Hot Weather Running

I have been asked to participate in a forthcoming Runners Round Table podcast on hot weather running, and feel myself to be well prepared owing to having run for over fourteen years in hot and humid Houston. With the caveat that I am not a physician, and am simply sharing my personal experiences, the following are my related thoughts.

Heat acclamation helps, but it is no panacea:

The heat-acclimated runner quickly produces large volumes of sweat upon the initiation of exertion in hot conditions. This quick and voluminous sweat response results from the body's prior training in similar conditions, and the resulting trained response provides essential cooling through the typically highly efficient heat transfer mechanism called evaporation. However, under high humidity the effectiveness of evaporation is severely diminished, and as such the runner quickly becomes at risk of overheating due to their rising core temperature.

Repeated stimulation and use the sweat glands allows them to become much more effective at minimizing the losses of critical and life-sustaining sodium and potassium electrolytes. Though a heat acclimated runner's sweat is therefore less salty than a non heat-trained runner's, a dangerous low blood salt condition called hyponatramia is a serious risk for all runners. This risk is heightened amongst relatively new marathoners who are often unwisely instructed to "drink even if you're not thirsty and at every aid station" so dilute their body's electrolytes by excessive fluid consumption, compounded by inadequate heat acclimation resulting in greater electrolyte losses through their sweat.

Personally, while running in hot and humid conditions over ten miles in length I supplement my body's electrolyte stores by taking Succeed S!Cap (a buffered sodium and potassium capsule) approximately every 30 minutes. As it is important to take the capsule with fluids I align the supplement's timing based on Gatoraid or water availability. Conversely, for shorter mid-week runs I drink only water and do not take any electrolyte supplements, though do incorporate additional salt into my diet based on taste.

Humidity is the silent killer:

While for most Americans the absolute temperature is the primary determinant of whether the outside conditions are perceived as hot, for experienced hot weather runners they look first at dew-point. Why?

As noted previously the reason is that our bodies reject heat primarily through sweating. An interesting fact is that human beings have more sweat glands then any other mammal, and it is generally believed that this physiological attribute provided our distant ancestors a survival advantage. Again, when the dew point is high, the resulting high humidity renders sweating as a cooling mechanism largely ineffectual.

Nevertheless, there are solutions available that allow us to continue running, with due caution:

1. Slow down!  Pace yourself for the hot conditions, and not for the cooler conditions which most of us remember our ideal pace to be. In this regard slavish obedience to one's target pace set on our Garmin Forerunner or GPS-integrated smart phone App can quickly lead a runner into heat problems, simply by running at a too fast target pace that is not adjusted for the conditions. Personally, though a cool weather low 8 minute per mile runner, under hot and humid conditions I routinely run between a 9:30 and 10 minute mile pace.
2. Negative Split!  As a dutiful follower of Kenyan Way's Coach Sean Wade, I follow his prime negative split mantra, i.e. to run the second half of every race and training run faster than the first half. Doing so, especially in hot conditions, provides its greatest physiological and race-tactical benefits.
3. Listen to your body and only drink to thirst!  Nevertheless be aware of your sweat rate per mile run, and how this varies based on weather conditions. Towards this end I suggest maintaining a log of your hot weather runs, capturing temperature, dew-point, wind speed, the amount of sun, and finally your weight (without shoes) at the beginning and end of each run. By doing so you can easily determine how many ounces that you had sweat per mile run via your measured weight difference (in pounds) multiplied by 16 then divided by the number of miles run. As I always drink only to my thirst I have never returned from a run weighing more than I had at its beginning (an indication of potential hyponatramia.) In the rare event that my weight loss is greater than 2-3% of my pre-run weight (an indication of dehydration) I realize that I simply hadn't had access to sufficient fluids to fully quench my thirst.
4. Stop for for cold fluids roughly every two miles.  Running with the Kenyan Way group this is easy, as the coach supplies us ice cold drinks every two miles. On your own self-made runs I suggest immediately beforehand pre-deploying along your route frozen bottles of Gatorade, or carrying a frozen drink whose contents are thawed while simultaneously cooling your hands.
5. Run at the coolest time of the day, dawn.  Not only will the early morning cool conditions make your run easier, but from a health perspective you will breathe far less potentially lung damaging ozone and particulates. Runners are typically unaware of their lungs' vulnerability to ozone damage, which results from the massive volumes of ambient air respired.  As ozone is typically found in the highest concentrations in the late afternoon, which results from the effect of UV radiation upon ozone precursors in the atmosphere, this is one more reason to run in the early morning.  Besides being the coolest time of day, the vehicular traffic is lowest as well.
6. Dress cool!  I once conducted an experiment on myself on a hot and humid day in which I ran the outbound half of a training run wearing a thin singlet running shirt while monitoring my pace and heart rate. I then removed my shirt and ran bare chested back to the starting point, while running at such a pace as to hold my heart rate at the same approximate level as it had been beating during the outbound leg. Afterward, the benefit of dressing cool was obvious as the return leg's pace was at least 15 seconds per mile faster!
7. Run on a shady path with maximum exposure to wind.  If necessarily exposed to the sun, periodically reapply a broad spectrum sunscreen and wear sunglasses with UV protection. Use extra caution at higher elevations.
8. What to do when you get too hot nevertheless?  Find shade and place ice packs to the neck, fold of the elbows and behind the knees. This will cool the blood passing through the major arteries and veins, thus cooling the target organs. Don't force-feed cold water, instead, allow water to be slightly cool before drinking. Forced ice cold water can create the condition of shock.

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Good input to the article from a good friend, former assistant coach and long-time runner: 

I think point #3 is the most important to know your sweat rate.  I understand that every pint of sweat typically has 500 mgs of sodium therefore after loosing a few pints of sweat the sodium replacement becomes critical.  While an experienced runner may be secreting less sodium, as a rule of thumb I try for one Succeed S!Cap (330 mgs of sodium) per pint of water.  

Another suggestion is for longer runs if you carry a water bottle with an electrolyte drink (gatorade), spiking the fluid with a 1/4 teaspoon of sodium is a good way to get sodium in the body early and easily.  There has very little impact to the flavor of the gatorade by adding a little sodium.  I usually use a 1/4 teaspoon for a 20 oz bottle, so if the fuel belt is used then perhaps an 1/8  would be better.  By getting the sodium in the body early on a long run defers any sodium deficient problems.  

This affects people the most who perspire heavily.  Signs are those whose clothes are soaked before others, shoes get soggy wet and those who cramp.  Dizziness and cramps are signs of dehydration and when attempting to re-hydrate either on the run or after the run, sodium is a must either in pill form or food.  Salty but tasty foods - tomatoe based foods like pizza, red pasta sauce, salsa, store or restaurant made soups.  Potato chips are salty but not really healthy. 

Additional good input from a physician and friend:

The body functions best within a certain temp range 98.6+\- 1.4. As body temp rises the risk of organ failure increases. The body keeps cool primarily by evaporation conduction convection and radiation. When you exercise in hot temps the latter 3 are out and when it's too humid there's no vaporization gradient for sweat to evaporate so that becomes less effective. If you're "conditioned" you can tolerate the effects better but invariably you too will succumb to the heat. The risk is much higher in the first couple of weeks of training in the heat and any acclimatization is gone after only a few weeks of being back in a temperate environment. As you note running at dawn when humidity is less as is temp is safest. There's a reason why marathons are run mostly in 60-70 degree weather.