Hormonal
Fluid ingestion during high-intensity cycling in the heat prevents performance impairment caused by low-level dehydration (<2% body mass loss), primarily by reducing ratings of perceived exertion (RPE) and plasma vasopressin (ADH) rather than by altering core temperature or plasma volume.
If you are cycling hard (near race pace) in the heat, do not wait until you are thirsty or feel dehydrated. Even a small loss of body weight (under 2%) will increase how hard the exercise feels (RPE) and cut your performance short. Drink a sodium-containing solution (20 mmol/l) in small, frequent amounts (e.g., 120ml every 10 minutes) during your warm-up or moderate-intensity phase to ensure you have fluid available for the high-intensity finish. This prevents the mental and hormonal strain of dehydration, allowing you to sustain power longer.
These results suggest that high-intensity (90% VO2peak) cycling time to exhaustion is impaired even at low (1.8%) levels of dehydration, and that this is associated with increases in RPE.
Why this rating
Randomized, counter-balanced, controlled trial with trained subjects, though small sample size (n=6) limits generalizability.
Source
Impaired High-Intensity Cycling Performance Time at Low Levels of Dehydration
Ryan Walsh et al. · International Journal of Sports Medicine · 1994
DOI 10.1055/s-2007-1021076
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