Some like it hot…but most of us don’t. Running at any intensity level in the heat can dehydrate you, and severe dehydration can lead to reduced performance and, of course, other health related issues. In this tip I will talk about understanding heat and how you can measure your personal Sweat Rate to ensure you are getting the fluids your body needs.
Heat vs Humidity
Before we dig in too deep, it’s worth noting that the number on your outdoor thermometer isn’t the only one worth considering. As an endurance athlete, you need to consider the relative humidity as well.
Humidity is the amount of water vapor in the air and can be described in different ways, including “relative humidity.” Relative humidity is the amount of water vapor in the air compared with the amount of vapor needed to make the air saturated at the air’s current temperature.
In other words, if it’s sunny and 90 degrees out with little to no humidity, it’s hot but solving that is easy (shade, appropriate clothes, etc.). Once you up th the humidity, however, things get far more complicated. The additional moisture in the air means you are more likely to sweat more, breathing can feel difficult, and the planet just seems stuffy in general. Since high humidity rate can happen even at moderate temperatures, like 70 to 80 degrees Farenheight, your next run could be hotter than you think.
How To Measure Your Sweat Rate
There’s some work to be done here, but once it is part of your routine it should be no inconvenience at all. How much effort you put into gathering this data will depend on how serious you about performance on race day. That said, I recommend that most people save this for their critical runs, say over the last 6 weeks leading into race, as their fitness and weather conditions won’t be changing _that_ much.
Weigh yourself at the last minute before going out – and after that last-minute visit to the toilet. Do this without any clothes. Note this number.
Then weigh yourself as soon as you get back again and note the two measurements in your training diary.
Add anything you had to drink during the run to the amount of weight lost (#2). It’s best to use a bottle with calibration marks on the side, so you know the ounces of fluid without having to weigh it.
Note: There are some errors in these calculations, as you will also be using up some stored fuels in the form of carbohydrate and fat, but these can be ignored with no great loss of accuracy.
As an example Joe Runner weighs 165 lbs pre run, and returns at 160 lbs after a 90-minute run, having consumed 16oz of fluid. Joe’s total loss is 165lbs minus 160lbs, or 5lbs. To that number we add the weight of the water consumed, 16oz (1 lb), so on that run the total loss was 6lbs.
How To Address
There is no way to directly replace all the fluids lost during competition. And in many cases trying to do so would be more detrimental than helpful; think extra weight at the end of a race or just trying to choke down a ton of fluids when your whole system is on the ropes.
A general rule of thumb for endurance athletes is to try an keep fluid losses to no more than 1-3% of pre-race weight.
In the example above, Joe Runner lost 6lbs over 90 minutes, or 4lbs per hour. The total loss is approximately 3.6% of his total body weight, so he clearly needs to drink more. The loss represents about 4lbs per hour, while consuming 10.5 oz per hour.
In order to keep his sweat rate to the manageable 1-3% range, Joe can only afford to lose about 5 lbs in any given run. In the case of this 90-minute effort, he will need to consume another 16 oz (1 lb) of fluids over the 90-minutes, or another 10.5oz per hour…effectively doubling his initial fluid intake.
Make It Personal
Calculate your “manageable” sweat rate by multiplying your weight in lbs by “0.03.” The outcome, in lbs, is the high end of the amount of weight you can lose on any run. Compare your run calculations vs the number and then adjust accordingly.
More on hydration coming soon…what are your sweat rate tricks?
Hi, I'm Patrick McCrann. 
