Exercise in warm environments increases thermal/cardiovascular strain and decreases gastrointestinal (GI) integrity and endurance performance. However, laboratory-based studies have provided little to no facing airflow, potentially exacerbating these effects, particularly for cycling, where convective cooling may be a major contributor to thermal balance. Purpose This study investigated the effect of cycling in a warm versus temperate environment with sufficient facing airflow on exogenous glucose use, performance, and GI responses. Methods Ten trained male cyclists/triathletes (36 ± 6 yr; 55 ± 6 mL·kg-1·min-1) completed V?O2peak and familiarization trials, and two experimental trials in 19°C (TEMP) and 32°C (WARM). Experimental trials involved 2-h cycling at ~50% Wpeak (preload) and an ~15-min time trial (TT) with fan-provided airflow covering the cyclist (preload ~29 km·h-1, TT ~35 km·h-1). A glucose drink containing [U-13C]-glucose was consumed every 20 min during the preload (72 g·h-1). Results Average 40-120 min (TEMP 0.56 ± 0.13 g·min-1; WARM 0.48 ± 0.12 g·min-1; 15%; P = 0.015) and peak (TEMP 0.79 ± 0.18 g·min-1; WARM 0.68 ± 0.14 g·min-1; 14%; P = 0.008) exogenous glucose oxidation rates were reduced in WARM. TT performance was 15% slower in WARM (TEMP 819 ± 47 s; WARM 961 ± 130 s; P = 0.002). GI temperature (P = 0.007), heart rate (P < 0.001), and rating of perceived exertion (P = 0.046) were greater during WARM. GI comfort (P = 0.659) and intestinal fatty acid binding protein (P = 0.094) were not different between trials. Conclusions These data demonstrate that the ability to use glucose provided in drinks was impaired during prolonged cycling in WARM. WARM ambient conditions impaired laboratory-based cycling performance, even with facing airflow approximating outdoor conditions, likely via impairments of thermoregulatory, cardiovascular, and metabolic function.
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