Training in the Heat: Fatigue, Dehydration, and How to Adapt

Summertime and the livin' is easy......except when the heat index is 103 degrees. It feels more like a blast furnace while riding on the sizzling blacktop and the thick, syrupy air makes for soaring heart rates and fumbling feet while trundling through the woods.

The local and national weathermen and newscasters are warning people to stay indoors and avoid the heat. Crank the AC, pour an ice cold glass of coke, pull the shades, stream a movie, and sit under the ceiling fan or else risk dehydration, heat stroke, heart failure, and death. 

It seems the general public is afraid of the uncomfortable. Yes, running on a hot humid day will cause sweat to drip down your brow and into your eyes. Yes, running in the heat will cause your heart to beat rapidly and you pace to slow. Yes, running in the heat will make your fingers fat, your mouth dry, and your mind focused on the cold coke waiting in the fridge. You did remember to put the Coke in the fridge? Sadly, I forgot.

Actually, my house has no air conditioning and after a few days of hot temps begins to feel something like this:

If you are smart enough to avoid the health warnings on a day like today and decide to risk life and limb by running you will inevitably be confronted with the sports drink dilemma. Most "research" done by sports drink manufacturer's is questionable at best. But that's a topic for another day. 

Three factors to consider when exercising in the heat.

1. Dehydration – Jonathan Dugas and Ross Tucker over at www.sportsscientists.com have written extensively on the topic of dehydration, fluid consumption, and exercise. They conclude, “should you choose to ignore your thirst, you will not collapse from “heat illness”, and nor will you die from heatstroke. However, you will be miserable and you will run slower than you would like.” Noakes, in the Lore of Running, recommends to “drink as your thirst dictates…..with fluids that contain the appropriate amount of sodium” and concludes that higher rates of forced ingestion of fluids is not beneficial. He does not mince words when referring to the ACSM position stand on fluid replacement “the scientists who developed these guidelines would be best advised to warn runners and cyclists that drinking too much can kill you, and that there is seldom, if ever, the need to drink more than 400-800ml per hour during exercise”.

2. Heatstroke – According to Dugas and Tucker, metabolic rate, not dehydration, is the best indicator of core temperature. In other words, the faster you run the greater the chance you have of overheating. This fact makes hot, shorter races more dangerous than hot, long races, provided one’s internal temperature controls work properly. Noakes says that if your body begins to overheat its internal regulatory system will usually cause you to slow down before reaching heatstroke levels. 

This brings us to the most important factor regarding exercising in the heat:

Adaptation – Noakes says that heat acclimatization can occur within 7-14 days after first exposure and can be retained for up to a week after returning to cooler climates.  Dr. Larry Kenny in “Exercise Physiology 5^th edition” says that in football players the single most important predictor for heat illness is lack of acclimation to hot conditions. Major changes occur during the adaptation stage including: decreases in exercising heart rate, body temperature, salt sweat content, metabolic rate, blood lactate accumulation, and carbohydrate and muscle glycogen utilization.

Should you exercise in the heat? After reading that, I can only answer with a resounding YES!

During the course of today's hot, humid 3.0 hour run/ride I drank 1,600ml of water and had a few swigs of my homemade energy gel mix as needed.

Comparison of today's technical trail run on the white rocks/Appalachian Trail versus the same course last week:

                    Temp: 92+     Temp: low 80's

Distance	10.00 mi	10.13 mi
Time	2:01:58	1:55:50
Avg Speed	5.0 mph	5.3 mph
Avg Pace	12:06 min/mi	11:23 min/mi
Heart Rate	153	142

My pace was :43 seconds per mile slower while my heart rate was on average 11bpm higher. Were this a race, I would have suffered greatly. But it's this type of training that will provide the stimulus to acclimate my body for the sure to be hot and humid Catoctin 50k at the end of July.

Recovery meal of beef, chorizo, salsa, avocado, scrambled eggs, cheddar cheese, on crispy corn tortillas

Too bad I didn't have a cool mountain stream to dip my toes into while eating my tasty dinner

Instead the largest 1-day street fair on the east coast, Jubilee Day, is going on outside my front door. The live bands are blasting, the sweaty people are eating funnel cakes, drinking blue raspberry slushies, and participating in their own version of acclimating to the heat and humidity.