Measuring Electrolyte and Sweat Loss In Athletes

Measuring Electrolyte and Sweat Loss In Athletes

Feb 17, 2023Tom Parker

I am not personally the biggest perspirer I know but I sweat a lot when I play tennis.  Have you noticed how Rafa Nadal is just drenched but Novak Djokovic or Roger Federer appear ready to party in the middle of a tennis match?  And unless you are an elite athlete, you are much more likely to be like Rafa than Roger.  But you very well may be an elite athlete and sweat like it’s your job.

Exercise Related Sweat Loss

If you are not a tennis fan, pick your favorite sport and athletes and you will notice there is a wide range of fluid and mineral loss.  In fact, there has been significant research on fluid and electrolyte loss and the results demonstrate just how difficult it is to determine the replacement needs of an individual athlete.  As you will see, I am going to provide a gauge below that may help you make a reasonable calculation of your electrolyte and sweat loss based on the many factors that affect how much you lose.

Let’s dispense with normal sweating as normal physiology of the human body.  It is simply a mechanism by which heat is dissipated.  The heat to be dissipated is generated by the work of contracting muscles just as the radiator in your car helps dissipate the heat generated by the engine.  There are some disorders that are marked by excess or deficient amounts of sweat but we are only concerned with healthy sweating in this blog.

How can I say confidently that you do not know what replacement fluids and electrolyte would match your loss?  You cannot possibly know on any given day because there are so many factors that influence loss.  The available studies will not have data on somebody who is your sweat doppelganger!  Among the factors that affect sweat volume and content are age, weight, temperature and sport.  Percent body fat is a big factor as well; fatty tissue is a great insulator, so it makes dissipation of heat much harder.  The lean athlete with low percent body fat will dissipate heat more efficiently but be cold when the not-lean athlete is sweating at rest.

What we do know from the scientific research on sweat is that there is a wide range of sodium and potassium loss in sweat and, likewise, there is a wide range on the volume of sweat for which replacement drinks attempt to replenish. 

Measuring Sodium and Potassium Loss

To make an estimate for your personal calculations you will want to pay attention to your body weight, before and after exercise.  Ideally, you would urinate, weigh yourself nude before an exercise period and, as soon after as possible, urinate, remove your exercise clothes, and then weigh yourself.  Alternatively, weigh before and after exercise with your clothes on but weigh your clothes before and after so that you can determine how much fluid has been absorbed by your clothing.  Keep track of your fluid intake in milliliters or ounces during the exercise period. Recognizing that you have had no other losses during your exercise, the difference between the pre- and post-exercise weights reflects fluid loss.  If you had an episode of vomiting or diarrhea during the exercise period, of course, the weight change would not all reflect sweat fluid loss, but still, fluid loss none-the-less.

Fluid loss = (pre-exercise weight – post-exercise weight) + (weight of sweaty clothes – weight of dry clothes) 

As an example, before exercise, an athlete weighs 150 pounds and clothes weigh 2 pounds.  After exercise, the athlete weighs 145 and clothes weigh 5 pounds (reflecting sweat absorbed by clothes, socks and shoes.)  This athlete lost 8 pounds of fluid.  Don’t forget to urinate before the post-exercise weight since the fluid in the bladder is also lost during the exercise period.

Don’t be surprised to find that you have lost several pounds (of fluid, not fat) while you exercised.  In many of the endurance and explosive sports, fluid loss of 10 pounds is not uncommon among athletes who do not rehydrate ambitiously, and significant fluid deficits of several liters can occur even in the athlete who attempts to rehydrate.  The concern for the athlete with the fluid loss is related to a drop off in performance that occurs with dehydration and electrolyte imbalance.

Recognizing that 2.2 pounds represents 1 liter of fluid we can move to calculate how many liters of fluid you lost.  In the example above the athlete lost 8 pounds which is the equivalent of 3.6 liters.  (8 lbs/2.2 pounds per liter)  So now that you have some data on your personal weight change (same as fluid replacement needs) we can move on to making a rough estimate of your electrolyte replacement needs.

Because we are not going to measure the sodium and potassium content of your sweat we will want to use the information that scientists have gathered on athletes who are like us.  And this is where the information becomes less and less precise in making our calculations.  We can look up and see that healthy 20 year olds can lose 1438 mg sodium and 216 mg potassium per liter of sweat with treadmill exercise.   We don’t have information on other age groups  however so we will have to extrapolate.  But sodium loss is different depending on the sport.  Available data shows these losses but they are reported as mg of sodium lost per hour, not per liter:

  • Football: 1510 mg/ hour
  • Endurance: 1280 mg/ hour
  • Basketball: 950 mg/ hour
  • Soccer: 940 mg/ hour
  • Baseball: 830 mg/ hour

The wide range of sodium loss depending on the sport likely reflects the age of the athletes, temperature, conditioning and intensity of the exercise. 

Calculating Sodium and Potassium Replacement

So how do you calculate your sodium and potassium replacement needs?  At best we can only make an approximation by using the weight change reflecting fluid loss and selecting some empiric number for sodium and potassium loss.  The good news is that the human body is remarkably adept at adjusting to these sorts of metabolic challenges but correcting the deficits can take time.   And if the correction takes too long, athletic performance can be impaired.  The correct course of action, then, is to do everything possible to keep hydrated and maintain electrolyte balance while exercising.

Based on the range of sodium loss described above, I am taking the liberty of suggesting that sodium loss in sweat is in the range of 1000-1500 mg/liter and potassium is 200 mg/liter.  These numbers may be wildly off for the elite athlete but will apply to the average athlete.

Let’s apply these numbers to the example above and calculate the free water, sodium and potassium deficits.  We will assume initially that the athlete was not rehydrating and then do the calculation with the athlete hydrating.

    • Not hydrating:  Weight loss of 8 pounds reflects a fluid loss of 3.6 liters (0.95 gallons), sodium and potassium deficit of 3.6 grams and 0.720 grams respectively.

  • Hydrating (with HydroHelix) at 2 liters per hour:  Weight loss would only be 3.6 pounds and a fluid loss of 1.6 liters (0.43 gallons), sodium and potassium deficit of 2.0 grams and  0.320 grams.  But remember, HydroHelix contains 800 mg and 200 mg of potassium respectively so there will still be a 400 mg deficit of sodium that was not matched by HydroHelix replacement and this will be added to the 2.0 deficit to reflect a more accurate deficit.  The deficit then would be 2.4 grams of sodium.

The athlete who wants to maximize her performance will want to pay attention to the fundamentals of fluid and electrolyte replacement as described above.  

Use This Spreadsheet to Calculate Sodium and Potassium Replacement Needs

There is no doubt that performance can be degraded by mismanagement.

The description of fluid and electrolytes described here is a simplified summary.  It is not an essay that would serve as preparation to take the American Board of Nephrology certifying exam and so if you find that the assumptions I have made do not apply to you, make the necessary adjustments and recalculate. 

- Tom Parker MD, CEO, Body Helix

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