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.
revns editorial · Published November 12, 2026 · 7 min read
A 2026 Journal of Physiology trial in 17 cyclists found exogenous carb oxidation fell ~27% at 40°C vs 20°C; heat acclimation helped comfort, not drink fuel use. Keep carbs, add cooling and fluids.
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 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). 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:
- COOL: 90 minutes at 20°C and 20% relative humidity.
- HOT: 90 minutes at 40°C and 20% relative humidity.
- 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).
- 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).
- 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).
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:
- 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.
- Practice fueling in warmth before race day. Gut training still helps tolerance; this paper argues the limit in extreme heat is beyond the stomach.
- Treat fluids and electrolytes as performance tools. Use the water intake calculator as a starting reference, then adjust to sweat rate. In long hot efforts, sodium matters; see electrolytes.
- 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.
- Adjust pace to temperature. Zone 2 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 as feedback, not as a score to chase.
- 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 beside sleep and recovery trends 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.
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
- How much protein do you need a day?
- Water intake calculator
- Heart rate zone calculator
- Meal tracking without counting calories
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
- 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
- University of Birmingham. Research publication page, 2026
- Mougin L et al. Heat stress impairs exogenous carbohydrate oxidation during prolonged running when maintaining euhydration. J Appl Physiol 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
- 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
Terms in this post
revns publishes training and nutrition guidance, not medical advice. See a clinician for symptoms, injury or a medical condition. Accurate as of November 12, 2026.
