DOI: 10.1249/mss.0000000000003632 ISSN: 1530-0315

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

Kirsty M. Reynolds, Mark P. Funnell, Adam J. Collins, Stephen A. Mears, Jamie N. Pugh, Lewis J. James

ABSTRACT

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 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.

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