Managing the Texas Heat

By Shannon Mitchel, M.D., P.T. – June 4, 2012

As summer approaches, it is impossible to not recall the plethora of triple-digit sweltering days we suffered last year. As much as Austinites love to be outdoors many were forced to stay inside to avoid the oppressive heat and humidity. For those who make their living out in the elements, this is not a choice. Others, who find exercising indoors to be the work of the devil, also brave the tough conditions to get their fix.

As a physician who treats people injured on the job, I see many each year that are afflicted by one of the heat-related illnesses. Serving as medical director for ultramarathons, it is far from uncommon for me to have to treat runners during the summer races that are seemingly unaware of how to manage the heat. Often, they are in serious trouble and end up needing more than just a cool drink and a comfortable chair. I wish I could say that my experience seeing this has made me very wise when it comes to dealing with heat. Unfortunately, I am not immune to the blistering heat and poor decisions.

Even now, a decade later, I vividly remember the IV I needed at the end of a race I ran in high heat. With my background in medicine, there was no way this should have happened to me. I was so embarrassed. However, the reality is that problems with heat can happen to anyone regardless of their skill level or knowledge of what heat can do. Furthermore, if you wait until you are thirsty to drink while active in 100-degree heat, the damage is already done. Chances are you are playing a game of catch-up you are likely to lose. But with a little planning, anyone can make training for that early fall marathon manageable—even in Austin, Texas.
 

Let’s start with physiology

Fortunately, the body can adapt to almost anything with proper training. If you want to cycle for 100 miles, then you get on the bike. If you want to run a marathon, start by lacing up your shoes. If you want to do either of those in Austin, Texas, in August, start acclimating yourself to the heat early. Repeated exposure to heat over time will result in acclimation to that heat. Once this occurs, you will be able to be active in higher temperatures with a lower body core temperature. Your body will actually produce less heat even as you remain active for longer periods of time. In addition, your skin will “learn” to sweat earlier and in greater quantities to help cool yourself. Even better, all this extra effort will be done at a lower heart rate. When all is said and done, you will not only increase your performance level but be more comfortable while you do it.
 

Great! So how do I do that?

According to Occupational Safety and Health Administration guidelines (OSHA – the entity that helps ensure workers have a safe work environment), this acclimation process will take about two weeks. Exposure to the heat for two hours a day for 14 days will result in physiological acclimation. On the flipside, the benefits you gain from acclimating will start to decline after one week with no exposure. After three weeks of no exposure, those gains are completely gone. Exercising in the heat is the best means of acclimation but any repeated exposure to the heat is better than nothing. Remembering to do this in moderation and that it will take time to acclimate is the most important thing.
 

This is how it works

During days one through five of heat exposure, your body starts to retain salt. Because water follows salt in your body, the volume of plasma (the “watery” part of your blood) increases. There is a re-direction of your blood flow towards your skin. Meanwhile, all kinds of stress hormones (cortisol) will be released, making your body retain even more salt. As your body is still learning how this acclimation works, it will just throw that extra salt into your sweat and urine. This “salt dumping” is what makes days one through five the hardest part. Your blood flow is being directed towards your skin to help you learn how to sweat better rather than the other places it would normally go during activity (like muscles). As such, this makes your perception of exertion higher.

However, after this initial phase, there is a change. In days five through nine, your blood flow is directed from the skin back toward the blood stream again, just in time to see your blood volume start to decrease. As the stress hormones level out (your body is starting to get used to the heat rather than responding as though you have been stranded in the desert) the amount of salt in your sweat and urine returns to pre-acclimation levels. Eventually, your body is able to work in the heat with greater ease. You are sweating more efficiently and in larger volumes but you are able to work with a lower heart rate and keep your core temperature lower.
 

What should I drink and what about salt?

