Abstract
Body water losses of >2 % of body mass are defined as hypohydration and can occur from sweat loss and/or diuresis from both cold and altitude exposure. Hypohydration elicits intracellular and extracellular water loss proportionate to water and solute deficits. Iso-osmotic hypovolemia (from cold and high-altitude exposure) results in greater plasma loss for a given water deficit than hypertonic hypovolemia from sweat loss. Hypohydration does not impair submaximal intensity aerobic performance in cold–cool environments, sometimes impairs aerobic performance in temperate environments, and usually impairs aerobic performance in warm–hot environments. Hypohydration begins to impair aerobic performance when skin temperatures exceed 27 °C, and with each additional 1 °C elevation in skin temperature there is a further 1.5 % impairment. Hypohydration has an additive effect on impairing aerobic performance in warm–hot high-altitude environments. A commonality of absolute hypovolemia (from plasma volume loss) combined with relative hypovolemia (from tissue vasodilation) is present when aerobic performance is impaired. The decrement in aerobic exercise performance due to hypohydration is likely due to multiple physiological mechanisms, including cardiovascular strain acting as the ‘lynchpin’, elevated tissue temperatures, and metabolic changes which are all integrated through the CNS to reduce motor drive to skeletal muscles.
Highlights
Key PointsAthletes 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
The decrement in aerobic exercise performance due to hypohydration is likely due to multiple physiological mechanisms, including cardiovascular strain acting as the ‘lynchpin’, elevated tissue temperatures, and metabolic changes which are all integrated through the CNS to reduce motor drive to skeletal muscles
No single review has examined the importance of body water deficits on environmental tolerance and aerobic exercise performance during exposure to heat, cold, and high-altitude environments
Summary
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
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