Hydration during exercise. Effects on thermal and cardiovascular adjustments.

Area of Science:

• Environmental Physiology
• Exercise Science
• Sports Nutrition

Background:

• Understanding rehydration strategies is crucial for athletes and individuals exercising in hot environments.
• The impact of fluid osmolarity on physiological responses during prolonged exercise and heat exposure requires further investigation.

Purpose of the Study:

• To investigate the effects of different fluid osmolarities on rehydration and physiological responses during exercise in heat.
• To compare the efficacy of spring water, hypotonic, isotonic, and hypertonic solutions in restoring fluid balance.

Main Methods:

• Five young, unacclimatised subjects exercised on a bicycle ergometer for 4 hours at 34°C.
• Experimental sessions included no rehydration (NO FLUID) and rehydration with spring water (WATER), hypotonic (HYPO), isotonic (ISO), and hypertonic (HYPER) solutions.
• Physiological parameters measured included body weight loss (delta Mb), heart rate, sweat rates, plasma volume, and plasma osmolarity.

Main Results:

• No significant differences were observed in body weight loss or hourly sweat rates across rehydration conditions.
• Heart rate was higher in the NO FLUID condition.
• Isotonic rehydration (ISO) rapidly led to plasma isoosmotic hypervolemia, while spring water (WATER) resulted in slightly hypoosmotic normovolemia. Dehydration (NO FLUID) induced hyperosmotic hypovolemia.
• Sweating sensitivity was lowest with dehydration and highest with spring water.

Conclusions:

• Ingesting an isotonic electrolyte-sucrose solution effectively restores and expands plasma volume during exercise in heat, even with intake less than evaporative heat loss.
• Fluid osmolarity influences sweating sensitivity but has minimal impact on core temperature regulation within the observed plasma volume changes (-6% to +4%).