hot-conditions/) - [University of Birmingham. Research publication page, 2026](https://research.birmingham.ac.uk/en/publications/heat-acclimation-does-not-alleviate-the-reduction-in-exogenous-gl/) - [Mougin L et al. Heat stress impairs exogenous carbohydrate oxidation during prolonged running when maintaining euhydration. J Appl Physiol 2025](https://doi.org/10.1152/japplphysiol.00873.2025) - [Salame A et al. Carbohydrate supplementation for endurance exercise in the heat: a systematic review with practical recommendations. J Int Soc Sports Nutr 2026](https://doi.org/10.1080/15502783.2026.2669307) - [Wang X et al. Effects of acute carbohydrate-electrolyte supplementation on exercise performance in moderate-to-hot ambient conditions: a systematic review and meta-analysis. Nutrients 2026](https://doi.org/10.3390/nu18193200) ## Questions ### Does heat stop your body from using sports drinks and gels? In controlled lab heat, yes, partly. A 2026 study in The Journal of Physiology found trained cyclists oxidized about 27% less of the glucose they drank during a 90-minute ride at 40°C than at 20°C. Total carbohydrate use stayed similar, so stored muscle glycogen carried more of the load. Gut absorption was not the limit; the bottleneck appeared after glucose entered the blood. ### Should you eat fewer carbs during a hot race? Lead author Tim Podlogar and the Birmingham team told reporters not to cut carbs in the heat because the gut still absorbs them. High heat changes how much ingested carbohydrate becomes fuel, not necessarily how much you can absorb. Dropping carbs risks emptying glycogen faster. Focus on fluids, electrolytes, pacing and cooling so core temperature stays as low as your event allows. ### Does heat acclimation fix carbohydrate use in the heat? In this trial, nine days of structured heat acclimation lowered heart rate and rectal temperature during exercise and made hot sessions feel easier, but exogenous carbohydrate oxidation stayed as low as before acclimation. Acclimation still matters for safety and performance, but it did not restore drink-fuel use in this design. ### How much carbohydrate did the study use? Participants drank glucose at 90 g per hour, a rate common in long cycling research and elite triathlon fueling. That is higher than many recreational runners need, but the relative drop in oxidation in heat still applies to anyone sipping less. ### Who was in the study? Seventeen trained endurance cyclists, ten men and seven women, average age about 24 years, with mean peak oxygen uptake around 52 mL/kg/min. Everyone rode 90 minutes at 45% of their maximum power in either 20°C or 40°C air at 20% relative humidity. ### When should you stop exercising in the heat and get help? Stop and seek urgent care for confusion, fainting, vomiting that will not settle, headache with a very high temperature, skin that is hot and dry, or you have stopped sweating while still overheated. These can be signs of heat illness. This article is about fueling science, not emergency treatment. --- Canonical: https://revns.com/blog/heat-exercise-carbohydrate-oxidation-study-2026 Index of everything: https://revns.com/llms.txt revns gives training guidance, not medical advice.