PHYS-30 · Principle

Why do I sweat so much more than my friend?

Sweat rate and its salt content vary widely between people, even in one sport, and with heat and effort. It is cooling, not fat burned (aerobic training and heat acclimation raise it); weigh before and after a ride for yours.

How the body works, agreed across the reviews below; a study entry shows what one experiment measured.

established physiologyApplies to everyoneEstablished — the spread in sweat rate and sodium, overdrinking as the primary cause of hyponatraemia; well supported — the heat-dependent cost of a deficit (a blinded trial in the heat agrees, REF-226; against it, a…

Practical Application

Why do I sweat so much more than my friend? Sweat rate and its salt content vary widely between people, even in one sport, and with heat and effort. It is cooling, not fat burned (aerobic training and heat acclimation raise it); weigh before and after a ride for yours.
What it means for training: drink to thirst, with a weighed sweat rate as the ceiling, never a target (SYNTH-68, REF-776); fit sodium to the fluid actually replaced (REF-220).
What it does not mean: that more sweat means more fat burned, that a population average is your number, or that salt protects someone who overdrinks (REF-776).

Mechanism

Evaporating sweat removes heat; humid air lowers how much (REF-711).
Sweat rate varies widely between athletes, and body-mass change is the simplest accurate measure (REF-221, REF-916); known factors explained only about a fifth of sweat-sodium variation (REF-222).
A deficit over 2% of body mass usually impairs aerobic performance in warm-hot conditions, sometimes in temperate ones, not in the cold (REF-062), through lost volume (REF-1354).
Exercise-associated hyponatraemia, and every known death from it, comes from drinking beyond losses (REF-776); salt loss alone rarely does, usually after more than 8 h of heavy sweating (REF-1353).

Common misreading

"Sweat out toxins": next to liver, kidneys and gut sweat's part seems minor, and direct evidence of detox is lacking (REF-1353). "Drink ahead of thirst to be safe": overdrinking is the danger (REF-776).

Backs

SYNTH-68 "A deficit beyond ~2% of body mass costs performance mainly in the heat" · SYNTH-68 "sweat sodium is known only by testing"

Sources

Baker 2019 REF-1353 (B+) · Paper ↗
Hew-Butler et al. 2015 consensus REF-776 · Paper ↗
REF-1354 (B+)
REF-1365 (B+)
REF-1374 (B-)