The Secret To Triathlon Hydration

How a simple Sweat Test can help you avoid dehydration on race day…

In this week’s blog I want to introduce you to the secret of triathlon race-hydration, which is… knowing your own personal sweat rate. This is vital because your sweat rate has a direct impact on how much you need to drink during a triathlon. We are all different and therefore require different amounts of fluid.

You might be reading this blog and thinking: “What’s the big deal, I’ll just swig my bike bottle when I’m thirsty.”

​That’s fair enough, but it’s actually very difficult to listen to your thirst when you’re in the middle of a triathlon. The chances are you’ll perform better if you drink according to your personal sweat rate. I’ll explain how to do this below.

Three Reasons to Get Your Hydration Right

  • Studies have shown that even a small amount of dehydration can have negative effects on athletic performance and predispose you to heat-stress.
  • Dehydration slows your rate of gastric emptying (the speed at which your gut processes stuff) and puts you at greater risk of gastrointestinal distress (a bad tummy).
  • Hyponatremia. This is where the sodium (salt) levels in your blood become too low. This can be caused by either losing too much sodium through sweat or by drinking so much that the existing sodium in your blood becomes too diluted. Mild hyponatremia can cause bloating, dizziness and sickness.

A personalised hydration strategy is important because:

  1. We all sweat at different rates during a race and therefore need to drink different amounts in order to rehydrate
  2. Every race presents different environmental and physical demands which lead to different sweat rates

How To Use Sweat Test Results

Once you know your own sweat rate, you can use it as a guide to how much you should drink on race day. For example, if you sweat 500ml per hour in your test, that’s roughly how much you should aim to replace through drinking (little and often) during your race. In reality you’ll struggle to absorb as much fluid as you sweat out. Therefore you should practice your hydration strategy in training to get an idea of how much fluid you can handle versus your hourly sweat rate.

The sweat test instructions are below…  You should also check out this blog about sodium and carbohydrate requirements for race day. Good luck!

by Phil Mosley, Ironman Certified Triathlon Coach, Triathlon Plus Coaching Editor and owner of My Pro Coach.

My Pro Coach can take care of your race-hydration, nutrition, training and more. Click here to learn more about becoming one of our coached athletes.


INSTRUCTIONS: How Test Your Own Sweat Rate

NOTE: This is slightly long-winded. You may want to print it and read it later…

A sweat test involves weighing yourself, exercising, then weighing yourself again to see how much weight you lost. This helps you understand how much fluid you lose, as well as how much you may need to drink to replace it. By replicating race conditions during the test you can estimate your sweat rate for upcoming target events. This involves wearing race day kit, testing in typical race weather conditions and training at race pace during the test.
Before we show you how to conduct your own sweat test, it’s important to understand that the maximal sweat rate of the human body is greater than its ability to re-hydrate. In other words, during tough workouts you’ll sweat more than you can ever hope to replace at that time. So sometimes you’ll be dehydrated during exercise, no matter how much you drink. You just have to accept that you can’t replace every drop of sweat. So the amount you drink should normally be a little less than the amount you lose through sweat.

7 Steps To Testing Your Sweat Rate

  1. Have a good pee, then weigh yourself wearing little or no clothing in order to get the most accurate reading. It is best if no solid food is consumed during the test and that you are hydrated beforehand.
  2. After you’ve weighed yourself, exercise for one hour at your target race-day intensity, while keeping track of the amount of water you drink.
  3. After exercising, towel yourself dry and step onto the scales again, in exactly what you wore at the initial weigh-in.
  4.  Your weight before and after exercise as well as the amount of fluid that you drank during exercise will be used to determine your sweat rate. Remember that 500mL of water = 500 grams.
  5. Subtract the post-exercise weight from the pre-exercise weight in kilograms. The difference should be what you lost through sweat in grams of fluid loss.
  6. Then add to that number the amount of millilitres of fluid that were consumed during the exercise. This will determine how much sweat was lost during exercise.
  7. Divide the sweat loss by the duration of the exercise to determine total fluid loss per hour.Sweat Rate Calculator
    A. Body Weight pre-exercise
    ___________________ [kg]
    B. Body Weight post exercise
    ___________________ [kg] (A-B)
    C. Change in Body Weight
    ___________________ [grams]
    D. Volume of fluid consumed
    ____________________ [mL]
    E. Sweat Loss
    ____________________ [mL] (C+D)
    F. Exercise time
    ___________________ [mins or hrs]
    G. Sweat Rate
    ___________________ [mL/min or mL/hr] (E/F)
    The final figure (G) is your sweat rate, or the amount of fluid that you lose through sweat (usually expressed as litres per hour). This should help you determine the amount of fluid you should be drinking during and after your workouts.
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Phil Mosley (Coach & Founder)
Phil Mosley (Coach & Founder)

Phil is a recognised expert in the field, having featured on many endurance sports publications. He founded MyProCoach in 2010 to sell premium training plans complete with email coach support for triathlons, duathlons, running & cycling.

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