A warm environment reduces exogenous glucose oxidation and endurance performance during cycling with facing airflow

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 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 V̇ O_2peak and familiarisation 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, 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.