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 vs temperate environment with sufficient facing airflow on exogenous glucose use, performance, and GI responses.

Methods: Ten trained male cyclists/triathletes (36 ± 6 y; 55 ± 6 mL/kg/min) completed V̇O2peak and familiarisation 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, TT: ~35 km/h). A glucose drink containing [U-13C]-glucose was consumed every 20 min during the preload (72 g/h).

Results: Average 40-120 min (TEMP 0.56 ± 0.13 g/min; WARM 0.48 ± 0.12 g/min; 15%; P = 0.015) and peak (TEMP 0.79 ± 0.18 g/min; WARM 0.68 ± 0.14 g/min; 14%; P = 0.008) exogenous glucose oxidation 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 RPE (P = 0.046) were greater during WARM. GI comfort (P = 0.659) and Intestinal Fatty Acid Binding Protein (IFABP) (P = 0.094) were not different between trials.

Conclusions: These data demonstrate that 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.