What to Know About Exercising in the Heat

2010-04-10 DirtFest 1157As summer is approaching, the temperature is heating up.  Will you let the heat keep you from getting out and exercising?  I grew up in Redding, CA, where the average high temperature in July is 99°, and as a kid I wasn’t about to let the temperature stop me from playing outside.  As an adult, somehow I’ve managed to keep the same perspective, but there are a few things to take into consideration when exercising in the heat.  So for those of you who aren’t afraid to sweat a little, remember these three essentials to prevent heat sickness: hydration, acclimation, and perspiration.

Hydration – The body can loose a significant amount of water in the course of exercise and far more so during hot weather.  The risk of overheating is great anytime the heat index is above 90°.  Since perspiration is one of the body’s primary ways to stay cool, hydration is key.

According to the US Public Health Service short bouts of exercise (under 1 hour) require anywhere from 8-16 oz of water replacement.  For longer durations of exercise in the heat, the International Marathon Medical Director’s Association (IMMDA) recommends anywhere from 18oz to 36oz of water per hour, depending on the amount of fluid lost.

When it comes to rapid fluid loss, both dehydration and over hydration can have a negative impact on health.  Dehydration can lead to heat sickness, and over hydration can lead to depletion of electrolytes (hyponatremia).  The best strategy for staying hydrated in the heat is as follows:

  • If you plan to exercise in the heat, be sure to drink 16-32oz of water about an hour before exercising.
  • As you are exercising, pay close attention to your level of thirst and hydrate as needed.  This should be anywhere between 18 -36oz of water per hour (it all depends on the weather, level of exertion, fitness level, etc).  Listening to the signals your body sends you is the most effective way to ensure adequate fluid intake.  If you’ll be out in the heat for extended periods of time, bring adequate water!
  • Replenish with electrolytes (sodium and potassium).  Electrolytes are essential for proper muscle function, as well as blood volume and blood pressure.  One of the best electrolyte drinks is coconut water, which contains a high proportion of potassium.  There are also a number of other electrolyte sports drinks that contain both potassium and sodium.  Electrolyte replishment is especially important for long, endurance exercise (1 hour or more).

Acclimation – The human body is actually extremely good at adjusting to the heat, so don’t let the heat stop you from getting outside, just work your way into it.  Studies conclude that it takes the body about 7-10 days to adjust to physical exertion in the heat, but most of the adaptations occur between 4-6 days.  The physiological responses to repeated elevations in core body temperature include a lower heart rate, increased plasma volume, and increased sweating rate.

While the body will rapidly return to normal physiological responses after a period of not exercising in the heat, there may be some short term benefits of exercising in the heat before a competition.  One study found that trained cyclists who trained in the heat for ten days outperformed their competitors during a race held during cool conditions.  Heat acclimation improved the cyclists’ power at lactate threshold by 5%, as well as their plasma volume and cardiac output.

Perspiration – The final key to ensuring proper cool down during hot conditions is to maximize perspiration and evaporation.  Sweat evaporation is what removes heat from the body.  There are several things that can hinder proper perspiration and evaporation:  First of all, don’t wear antiperspirant — go for a natural deodorant instead.  Not only will antiperspirants prevent proper cooling, they also contain aluminium which may be linked to Alzheimer’s disease.  Secondly, choose a natural sunscreen that’s SPF 15 or lower.  Anything above SPF 15 can block the sweat glands and interfere with perspiration.  Instead of using a higher strength sunscreen, just be sure to reapply.  Finally, wear breathable clothes and change out of wet ones.  If your clothes aren’t breathable  they’ll hold body heat in and promote rapid overheating!  Believe it or not, wet clothes are not very breathable.  Although they might feel cool at first, wet clothes can actually act like a wetsuit, insulating the body.  If you’re working in the heat for a sustained period of time, be sure to bring an extra shirt or wear specially designed athletic clothes with high breathability.

Even if you take all the necessary precautions, exercising for extended periods of time or during the middle of the day can still lead to heat sickness, so be aware of the signs.  There are different stages of heat sickness, including: heat syncope (nausea, dizzyness, and increased heart rate),  heat cramps (usually caused by loss of electrolytes), and heat exhaustion (shock-like symptoms, including dilated pupils, weak pulse, and clammy skin) , and heat stroke (the most serious of all, characterized by extreme overheating and possibly lack of sweating, racing heart beat, and unconsciousness).

Don’t let the heat stop you from getting outside, but be smart about it!  If you are especially sensitive to the heat (youth and the elderly are more susceptible to heat sickness), or when just starting out, try to get outside in the morning or during the evening, when it’s a bit cooler!  Have fun and enjoy the sun, but know your limits and remember: hydration, acclimation, and perspiration!

References: Heat Acclimation Improves Athletic Performance, Exercising in Hot Weather, IMMAD’s Revised Fluid Recommendations, Time Course of Heat Acclimation and its Decay, Heat Acclimation

Originally posted 2013-06-14 01:06:40.

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