Carbohydrate supplementation for endurance exercise in the heat: a systematic review with practical recommendations

Endurance exercise performance is impaired by heat stress. Although the mechanisms are not yet fully understood, heat-induced increases in glycogenolysis may play a role. Therefore, carbohydrate supplementation recommendations may differ for endurance exercise in hot environments. This systematic review aims to investigate the efficacy of carbohydrate supplementation for endurance performance in hot environments.

Electronic databases, including PubMed, the Cochrane Library, MEDLINE, and SPORTDiscus were searched through to April 14, 2026 for studies evaluating endurance performance outcomes under heat stress in response to carbohydrate supplementation. Studies were included if the participants were healthy, aged 18-65 years, and at least recreationally active. A hot environment was defined as an ambient temperature >23 °C. Studies were required to involve an endurance-based and continuous exercise protocol lasting >30 min and a direct performance measure. This cutoff was selected based on evidence indicating glycogen stress at ~40 min under heat stress. In total, 3151 records were identified, and nine randomized, crossover studies met the inclusion criteria. Risk of bias was assessed using the Cochrane Collaboration's tool for assessing risk of bias. Primary outcomes (endurance performance and carbohydrate oxidation rates) and secondary outcomes (including gastrointestinal symptoms, thermoregulation, and hydration markers) were extracted. Mean carbohydrate intake rates were calculated, and a narrative synthesis was performed.

Mean carbohydrate ingestion rates ranged between 14 and 140 g·h^-1, and the exercise trial duration ranged between ~50 and 152 min. Carbohydrate supplementation resulted in equivocal benefits to endurance performance: five studies found significant (p < 0.05) positive effects for carbohydrates (increases of 13.4%-19.3% in time to exhaustion and 3.3%-12.7% in time trial), and four studies found no significant effects. Although nonsignificant, three studies reported average improvements of 6.7%-11.6%, which may be meaningful in elite athletes. Most studies reported no differences in the respiratory exchange ratio between trials, indicating a preferential reliance on glycogen, which is in line with the literature. Carbohydrate ingestion during exercise in the heat did not influence markers of hydration, thermoregulation, or fatigue compared with placebo. Given that prior research shows gastrointestinal symptoms is common in endurance events in hot environments, the absence of symptom investigations in seven out of nine studies reviewed has important implications for interpreting findings. Overall, the results suggest that carbohydrate intake may not reduce glycogen breakdown during endurance performance in heat. Future research should help to better understand the underlying reasons, including any moderating effects of gastrointestinal distress.

Carbohydrate intake during endurance exercise in the heat does not consistently improve exercise performance. Athletes may focus on maintaining hydration during exercise and practice gut-training, based on the wider literature. Future studies should fill the gaps in research, namely, the measurement of gastrointestinal symptoms, the mechanisms of exogenous carbohydrate use in heat, and different modes of exercise, including running, to enable the development of robust, evidence-based recommendations.