Athletes performing exercise in warm-hot conditions have high sweat rates and ad libitum fluid consumption is often not sufficient to fully replace sweat losses (“voluntary dehydration”) and results in cumulative body water deficits.

A body water deficit of >2 % of body mass (~3 % of total body water for the average athlete) is defined as hypo hydration.

Hypohydration does not alter aerobic exercise performance in cold-cool conditions, sometimes impairs aerobic exercise performance in temperate conditions, and usually impairs aerobic exercise performance in warm-hot conditions.

When skin temperature exceeds 27 °C (81 °F), hypohydration impairs aerobic performance by an additional ~1 % for every 1 °C (1.8 °F) skin temperature elevation.