Increasing temperature also means increasing sweat. Those who ran the Boston marathon experienced this first hand. Many runners didn’t perform up to their usual standard because of the hot racing conditions, which resulted in increased sweating and consequently, dehydration. This problem is common, so I am going to do a blog series on hydration. This week I will be focusing on fluid requirements during exercise.

Fluid requirements during exercise are extremely individualized. This variability is because fluid requirements during exercise depend on how much you sweat and also, your sweat composition. Sweat rate depends on many different factors such as heat acclimatization, exercise intensity, equipment, body weight etc. Each person sweats to differing degrees, and also at different rates depending on conditions.

Practicing my fluid plan

The overall goal of fluid intake during exercise is to prevent greater than 2% loss in body weight. More than 2% loss in body weight is associated with compromised sports performance. For instance, an athlete weighing 75 kg would want no more than a body weight loss of 1.5 kg (75 kg x 2%) during exercise.

The best way to make your own customized fluid plan for during exercise is by determining your own individual sweat rate. To determine your sweat rate:

  1. Weigh yourself immediately pre-exercise, either nude or with minimal dry clothing.
  2. If you are going to consume fluids during your workout, measure the amount of fluids.
  3. Start the clock as exercise begins and record the time at the end of exercise.
  4. Immediately after exercise, weigh yourself again either nude or in the same dry clothing (that you didn’t exercise in).
  5. Measure the amount of fluids remaining.

Based on the information gathered from above, fill in the following:

Calculation Example:
Pre-weight (kg): 75 kg
Post-weight (kg): 73 kg
Deficit (kg)= 2 kg
Deficit (kg) x 1000 mL= 2000 mL
Pre-fluids (mL): 1000 mL
Post-fluids (mL): 600 mL
Fluid intake (mL)= 400 mL
Sweat loss= deficit + fluid intake 2400 mL
Time (hr): 1.25 hrs
Sweat rate= sweat loss/time 1920 mL/hr

With the above information, you should also note the conditions (i.e. temperature, humidity, windy etc.), your exercise intensity, and even if you were dressed appropriately for the weather. Calculate and record your sweat rate across the seasons, differing conditions, and training intensities.

By calculating your sweat rate across these differing conditions, you can determine an optimal target for fluid consumption during a sporting event. For instance, if you’re training for an upcoming marathon and you consistently record your sweat rate at 1000 mL/hr when running at race pace, then you should aim for around 1000 mL/hr during your race. If there is a fluid station every 5 km, and it takes you about 30 minutes to run 5 km then you should be consuming around 500 mL at each fluid station. However, remember that the goal is to prevent greater than 2% loss of body weight. It’s okay to lose some fluid during exercise.

Next week, I will be covering what to drink during exercise, so stay tuned!

Categories: Megan Kuikman

Megan Kuikman

Hello! I’m Megan Kuikman. I’m a Registered Dietitian with specialized training in sports nutrition. My goal is to help athletes and active individuals achieve a healthy attitude towards health, training, and food. I empower athletes to fuel properly for training in order to restore their health and enhance performance. You can get in touch with me at:


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