How to Calculate Your Sweat Rate

This article is from Monique Ryan, MS, RD, LDN, CSSD, author of Sports Nutrition for Endurance Athletes.

Every triathlete anticipates, experiments, and maybe stresses a bit (or a lot) about race day nutrition, starting with the foundation of fluid replacement. They appreciate that dehydration can slow you down, wreak havoc with your gastrointestinal system, and lead to overheating. On the flip side, over hydrating increases your risk for developing exercise associated hyponatremia, a fluid overload condition characterized by abnormally low levels of blood sodium that can be serious and even fatal. To find your own hydration sweet spot for race day—not too much and not too little—you can set up a process to check your sweat rate throughout your training season.

Got thirst?
You may have heard recently that thirst should drive your drinking. While this is safe and prudent for the recreational exerciser to prevent fluid overload, we can all appreciate that Ironman events are not casual, no matter your race day goals. Just as you have a planned race day pace, you should also have a paced plan for hydration during the bike and run. However, endurance athletes will need to be prepared to adapt and adjust on race day as well.

Getting started
Start by collecting your own sweat data. You can start anytime, but should do this throughout the training season to capture changes in your sweat rate that occur with increased fitness, acclimatization, increased training intensity, and of course the full smorgasbord of potential weather conditions. Creating a flow sheet for tracking is wise as this allows you to review data from the past as you develop and refine your hydration. Race day weather can differ greatly from recent training conditions due to travel to other climates and venues famous for labile weather conditions. Looking back at your records allows you to confidently tweak your race nutrition plan accordingly as the race day weather forecast emerges.

How to check your sweat rate
Begin to think of your sweat losses as an hourly rate specific to the bike and to the run. Fueling guidelines are also described at carbohydrates per hour, so this is a good base for your full race nutrition plan development.

A simple way to monitor hourly fluid losses over a workout is to measure weight changes. This strategy has been validated against sweat loss measurements in a lab, but there are some ways in which errors can creep into your calculations.

While you are preparing for a long race, it is best to check sweat losses during shorter workouts- about 60 to 90 minutes. That’s because you burn stored fuel or muscle glycogen during exercise, contributing to the weight loss. Longer workouts mean more glycogen and water loss, so it throws off the data.

It is also important to be well hydrated prior to workouts when you are completing a sweat check. Weighing sweaty clothes and hair also throws off your calculations as does consuming solid or semi-solid products during the workout, so stick with liquids.

Here are some guidelines to follow:

  • Make sure that you are well hydrated for these test session
  • Weigh in before training but after urinating
  • Weigh in wearing minimal clothing- only underwear if possible
  • After exercising, wear the same minimal clothing used for the weigh in, and weigh in again after towel drying your hair and body
  • If you go pee during training, scrap the data collection and try again next time
  • Know exactly how much fluid you consume during the training session by measuring the amounts before and after training, or use a graded water bottle.

Important calculations
Every pound of body weight that you lose during training is equivalent to 16 ounces of sweat loss. Another easy conversion is one kilogram weight lost is equal to 1000 ml of sweat. Fluid consumed during the workout is sweat losses that you did replace, so you add it back in to the weight loss. You can also consume no fluid during these training sessions and skip that step.

Record the following: Weight before and after training; amount of weight lost; triathlon discipline; temperature; humidity; altitude; workout clothing worn; amount of fluid consumed during training; intensity of training; duration of training; date; time of day.

Your standard sweat check procedure is:

    • Check your weight before and after training, and calculate weight loss. Convert any weight loss to ounces or ml of fluid
    • Check/measure the amount of fluid consumed during training
    • Add the amount of fluid lost to the amount of fluid consumed to get total fluid losses
    • Divide the total amount of fluid lost by the number of hours of training to get fluid losses per hour

Example:

  • Workout: 90 minute bike workout at moderate intensity
  • Temperature: 65 degrees, low humidity, 300 feet above sea level
  • Weight before: 160 lb.
  • Weight after: 158 lb.
  • Fluid consumed: 30 ounces
  • Weight Loss: 160- 158 lb. = 2 lbs.
  • Convert to fluid loss: 2 lbs. = 32 ounces
  • Fluid consumed: 32 ounces consumed
    (2, 20 ounces bottles on bike, with 10 ounces remaining after ride)
  • Total fluid loss: 32 ounces lost + 30 ounces consumed = 60 ounces total
    60 ounces fluid loss divided by 1.5 hours is 40 ounces per hour

This triathlete sweats 40 ounces per hour on the bike in those specific weather conditions at that given stage in their training.

It’s not always possible, or perhaps not even needed to replace all fluid lost during each hour of training and racing. But knowing your sweat rate does provide a frame of reference for hydrating to minimize your sweat losses and of course sticking with a plan that does not result in fluid overload.

See what to eat and when with Sports Nutrition for Endurance Athletes. Ryan demystifies optimum daily nutrition and shows simple steps to make the best decisions about what you eat and drink.

Please Join Our E-mail List!

Fields marked with a * are required

Books are perfect gifts! Take a look at the 2017 VeloPress gift book collection. FREE U.S. Shipping on Orders Over $30! Dismiss