Highlight
– Endurance exercise in hot conditions increases carbohydrate oxidation (SMD 0.29, P = 0.006) and muscle glycogen use (SMD 0.78, P = 0.006) compared to temperate conditions.
– Dehydration increases carbohydrate oxidation (SMD 0.31, P = 0.002) and glycogen use (SMD 0.62, P = 0.003), but this effect is most evident when exercise is performed in the heat (oxidation SMD 0.37, P = 0.001) rather than in temperate environments.
Background
Substrate selection during prolonged endurance exercise is central to performance, recovery, and clinical metabolic responses. Carbohydrates are the most rapidly mobilized and oxidized fuel source for moderate-to-high intensity exercise, and depletion of muscle glycogen is a key limiter of prolonged performance. Environmental stressors such as heat and systemic dehydration are common in sport, military, and occupational settings and can alter physiology in ways that change substrate use. Determining whether heat exposure and dehydration independently or interactively shift the balance toward greater carbohydrate reliance is important for practical recommendations on hydration, carbohydrate intake, pacing and risk management—especially for people with metabolic disease or on medications that alter thermoregulation or glycemia.
Study design
The article summarized here is a PRISMA-compliant systematic review and random-effects meta-analysis (Mougin et al., Sports Med. 2025). PubMed/MEDLINE and SportDiscus were searched through November 2024 for primary studies involving healthy, active, trained adults (>18 years) performing exercise ≥15 minutes. Interventions compared exercise in hot versus temperate conditions and/or dehydrated versus hydrated states. Main outcomes were (1) respiratory exchange ratio (RER), (2) carbohydrate oxidation (indirect calorimetry or tracer methods), and (3) muscle glycogen use (biopsy). Standardized mean differences (SMDs) were pooled; heterogeneity was assessed with χ2 and I2, and significance was P ≤ 0.05. The analysis included 51 studies with 502 participants (31 females), reflecting the historical male bias in exercise physiology literature.
Key findings
Primary pooled effects
Across studies, exercising in the heat (vs temperate conditions) increased carbohydrate oxidation (SMD 0.29; P = 0.006) and accelerated muscle glycogen utilization (SMD 0.78; P = 0.006). The magnitude suggests a small effect on whole-body carbohydrate oxidation and a moderate-to-large effect on muscle glycogen depletion.
Dehydration (vs hydrated) independently increased carbohydrate oxidation (SMD 0.31; P = 0.002) and glycogen use (SMD 0.62; P = 0.003). These are consistent moderate effects and indicate that volume loss has meaningful metabolic consequences during prolonged exercise.
Interaction of heat and dehydration
When stratified by environmental temperature, the increase in carbohydrate oxidation with dehydration was evident in hot conditions (SMD 0.37; P = 0.001) but not in temperate environments (SMD 0.27; P = 0.199). This pattern suggests a synergistic or at least additive interaction: heat amplifies the metabolic consequences of dehydration.
RER and methodological notes
Changes in RER paralleled oxidation data in several studies, but RER can be confounded in non-steady-state conditions or by altered ventilation in heat exposure. Muscle glycogen outcomes—derived from biopsies—strengthened the physiological plausibility of increased carbohydrate reliance under heat and when dehydrated.
Sample characteristics and heterogeneity
The pooled dataset included 502 participants but only 31 females, limiting generalizability across sexes. Exercise modalities, intensities, exposure temperatures, dehydration protocols (e.g., passive vs exercise-induced), and nutritional status varied across studies; consequently, heterogeneity (I2) differed by outcome and subgroup. Authors used random-effects models to account for between-study variability.
Mechanistic interpretation
Several physiological mechanisms plausibly explain increased carbohydrate reliance in heat and with dehydration:
- Increased core and muscle temperature raise metabolic rate and preferentially accelerate carbohydrate flux through glycolysis and glycogenolysis.
- Heat and dehydration increase cardiovascular strain (elevated heart rate, reduced stroke volume), which can reduce muscle perfusion for fat mobilization and favor carbohydrate oxidation that requires less oxygen per ATP produced.
- Elevated circulating catecholamines with heat/dehydration stimulate hepatic glucose output and muscle glycogen breakdown, increasing carbohydrate availability and utilization.
- Impaired adipose tissue blood flow in heat and volume-depleted states reduces free fatty acid delivery to working muscle, limiting fat oxidation during exercise.
Collectively these mechanisms support why heat is a dominant driver and why dehydration contributes mainly by heightening thermoregulatory and cardiovascular strain.
Clinical and practical implications
For athletes and physically active individuals:
- Expect higher carbohydrate needs when training or competing in hot environments. Pre-event glycogen loading and intra-event carbohydrate provision may need to be increased or timed differently to maintain performance.
