My Sweat is not your Sweat is not her Sweat is not his Sweat

My Sweat is not your Sweat is not her Sweat is not his Sweat

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This is why understanding your personal hydration requirements is so important!

In this blurb I talk about a very valid reason why you should understand YOUR PERSONAL exercise hydration requirements rather than accept a set plan or schedule, or simply copy those of your mates. There are obvious pitfalls in taking those approaches with potential to end up dehydrated, or worse, Hyponatremic (over hydrated). Previously we have written about the importance of correct hydration during exercise, particularly with respect to Hyponatremia (https://adventurethon.com.au/why-do-we-need-to-hydrate-correctly/), a problem that medically and recovery-wise is far more serious for your health and well-being than dehydration.

So lets get to the subject in question, liquid ambition. That juice that pours from your body during exercise, SWEAT. Sweat is clearly a loss of liquid, but not liquid alone as most of us realise when it gets in your eyes or mouth; it tastes salty because it contains salt (sodium and chloride). Therefore, replacement of lost liquid and sodium is imperative to maintain a correct electrolyte balance which is crucial for cell health and for maintaining sound sporting performance. Ok, so at what point during exercise should you start to replenish lost fluid and electrolytes? Well, the answer to that comes back to understanding your personal hydration requirements because there are many external and internal factors that influence your fluid and electrolyte loss rates.

There are substantial differences in the rate of fluid and electrolyte loss, and progression toward electrolyte imbalance, between different individuals and different exercise routines. To illustrate, different environmental, exercise speed/intensity and time/distance conditions all affect rates of sweating. When you consider all the variables that impact on your personal sweat rate (you can test your own sweat rate, we outline the procedure at the end of this article) you start to understand why personalising your hydration regime is so important and why a hydration plan that works for your mate doesn’t necessarily work so well for you. Everybody has different sweat rates, and within that, everyone has different sodium concentrations in sweat! Consequently, the best plan is a personalised plan.

Obviously it assists in developing a personal hydration plan (PHP) if you have some idea of your sweat rate but that is not crucial. What is crucial are your past, and future, experiences. Weighing yourself before and after exercise is a great way to determine if you are replacing lost fluid at the correct rate; equal weight before and after shows a really good PHP in terms of liquid replacement (not necessarily electrolyte though), higher weight after exercise indicates you are on the dangerous pathway to Hyponatremia with your hydration plan, and lower weight after exercise indicates an under-replenishment of fluids. Having to pee during exercise can also indicate that hydration levels are relatively high. Muscle cramps can also indicate if you are not replacing sufficient sodium. There are other causes of cramps but these are not so common so a salt tablet or two will soon tell you if the cause was lack of sodium if the cramp/s ease somewhat after about 10 minutes. Your body should let you know if you are under-hydrating; you will start to feel thirsty, simple as that. With those simple bits of information and your experiences you can pretty quickly arrive at a PHP that will work effectively for you. Perhaps the most important point about your PHP should be to ‘drink to thirst, not to a schedule’.

Hydration can have a measurable effect on performance for all those who exercise, but it is far more important for endurance athletes because they are more prone to electrolyte imbalances because of longer exercise durations. Perhaps surprisingly, 12.7% (n = 488) of Boston Marathon participants tested as Hyponatremic (over hydrated) after the 2002 event. Of those, 100% had been on a hydration plan that meant drinking every mile, or every 2^nd mile. It is worth remembering though that as late as the mid-2000s, and still present on the web today, are lots of training plans that promote hydration schedules as the best way to guard against dehydration. The more enlightened coaches and sports medical groups seem to favour the medical literature that points to Hyponatremia resulting in a more serious clinical situation than hypernatremia (dehydration); both are bad for your immediate health and sporting performance but if you are going to err then it is less dangerous to err on the side of dehydration. These days drinking to thirst, rather than to a predetermined plan is the most frequently recommend approach. Think about what happens if the temperature is much higher, or much lower, than expected on race day and you were drinking to a schedule; how easy would it be to get it wrong?

Calculate your sweat rate

This formula will provide an estimate of your sweat rate alone, it does not measure sodium loss so is useful for fluid loss but not electrolyte loss.

This method is best suited to exercise sessions between 45 mins to 2 hrs. To get the best approximation of your sweat rate you should undertake the test multiple times and under different environmental conditions and keep a record of your results. It is also a good idea to note down things like temperature/humidity and any other factors that might have influenced your sweat rate during the exercise.

After repeating the measurements a few times you should start to get a fair indication of your average sweat rate and how it might vary under different environmental conditions.

Immediately before exercise:

1. Empty your bladder
2. Weigh yourself (best to do naked)
3. Weigh any drink bottles (full) that you are taking to drink during exercise

During exercise:

1. Record start and finish time
2. Try not to pee. If you need to pee and can collect output do so, if not use 300 ml as a rough estimate of volume.

Post-exercise:

1. Towel dry and weigh yourself (best naked because clothes retain liquid)
2. Subtract post-exercise weight from pre-exercise weight to get weight loss measure (A)
3. Weigh drink bottles and subtract post-exercise weight from full weight to estimate volume consumed (B)
4. Calculate sweat rate as: (A+B)/time
5. Subtract volume of pee from sweat rate, or 0.3 litres if volume was estimated to get final sweat rate (this step not necessary if you did not pee).

Worked example:

1. Pre-exercise weight = 75.2 kg
2. 2 fluid bottles weigh 1.5 kg
3. Exercise duration = 1hr
4. Post-exercise weight = 74.6 kg
5. Post-exercise fluid bottle weights = 0.6 kg
6. Weight loss (A) = 0.9 kg
7. Fluid intake (B) = 0.9 kg
8. Sweat Rate = (A+B)/time = (0.9 + 0.9)/1 = (1.8)/1 = 1.8 litres/hour
9. If you stopped for a pee and estimated volume you add 0.3 litres to your calculated sweat rate so it would become 2.3 litres/hour in this example.

Is this a normal sweat rate? For some people it is about correct but it does depend on the environmental conditions during the test. The bulk of people lose between 0.75 l and 1.6 l of sweat per hour but this can vary significantly for some of the population from some as low as 0.2 l up to 5.8 l per hour. So you can see how important knowing your hydration requirements can be.