We are told to eat a diet low in salt, yet the American College of Sports Medicine recommends that you take a salt replacement one hour prior to exercise in the heat if you are planning on two hours or more of activity. So what is the right thing to do? Honestly, if you have normally functioning kidneys and are active outdoors, summer is not the time to skimp on salt. (Those with kidney disease and high blood pressure are exceptions to this rule and should speak with their doctors individually.)

Dehydration is very common among those exercising in the heat but the question remains about how to figure out when you are dehydrated before you end up sitting in a chair with an IV in your arm. The simplest way to figure out your own personal dehydration rate is to weigh yourself both before and after the activity you have planned. A normal person can lose up to two percent of body weight without any noticeable problems. However, if you are losing three to four percent, you need to take a serious look at your fluid and electrolyte replacement strategies. If that number goes up to four to six percent of your body weight, something is awry and it would be wise to ramp back your training until you figure out what is happening.

When seven percent of your body weight or more is lost, chances are good a feeling of dizziness will come over you, followed by a headache. At this point, some are unable to drink and keep down the fluid they need and an IV is required. If sweating ceases, it is imperative for a person to get out of the heat as soon as possible and seek medical help immediately. As the body’s core temperature rises, the cells in the body become increasingly resistant to functioning properly. Soon, bodily functions will stop and the cells themselves will start to break apart. The proteins in the body will start to denature, becoming unfit for use in the body. Yes, this is as bad as it sounds and, without swift reversal, can lead to shock and eventually death.
 

What about electrolytes?

You do not sweat just water (or even just salt and water, for that matter). Electrolytes are also lost in the process of sweating. While most recommendations you will find say salt it is sufficient to replace salt alone, there are many products on the market to assist you in replacing the actual minerals lost when you perspire. Most energy gels and sport drinks have some electrolytes; so, as you are figuring out what exactly you need, remember to factor that in as well. I have provided a comparison of some of the major electrolyte replacements on the market. While I do not prefer or endorse one over the other, I will tell you that generally a person needs to replace what they actually lose in the amount they sweat.

You will notice that two of the products do not have magnesium or calcium. Calcium is necessary for muscle contraction and, once a person gets low on calcium, muscle cramps are sure to follow. As for magnesium, it is the trickiest electrolyte to deal with. If you replace too much, you can get diarrhea. But if the amount of magnesium gets too low, what doesn’t go down will come back up. Of course, everyone has a different constitution when it comes to how the gut handles any sort of problem. The only true way of finding out your own tolerance is through simple trial and error, even though this can be the most unpleasant of routes.

With regard to the 11 ounces of sweat, the number is mostly arbitrary. Given the amount of variables present, it is not possible to give more than a ballpark number. Some triathletes can sweat as much as two to three liters an hour. Going back to your weight pre- and post-activity is the best means for figuring out your own weight loss. And as I mentioned earlier, if you have normally functioning kidneys, when in doubt, opt for a little extra salt.
 

What color is your urine?

Being a doctor and an ultramarathoner, I sometimes forget that most people do not speak so freely about pee in mixed company. But when it comes to dealing with heat, monitoring one’s urine is the best way to make quick assessments of how the body is handling dehydration.

During activity, urine will rarely be completely clear. Sweat loss will see to that. So, during the rest of the day when you are not in the heat, I recommend drinking to the point that your urine is close to clear. This way you are at least starting your workout with a full tank, so to speak. The more yellow it gets, the closer you are to dehydration. There is nothing wrong with a urine color that is not completely clear but if it gets dark yellow or brown, there is indeed a problem.
 

Conclusion

It is fact that everyone responds differently to exercising in the heat. Some handle it better than others. The only way to see where on this spectrum you fall is by easing yourself into your workouts, monitoring your own system, and making the appropriate adjustments. It also never hurts to hit the pool or Barton Springs and relax once in a while.



Dr. Shannon Mitchel is a physician, physical therapist, and ultramarathon runner who practices occupational medicine at NOVA Medical Center in South Austin. She also serves as Medical Director for various endurance events and maintains a small sports medicine private practice in South Austin.
 

 
 

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