Heat, Hydration and the Human Brain, Heart and Skeletal Muscles
Key Findings
Unreplaced sweat losses during prolonged vigorous exercise impair endurance capacity alongside: hyperthermia, hyperventilation, cardiovascular strain with reduced brain, skeletal-muscle and skin blood perfusion, greater reliance on muscle glycogen, altered neural activity and, in some conditions, compromised muscle metabolism and aerobic capacity
The strain of progressive dehydration up to ~4% body-mass loss can be attenuated or prevented by (1) drinking during exercise, (2) exercising in the cold, and/or (3) working at intensities that use a small fraction of functional capacity
Functional demand decides the damage: cardiac output, limb perfusion and muscle metabolism are stable or increase during small-muscle-mass exercise or rest, but are impaired during whole-body moderate-to-intense exercise
Brain: progressive dehydration accelerates the drop in cerebral perfusion and oxygen supply during submaximal and maximal endurance exercise
Muscle suffers more than brain in aerobic terms because its functional oxygen-extraction reserve is much smaller
Practical Application
Dehydration's impact scales with functional demand: at rest or in small-muscle exercise cardiac output and limb perfusion hold up, but in whole-body moderate-to-intense exercise progressive dehydration (up to ~4% body mass) reduces brain, muscle and skin perfusion together with hyperthermia and hyperventilation — and the review's three counters are drinking during exercise, cooler conditions, or lower relative intensity. For a cyclist: the harder and hotter the ride, the more a fluid deficit costs; an easy ride tolerates the same deficit far better.
In heat, metabolic heat production and dehydration-reduced skin blood flow compound each other — the same deficit is worse hot than cool (the review's point 2).
Not from this paper; the base's own practice: for hard efforts in heat, drink during the session rather than only before it; the specific intake sits in REF-221 (sweat-rate norms) and your own sweat test.
Limitations
From the design as the abstract reports it: a narrative (integrative physiology) review, not a systematic review or meta-analysis.
It describes physiological strain with progressive dehydration up to ~4% of body mass loss; it is not a performance trial and the abstract gives no drinking prescription.
The effects are conditional: the abstract states that the impact of dehydration depends on the functional demand of the exercise and on environmental stress — stable or increased function in small muscle mass exercise or at rest, impaired function in whole-body moderate to intense exercise.
How this entry was checked and changed
Why this level
regraded 2026-09-27 from A-: narrative review from the leading group (González-Alonso), no systematic method
Content correction 2026-09-27
"Cerebral blood flow declines at ~60% VO2max; oxygen extraction increases until a 2-3% dehydration threshold" -> not in the abstract (which gives ~4% body-mass loss as the range reviewed); removed.
"At elevation ... effects magnified" and "non-negotiable for hard efforts >60 min" -> not in the abstract; removed / replaced by labelled house practice.
Level A- -> B+ (narrative review). Sport line "cycling" -> "general". Added the abstract's three mitigation routes and the functional-demand framing.
2026-09-28: added Limitations from the abstract (Europe PMC, DOI 10.1007/s40279-018-1033-y, PMID 30671905).