Preface: I am a marine scientist by trade, however, I have run 4 marathons and I am in the midst of training for a half marathon/full marathon during a South Carolina summer. I might not be an expert in studying the physiology of heat related illnesses, but I have enough experience to make it count.
The first time I experienced heat related illness I was 14 years old, a freshman in high school. I was at a summer soccer clinic and if you ask my mother what happened she'll tell you I was so nervous about playing with the varsity team that I made myself sick.
In reality, I had been playing soccer for the past 4 hours in the sun and my body was done with it. I was dizzy and felt nauseous and ended up having to sit out the rest of the practice. I went to the doctor and he diagnosed me as suffering from heat stroke and told me to drink more Gatorade and get more rest.
As I've progressed from soccer to long distance running, I've realized a few things. First, my body still does not enjoy doing strenuous activity in the heat; second, I actually didn't have full blown heat stroke that day when I was 14; and third, there are ways to better handle running in the heat.
There are three main forms of heat related illness. One usually progresses to the other but, if you're like me, you can skip stages. Heat related illness is caused by a combination of overheating and dehydration. Seems simple and yet it is one of the main adversaries runners face.
Running on a typical day goes something like this: your muscles and body begin to heat up internally the more you move and the further you run; as your body heats up more, you sweat more. In attempts to cool itself down, your body sends blood from your muscles to the skin to come into contact with the relatively cooler air and sweaty skin (that has hopefully been cooled down in the sweating process).
Issue number one, your muscles need blood to gain oxygen and therefore your blood cannot go get cooled down at the skin. Alternate issue: your blood goes to get cooled down and therefore takes away oxygen from your muscles - what's a blood cell to do?
Issue number two, this scenario with high air temperatures added; the blood that stays with the muscles gets extra toasty while the blood that gets away to the skin doesn't actually get cooled and sweat doesn't evaporate. Essentially, overheating results when the air temperature exceeds your body's release of excessive heat. This article has a great description of this tug-of-war process within your muscles.
Issue number three, your body is trying so hard to cool its skin down by sweating that you're becoming dehydrated; loosing water and electrolytes means your muscles have to work even harder to stay at the same activity level they would be if the temperature was cooler.
More factors than just air temperature go into how heated your body gets during this process. Air temperature is obviously a main factor, however, humidity plays a large role as well. Also, factors such as clothing, body weight, distance, pace, previous hydration, heat acclimation and personal predisposition to heat related illness all affect how your body responds to heat.
For example, I can drink the same amount of water as another runner, eat the same things, run the same distance at the same pace, in the same clothing but I will still be at a higher risk of heat stroke due to the fact that my body has a high affinity towards heat stroke (woohoo!).
All of these factors combine and lead to overheating and dehydration; overheating and dehydration combine and lead to a set of three main heat related issues. A short article found here does a good job of describing the various stages of heat related illnesses from a man who has run the race deemed hottest ever, The Badwater Ultramarathon, but here is a summary:
Heat cramps: Mainly due to deficiencies in your body's electrolyte stores. These feel the same as any other running cramp - sharp pain, tightening of the muscles, rarely work themselves out on their own. Typically in runners, heat cramps take place in the legs (predominantly the calves) however, if I do get heat cramps (which is rarely) they're in my shoulders.
Heat exhaustion: This is normally the first stage I experience when I begin to deteriorate from heat. It results from the loss of too much fluid and electrolytes without replacement. Heat exhaustion is characterized by a rise in body temperature, dizziness, nausea, vomiting, and a headache. Possible extra bonuses are weakness, lack of coordination, heavy sweating and goose bumps. Basically, an all around fun time. I would also add shakes, however, some people might classify those under weakness.
Heatstroke: The mother of all heat related illnesses. Your body temperature rises above 105 degrees F and it becomes a life-threatening situation. Most often, heatstroke results from untreated heat exhaustion, although it's very possible for heatstroke to come about with no signs of heat exhaustion. Heatstroke is characterized by extreme fatigue and weakness, confusion and odd behavior, disorientation and finally unconsciousness. Your body's regulatory system completely shuts down at this point, sweating ceases, and your skin becomes hot and dry. Unfortunately, it doesn't end there. Convulsions and seizures can occur as your brain begins to shut down; coma and death are possible in the worst situations. GET OUT OF THE HEAT IMMEDIATELY! Seek medical attention, get in the shade, drink water, etc anything to get cooled down! You do NOT want to get to this point.
Heat related illness is no small matter. But there are ways to prevent or lessen the effects of extreme heat. Hydration is the most important aspect to heat related illness prevention; consider drinking a sports drink (a short list of the most popular sports drinks can be found here) to replace fluids and electrolytes before, during and after a run in the heat. Personal tip: I freeze a Gatorade bottle before running – it warms up along the way giving you an icy cold drink for your run (or at least for the first 15 minutes of your run).
Consider pouring water on your head as well (I do not recommend doing this with Gatorade). Some sources state that the debate is still ongoing whether it is better to put water in you or on you. My personal rule of thumb is that if the water tastes warm, pour it on your head because it will still feel cool there.
Another prevention method is to know the signs of heat related illness and take heed! I am the worst at this one; I do not like succumbing to defeat. However, I’ve realized that finishing a training run is not as important as being healthy enough to make it to the next run. Also, wear lightweight light-colored clothing – material that wicks away moisture will be your new best friend.
Last helpful hint: know the heat index and decide whether it is safe to run based on that value. The heat index is a value of the apparent temperature felt by your body based on the air temperature and humidity. Caveats to the heat index – it is based on a person that is 5’ 7” in height, 147 lbs in weight, Caucasian, with a 98.7 degrees F body temperature, wearing long pants and a short-sleeved tshirt, in the shade, walking at a speed of 3.1 mph, with a breeze of 6 mph, and not dripping with sweat (read: nothing like what your body feels while running).
The National Weather Service has a great heat index chart - scroll down a little bit to find it. Pay attention to the color-coding; I tend to continue running in the ‘extreme caution’ range and have been known to run in the ‘danger’ range, although I do cut my runs short and don’t keep up as fast as pace as I would normally and tend to regret the decision to run after I’ve started in these ranges.
I’ve come a long way since that day at soccer camp. I rarely (although it does happen) get to the point of heat exhaustion and I am much better prepared to handle that situation and work towards preventing it. If you’ve learned anything from this post, know that running in the heat is no laughing matter. As my running partner always says, "I'd rather die of hypothermia."
About the Author: Caitlyn Zimmerman is a current masters student at Duke University’s Nicholas School of the Environment. She is working towards a Masters of Environmental Management degree with a concentration in coastal environmental management. She is beginning work on her masters project, focusing on ocean science communication using social media. You can follow her masters project process on her blog. She gained her expertise in running over seven years running a combination of cross country and just-for-fun and currently runs half and full marathon races. She tweets as @CaitlynZim.
The views expressed are those of the author and are not necessarily those of Scientific American.