REF-062 · Sawka et al., 2015 · Sports Medicine, Vol. · Paper

Hypohydration (>2% Body Mass) Impairs Aerobic Performance

Usually in Warm-Hot, Sometimes in Temperate, Not in Cold Conditions

B+moderate evidenceMeasured on a mixed groupWritten before October 2026, being re-checked

Overview

Sawka, Cheuvront and Kenefick review how body-water loss affects aerobic performance and why. Hypohydration is defined as body-water loss greater than 2% of body mass and can arise from sweating or from diuresis in cold and at altitude.

Its effect on performance depends on the environment: hypohydration does NOT impair submaximal aerobic performance in cold-cool environments, SOMETIMES impairs it in temperate environments, and USUALLY impairs it in warm-hot environments. Impairment begins when skin temperature exceeds 27 °C, and each additional 1 °C of skin temperature adds a further 1.5% impairment.

The decrement is attributed to several integrated mechanisms — cardiovascular strain as the "lynchpin", elevated tissue temperatures and metabolic changes — acting through the CNS to reduce motor drive.

Hypohydration adds to the impairment in warm-hot high-altitude environments; iso-osmotic hypovolemia (cold, altitude) costs more plasma volume per unit of water deficit than the hypertonic hypovolemia of sweat loss.

Key Findings

Definition: hypohydration = body-water loss >2% of body mass
Environment decides the effect: no impairment of submaximal aerobic performance in cold-cool, sometimes in temperate, usually in warm-hot conditions
Threshold and dose: impairment begins at skin temperature >27 °C; each further 1 °C of skin temperature adds ~1.5% impairment
Mechanism: cardiovascular strain (lynchpin) + tissue temperature + metabolic changes, integrated by the CNS into reduced motor drive
Additive with warm-hot high-altitude exposure

Practical Application

Per this review, losing more than 2% of body mass to sweat usually costs aerobic performance in warm-hot conditions, sometimes in temperate ones and not in the cold — the penalty scales with skin temperature (impairment starts above 27 °C and grows ~1.5% per extra degree), so the hotter the race, the tighter the drinking plan needs to be; in a cool race a modest deficit is not a performance problem.
Not from this paper; the base's own practice: measure sweat rate on a long ride (weigh before and after, add fluid drunk) and plan intake so the loss stays around or under 2% in the heat; typical intakes land around 400-800 mL/h but the individual's number governs.
Inference, not tested here: skin temperature is the operative variable, so anything that lowers it — wind on the bike, wetting the jersey, cooler start times — buys tolerance to a given fluid deficit.

Limitations

Narrative review; the abstract reports no pooled effect size for a given percentage deficit, so any "x% power loss per 2%" figure is not from this source.
The 27 °C / 1.5% per °C relationship is the review's synthesis, not a single trial's result.
Sport-general laboratory literature; race-specific data (wind cooling, self-paced effort) are limited.
How this entry was checked and changed
Why this level

regraded 2026-09-27 from A: narrative review from the leading group (Sawka, Cheuvront, Kenefick), no systematic method

Content correction 2026-09-27
Removed invented magnitudes: "3-5% power loss per 2% dehydration", "4% dehydration: 8-12% power loss", "+5-10% RPE" — none is in the abstract; replaced with the abstract's skin-temperature relationship (>27 °C onset, ~1.5% per additional °C).
Title and lead corrected: the review says hypohydration does NOT impair aerobic performance in cold-cool conditions and only sometimes in temperate ones, so "even 2% reduces endurance performance" overstated it.
Sweat-rate and intake advice labelled as the base's own practice; Sport "cycling" -> general; Level A -> B+ (narrative review from the leading group).