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Hydration & Sweat Rate

Photo Credit: Pedro Gomes

Hydration is extremely important as our bodies are approximately 60% water. Dehydration occurs when fluid loss (via sweat, urine and through respiration) is greater than fluid intake (via drinking and food). For the unconditioned athlete, just a small amount of dehydration can have a negative impact on athletic performance and may predispose them to heat illnesses. For the conditioned and acclimated athlete, withstanding a reasonable level of dehydration should be standard protocol on race day. This doesn't mean that you should neglect hydration in competition. It means that you don't need to attempt to replace 100% of your fluid loss.

What is a reasonable level of dehydration & how much of your sweat loss should you replace? It is impossible (and dangerous) to target 100% fluid replenishment on race day as it can lead to hyponatremia. Hyponatremia occurs when your blood sodium levels reach low levels resulting in dangerously uncomfortable symptoms.

Although athlete dehydration standards are highly individual based on ambient temperature, humidity, clothing selection, skin tone, genetics and heat acclimation, the sweet spot for acceptable dehydration level probably falls somewhere between 2-4% bodyweight loss. Note that environments and individual physiology also play a role in sweat loss. Nothing is absolute!


  • Sweat rate data is NOT an inflexible strategy for fluid and electrolyte replacement. Measuring your sweat rate is about getting a decent ‘ballpark’ figure for how much sweat (and sodium if you know your sweat composition) you’ll likely lose over a period of time, at a certain intensity and in a particular set of environmental conditions.

  • Sweat rates generally increase after 10–14 days of heat exposure, so sweat rate should be calculated following heat acclimatization.

  • Higher sweat rates are generally found in men (more mass) and those that are highly fit.

  • Temperature plays a role in sweat rate, so calculations should be done for different environments (winter vs. summer OR hot vs. cold spaces).

  • When first beginning an exercise routine in heat, your body loses more sodium through sweating, so slightly increase the amount of sodium in your diet until you’ve become adapted (after 10–14 days).

Sweat rate calculations determine how much fluid an athlete should consume during a training session by calculating the amount of fluid that is expelled through sweat during exercise. Sweat rate is specific to the discipline being tested, so athletes should conduct this test for swimming, biking and running.


Sweat Rate Calculator adapted from Asker Jeukendrup & Precision Hydration

1. Use the restroom and then record your body weight, ideally with no clothes on (that’s A).

2. Perform your session (I suggest 60’ @ 70.3 effort on the bike) and record exactly how much you drank. This is easy if you drink from a single bottle or two; simply weigh your bottles before you ride (that’s X) and after (that’s Y) and record the difference (that’s Z). *1 gram = 1 millilitre. *Make sure all units are in kg or liters.

3. After exercise, towel yourself dry and then record your weight (that’s B). Again no clothes on is optimal, as your clothes will hold some sweat.

4. Now subtract your post-exercise weight (B) from your pre-exercise weight (A) to get the weight you lost during the session.

Weight lost (C) = A-B

5. Also subtract the weight of the bottle(s) before (X) and after (Y) to obtain the amount you consumed (Z).

Volume consumed (Z) = X-Y

6. You can now calculate your sweat rate using (C+Z) / time.

Record your data on a spreadsheet to help you determine the amount of fluid you should be drinking during and after your workouts. Note the temperature and environment that you performed the sweat test so that you can learn from your data.

Still curious about what we recommend to Toro Athletes? Here is insight into our fueling protocol.

Photo Credit: Jeff Yingling

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