Background: Carbohydrate metabolism during prolonged endurance exercise can be influenced by heat stress and dehydration. While heat exposure and dehydration have been shown to independently affect glycogen use and carbohydrate oxidation, their combined impact remains unclear. No previous review has systematically evaluated the effects of these factors on carbohydrate metabolism during prolonged endurance exercise or undertaken a meta-analysis. Objective: The aim was to systematically review the literature and meta-analyse the effects of heat stress (hot compared to temperate conditions) and dehydration (dehydrated compared to hydrated status) on (1) respiratory exchange ratio, (2) carbohydrate oxidation and (3) glycogen use. Methods: A Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA)-compliant systematic review with meta-analysis was completed (https://osf.io/uq8n5). PubMed/MEDLINE and SportDiscus databases were searched for original articles (published up to November 2024) that assessed changes in (main outcomes) (1) respiratory exchange ratio, (2) carbohydrate oxidation or (3) glycogen use. The population included healthy, active, trained adults (> 18 years). Interventions involved exercise in hot conditions compared to temperate conditions and/or dehydration compared to a hydrated state. The exercise duration was required to be = 15 min. Meta-analysis was performed using a random-effects model to calculate standardised mean differences (SMDs) between experimental conditions (hot compared to temperate conditions and/or dehydrated compared to hydrated statuses). Heterogeneity was assessed using ?2 and I2 statistics, with significance set at P = 0.05. Results: Fifty-one studies (502 participants; 31 females) were included. Carbohydrate oxidation (SMD 0.29, P = 0.006) and glycogen use (SMD 0.78, P = 0.006) were greater in hot conditions compared to temperate conditions. In a dehydrated state, carbohydrate oxidation (SMD 0.31, P = 0.002) and glycogen use (SMD 0.62, P = 0.003) were greater compared to in a hydrated state. Greater carbohydrate oxidation in a dehydrated compared to a hydrated state was observed in hot (SMD 0.37, P = 0.001) but not in temperate conditions (SMD 0.27, P = 0.199). Conclusion: Carbohydrate utilisation increases during prolonged endurance exercise in hot conditions. Dehydration appears to increase carbohydrate use, especially when combined with heat stress; however, these effects are not consistently observed under temperate conditions. Consequently, dehydration does not appear to be the primary driver of elevated carbohydrate utilisation but may play a significant role by affecting thermoregulatory responses.
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