The other day it was 100F with a heat index up near 110F. Pretty much the lower half of the country was experiencing the heat wave.
This got me thinking about heat acclimation to improve fitness. I started reading all sorts of articles about acclimation and how it is supposed to improve the body’s ability to control core body temp, increase sweating, and increase blood flow near the skin, and increase blood volume, so the heart can then pump more blood to exercising muscles, organs, etc. Think of it as the natural alternative to blood doping.
Yeah, but how HOT does it need to be? I started looking through some of the studies. Most of the studies that I read use 100F in a heat chamber for the control group. The humidity was set at only 30%. In my neck of the woods we call 30% humidity "Not humid". So, it would appear that the purpose of these studies was to create an absolute 100F, where "100F feels like 100F" with the heat index.
How hot is too hot? Most places say that a heat index of 105F is the tipping point where you’re going from heat acclimation to the Darwin award winner on the 5 o’clock news. This seems like a reasonable place to draw the "go" or "no go" line with exercising outside, especially for longer than an hour. Repeated for dramatic affect: If the heat index is 104, don’t go outdoors …(yes that was directed at you;).
You’ll also want to know how much fluid you’ll need to offset your fluid loss. Since it was fiendishly hot on Tuesday, I figured it was a good day to figure this out.
To calculate Sweat rate:
Sweat Rate Calculator
Pre Exercise…..178.2 lb
Post Exercise….174.4 lb
Fluid Intake…….42oz (Gatorade)
Exercise Time…120 minutes
Sweat Rate: 50oz/hr
In other words, even though I drank 42oz of Gatorade over a 2 hour period, I still dropped 3.8 lbs from fluid due to sweating! Dangerous levels of dehydration occur once you lose 2.5% of your body weight(4.45 lbs for me). I would have needed to drink 50 oz/hr to maintain and/or use other supplements like Endurolytes and EFS.
Oh yeah, and this was all done indoors at the gym at a constant 75F. If I would have done the same workout (1hr Run,1hr Bike) outside, I’m sure I would have been at that 2.5% dehydration zone, taking it easy for the next 3 days, and missing other important workouts.
Thanks for Reading!