A Warm Environment Reduces Exogenous Glucose Oxidation and Endurance Performance during Cycling with Facing Airflow.

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 V̇O 2peak and familiarization trials, and two experimental trials in 19°C (TEMP) and 32°C (WARM). Experimental trials involved 2-h cycling at ~50% W peak (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- 13 C]-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.