I have noticed that Dexcom really bonks on me after about an 1 to 1 1/2 hours of running. This is because of the amount of sweat loss I have, which reduces the effectiveness of the interstitial fluid for measuring BG.

I occasionally check my sweat rate to know how much I should be drinking to replace lost fluids.

**Here is how to do it:**

(this is a very close approximation, but for simplicity, it ignores the small difference between Avoirdupois weight in ounces and fluid ounces…):

- Do a bit of a warmup, up to the point where you will start to sweat.
- Empty your bladder completely. Don’t pee again until you’re finished with the test!
- Weigh yourself nude or almost nude right before you begin your exercise. Don’t forget to put clothes back on when you head out for your run or exercise, otherwise you might get arrested.
- Do your exercise and record how long you are exercising and how much you drink in ounces during the exercise. A longer period will give you more accurate results.
- After the run, strip down, towel off any sweat, and weigh yourself nude again. Remove any wet clothes!
- Subtract your post-exercise weight from your pre-exercise weight and convert to ounces (multiply pounds x 16).
- Add the number of ounces you lost to the number of ounces of liquid you consumed. For example, if you lost a 1/2 pound (8 ounces) and drank 6 ounces of fluid, your total fluid loss is 8 + 6 = 14.
- To calculate rate lost per minute, or hour, or mile if you are running, you can divide your ounces lost by the time you exercised or the distance you ran.

Here is a quick example:

A person’s pre-exercise weight = 150 pounds

The person drank 10 ounces while exercising and exercised for 1 hour

Their post-exercise weight = 144 pounds

Weight lost in pounds 150 - 144 = 6 pounds

6 pounds = 96 ounces (multiply pounds x 16)

Ounces lost plus ounces drank = 96 + 10 = 106

In this example, the person loses 106 ounces of sweat in an hour. That could mess with your CGM!

Don’t want to do the math? You can go here and let them do it for you:

http://www.triharder.com/thm_swrate.aspx

(make sure you select the correct units, lb/oz versus kg/ml)

**Couple of points:**

- Use an accurate scale
- Temperature plays a big role in sweat rate, so calculations should be reexamined as the temperature changes from season to season.
- It takes a few weeks to get acclimated to changes in temperature.
- Sweat rates generally increase after 10-14 days of heat exposure, so sweat rate should be recalculated after sufficient heat acclimatization.
- Higher sweat rates are generally found in men versus women.
- Higher sweat rates are generally found in those that are highly fit.

So there you have it. If you notice a bonk in your CGM after a certain amount of exercise, you can start to figure it out in terms of how much sweat it takes to cause that…

Have **run**!

(no, I meant to say that )