Research Review: Perceived Hydration Status Influences Endurance Cycling Performance: Psychological Expectation, Not Physiological Dehydration, Drives a 6% Decline in Output

By Jackson Mann

BLUF (Bottom Line Up Front)

Endurance cycling performance in the heat was significantly impaired (~6% decrease) when participants believed they were dehydrated, even though their actual hydration status was identical across trials. This study revealed a clear nocebo effect: psychological perceptions of hydration meaningfully influenced performance under mild dehydration (~1.6% body mass loss), without corresponding changes in physiological strain.

Purpose of the Study

The study sought to determine whether perceived hydration status, rather than actual physiological dehydration, could independently impair endurance performance. While prior research shows that ≥3% body mass loss can hinder performance regardless of awareness, this study focused on mild dehydration (<2%) and explored whether believing one is dehydrated can itself reduce output.

Methods

Nine recreationally active males (mean age: 25 years; V̇O₂ peak: 52.5 ± 9.1 mL·kg⁻¹·min⁻¹) participated in a randomized, double-blind, counterbalanced crossover trial. Each completed two 135-minute trials in a hot environment (34°C, 50% RH). Both trials began with a 120-minute intermittent preload: 8 × 10-minute cycling intervals at 50% peak power output (W_peak), each separated by 5 minutes rest. This was followed by a 15-minute self-paced cycling time trial to assess performance.

To manipulate perceived hydration without changing actual hydration, participants consumed identical fluid volumes in both trials but were told they were either euhydrated (“PER-EUH”) or dehydrated by 2% body mass (“PER-DEH”). Fluids were delivered both orally and via nasogastric/orogastric infusion. Pre- and post-trial measurements included body mass, gastrointestinal temperature, heart rate, rating of perceived exertion, thirst sensation, oxygen uptake (V̇O₂), and fat/carb oxidation. Post-study interviews confirmed participant beliefs and assessed blinding efficacy.

Key Findings

Despite nearly identical hydration levels (mean body mass loss: −1.6 ± 0.3% in both trials), participants who believed they were dehydrated (PER-DEH) produced significantly less work—5.6 ± 6.1% lower—than in the PER-EUH condition . Seven out of nine participants experienced a performance decline under perceived dehydration.

Crucially, no significant differences were observed in physiological markers: heart rate, gastrointestinal temperature, rating of perceived exertion, or thermal sensation all remained statistically similar between conditions. However, thirst sensation was significantly higher in the perceived dehydration condition at the end of both the preload phase and the 15-minute time trial, despite identical fluid intake and body mass loss. This suggests psychological framing, not hydration status, influenced thirst perception and possibly performance.

VO₂ and fat oxidation were modestly higher during the preload in PER-DEH, though not statistically significant. Post-trial interviews confirmed participants generally believed 1–2% dehydration impairs performance—highlighting that their expectations likely contributed to the outcome.

Interpretation & Significance

This study demonstrates a powerful nocebo effect: the mere belief of being dehydrated led to reduced cycling performance, independent of any measurable physiological strain. Objective performance decline occurred without corresponding changes in core temperature, HR, or exertion—indicating that psychological expectations, shaped by hydration beliefs, directly influenced output. This means that athletes might underperform if they believe they’re dehydrated—even when physiological indicators suggest they’re within safe hydration limits.

Conclusions

Under conditions of mild dehydration (~1.6% body mass loss), psychological perception of hydration—not physiological dehydration—was the primary driver of reduced endurance performance. The results underscore the importance of athlete mindset, expectation management, and avoiding overly rigid narratives around hydration. Coaches, trainers, and practitioners should account for psychological framing when educating individuals on fluid intake and performance.

Source

1. Funnell, M. P., Moss, J., Brown, D. R., Mears, S. A., & James, L. J. (2024). Perceived dehydration impairs endurance cycling performance in the heat in active males. Physiology & Behavior, 276, 114462.