- Dehydration compounds carbohydrate demands principally in the heat; therefore, optimal hydration strategies are important not only for cardiovascular stability but also for moderating metabolic cost and delaying glycogen depletion.
- Pacing strategies should consider increased metabolic costs in heat; athletes may need to adopt conservative pacing or increase carbohydrate intake if attempting to maintain higher intensities.
For patients and clinicians:
- People with diabetes or glucose-handling disorders exercising in hot environments may experience altered glycemic responses and higher carbohydrate turnover. Medication timing (e.g., insulin) and carbohydrate supplementation plans should be reviewed in the context of heat exposure and hydration status.
- Occupational guidelines for workers in hot environments should account for increased carbohydrate turnover, particularly if dehydration is likely; scheduled hydration and carbohydrate access may support sustained work capacity and safety.
Expert commentary and limitations
Strengths of the meta-analysis include comprehensive search, use of both indirect calorimetry and biopsy outcomes, and stratified analyses for environmental conditions. Limitations worth noting:
- Female underrepresentation (31 of 502 participants) restricts sex-specific conclusions. Hormonal differences, size, and sweat rates may modulate responses.
- Study heterogeneity in exercise intensity, acclimatization status, humidity, clothing, dehydration magnitude, and pre-exercise nutrition limits pooled interpretability for specific field situations.
- Measurement challenges: RER is sensitive to hyperventilation and non-steady-state exercise; muscle biopsies are direct but infrequent and localized; tracer studies are more precise but less common.
- Dehydration protocols varied (e.g., percent body mass loss, method of inducing dehydration), making dose–response insights limited.
Future work should prioritize balanced sex representation, standardized dehydration and heat protocols (including humidity reporting), and combined methodologies (tracers plus biopsies and indirect calorimetry) to refine quantitative estimates of increased carbohydrate needs in different contexts.
Recommendations for practice
- Anticipate modest increases in whole-body carbohydrate oxidation and larger increases in muscle glycogen use when exercising in heat; plan for additional carbohydrate intake during prolonged (>60–90 min) exercise in hot conditions.
- Prioritize hydration strategies to limit the additive metabolic impact of dehydration in heat; targeted rehydration can reduce cardiovascular strain and may attenuate the rise in carbohydrate reliance.
- For clinicians advising patients with diabetes, consider individualized adjustments to carbohydrate and medication strategies for exercise in heat, and counsel on hydration monitoring.
- Event and occupational planners should incorporate both hydration and carbohydrate access into protocols for prolonged work or competition in hot environments.
Conclusion
The systematic review and meta-analysis by Mougin et al. (2025) provide robust evidence that heat exposure increases carbohydrate oxidation and accelerates muscle glycogen depletion during prolonged endurance exercise. Dehydration appears to increase carbohydrate use as well but acts mainly by exacerbating thermoregulatory and cardiovascular strain in the heat. These data support practical adjustments to fueling and hydration strategies in hot environments and identify important research gaps—most notably the need for more female inclusion and standardized protocols to refine operational guidance.
Funding and trial registration
Funding and trial registration details should be consulted in the original article: Mougin L, Macrae HZ, Taylor L, James LJ, Mears SA. The Effect of Heat Stress and Dehydration on Carbohydrate Use During Endurance Exercise: A Systematic Review and Meta-Analysis. Sports Med. 2025 Nov;55(11):2825-2847. doi: 10.1007/s40279-025-02294-3. PMID: 40835807; PMCID: PMC12559103.
References
1. Mougin L, Macrae HZ, Taylor L, James LJ, Mears SA. The Effect of Heat Stress and Dehydration on Carbohydrate Use During Endurance Exercise: A Systematic Review and Meta-Analysis. Sports Med. 2025 Nov;55(11):2825-2847. doi: 10.1007/s40279-025-02294-3. PMID: 40835807; PMCID: PMC12559103.
2. González-Alonso J, Teller C, Andersen SL, Jensen FB, Hyldig T, Nielsen B. Influence of body temperature on the development of fatigue during prolonged exercise in the heat. J Appl Physiol (1985). 1999;86(3):1032-1039.
3. Montain SJ, Coyle EF. Influence of graded dehydration on hyperthermia and cardiovascular drift during exercise. J Appl Physiol (1985). 1992 Nov;73(4):1340-1350.
AI thumbnail prompt (for designers)
A high-resolution photograph of an endurance athlete (runner or cyclist) exercising under bright sunlight, visible sweat, slightly flushed face; overlaid semi-transparent line graphs showing an upward trend labeled “Carbohydrate Oxidation” and a glycogen icon; warm color tones (orange/red) to convey heat; clean, modern typography space for headline; realistic, dynamic, energetic composition.