Figure out your sweat loss for CGM

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…):

  1. Do a bit of a warmup, up to the point where you will start to sweat.
  2. Empty your bladder completely. Don’t pee again until you’re finished with the test!
  3. 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.
  4. 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.
  5. After the run, strip down, towel off any sweat, and weigh yourself nude again. Remove any wet clothes!
  6. Subtract your post-exercise weight from your pre-exercise weight and convert to ounces (multiply pounds x 16).
  7. 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.
  8. 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:
(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 :wink:)


@eric - this is awesome, I’ve often thought about this as well.

Thanks for posting.

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My CGM is garbage after an hour of running - often (but not consistently!) lower than a finger stick by a full 100 points and definitely not usable for making treatment decisions.

Actually, it appears to be at its worst immediately following an intense run in which I have consumed some carbs. If I did go low during the run, the CGM will continue to report low well after I’ve bounced back.

I wish I understood how to interpret this device’s underlying behavior better so that it was more useful to me.


Do you figure it is due to dehydration?

Dexcom support asks me about hydration every time we have a call about poor readings.

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Same for me. It really isn’t very useful for running.

Make yourself a BG meter holder for running. I can make you a strip holder.


This is interesting. Mine is very accurate even after a 5 mile run (that’s about my max). And I sweat pretty good on runs.

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Ah ha! I believe this is a weekend project. I think though, I said that last weekend too.


It’s possible but not likely… I drink a pretty large amount of water throughout the day, some before the run, and then use drinking fountains at 2.5 and 4 miles. The weather here is temperate (neither hot nor dry).

I guess I could measure my water loss using the exact procedure above (if I ran with a cup which I could use to measure the drinking fountains). I’m happy to try it.

Honesty I suspect Dexcom knows full well that their technology doesn’t remain accurate during exercise and would rather blame the user than admit it. :slight_smile:


Is it? That is interesting!

I suspect mine becomes inaccurate because I go both low and high (this month my running has features new, record-breakingly poor blood sugar control). I think it gets disoriented by rapid changes in blood sugar (lows due to exercise and highs from Gu). Would you say your sugar is mostly stable or that changes in sugar are gradual?

Interestingly I used to run with no problem, but this month has been super bad. I’m working on it of course (with significant help from the folks on this forum and my girl).