Beat the Heat!!

Combined effect of heat stress, dehydration and exercise on performance

The effects of heat stress and dehydration on the body: What does it all mean?
Combined heat stress and dehydration has been shown to impair aerobic performance.  There are several changes that occur in the body due to dehydration. There is an increase in both heart rate and core temperature. During prolonged exercise in the heat, there is a reduction in blood flow to the active muscles. The rise in muscle temperature has been shown to reduce isometric endurance as well as lead to shorter contraction and relaxation times causing fatigue to occur earlier. Dehydration can also cause a decreased sweat rate and decreased heat dissipation.

Ways in which heat stress can affect your body: radiation, convention, conduction, evaporation
Environmental heat stress is determined by the ambient temperature, relative humidity, wind velocity and solar radiation. During exercise, working muscles produce heat at a high rate and body temperature rises. If the skin is hotter than its surroundings, heat is lost from the skin by physical transfer by evaporation, convention or conduction. If the environment is hotter than the skin, heat is gained by convection and conduction. High humidity can be dangerous because it can compromise the evaporative loss of sweat. You must be careful exercising in the heat because once you feel the sensation of thirst, you have already lost 2% of body weight through sweating.

Recommendations: So what can I do?
For starters, thirst is not a good indicator of the degree of hydration for reasons mentioned above. Ideally, enough fluids should be consumed to make body weight remain fairly constant before and after exercise. The American and Canadian Dietetic Associations recommend that roughly 500ml of fluid be consumed 2 hours before exertion and another 500ml be consumed about 15 minutes before prolonged exercise. In hot and humid environments, every 15 to 20 minutes of 120 to 180 ml of fluid is recommended. When rehydration is the main priority, it is best to choose a fluid composed of carbohydrate, glucose and sodium so most commercial sports drinks will do the trick.

Reference: Coudreuse J, Ftaiti F, Grelot L, Nicol C: Combined effect of heat stress, dehydration and exercise on neuromuscular function in humans. Eur J Physiol 2001; 84: 87-94

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