Nutrition Tuesday: Hydration

Water and hydration is essential to life. Water makes up about 66% of the human body. Water runs through the blood, inhabits cells, and everywhere in between. Our brains are actually composed of about 85% water and proper hydration plays a role in memory functions (2). So start your morning off with a big old glass of H2O!

Proper hydration is also essential to sports performance. Studies have found that athletes who lose as much as 2% of their body weight through sweating can have a drop in blood volume which results in the heart having to work harder to circulate the blood. A drop in blood volume can also lead to muscle cramps, dizziness, fatigue, and heat illnesses such as heat exhaustion or heat stroke (3).

Common causes of dehydration in athletes are:

  • Inadequate fluid intake
  • Excessive sweating
  • Failure to replace fluids losses during activity and post-activity
  • Exercising in dry, hot environments
  • Drinking only when thirsty

Common ways athletes lose water:

  • High Altitude – Exercising at altitude increases your fluid losses and thus increasing your fluid needs.
  • Temperature – Exercising in the heat increases your fluid losses. Also, exercising in cold temperatures can impair your ability to recognize fluid losses and more fluid is lost during respiration.
  • Sweating – Everyone has their own unique sweat lost rate. It is important to know your sweat lost rate to be ability to calculate your hydration needs for daily activities, during exercise, and post-exercise recovery.
  • Exercise duration and intensity – The longer the the exercise period, the more you need to drink.

How to calculate your sweat rate:

  1. Empty your bladder and weigh yourself nude. Record weight.
  2. Do workout and drink as you normally would.
  3. Record approximate volume of fluid intake during exercise.
  4. Towel dry, empty bladder, and weight yourself nude again. Record weight.
  5. Subtract post-exercise weight from pre-exercise weight to get the number of pounds you lost during exercise.
  6. To find out how much fluid ounces of water you lost, multiply pounds lost by 16 to get the fluid ounces of water you lost during exercise.
  7. To determine hourly fluid intake needs, add to ounces of fluid you lost during exercise to the number of fluid ounces you consumed during exercise and divide by the number of hours spent exercising.

Note: Sweat rate can vary due to heat, humidity, and an elevated heart rate. Humidity levels over 75% will contribute to an increased risk of heat injury. When heat and humidity are above 75 degrees and 75%, respectively, multiple your hourly fluid needs by a factor of 1.2-1.6 (4).

Here’s a great handout from the University of Arizona on fluid needs and recovery: Sweat Rate and Fluid Replacement Calculations with Recovery Plans

General Guidelines for Hydration (3)

Hydration throughout the day:

  • Drink about 6-8 8 oz of water a day (whether you plan on exercising or not)

Hydration pre-exercise:

  • Drink about 15-20 fl oz, 2-3 hours before start of exercise
  • Drink 8-10 fl oz, 10-15 min before start of exercise

Hydration during exercise:

  • Drink 8-10 fl oz every 10-15 min during exercise
  • If exercising longer than 90 mins or 60 minutes at high intensity, drink 8-10 fl oz of sports drink every 15-20 minutes 

Hydration after exercise:

  • Drink 20-24 fl oz of water for every pound lost during exercise
  • Consume a 4:1 ratio of carbohydrate to protein drink within 2 hours of exercise to replenish glycogen stores

Now, in rare cases, it is possible to consume too much water. This is called hyponatremia, or water intoxication. Drinking excessive water can cause a low concentration of sodium in the bloodstream and could possibly lead to death.

I have a few posts in the works on sports drinks and recovery drinks so look for those in the near future!

Have a happy, safe, and hydrated 4th of July! Remember, alcohol actually dehydrates you so drink lots of water!


1. Gowin J. Why Your Brain Needs Water. Available at: Accessed July 3, 2012.
2. Exercise and Fluid Replacement, ACSM Position Stand, American College Of Sports Medicine, Medicine and Science In Sports & Exercise, 2007.
3. How to Caluclate Your Sweat Rate. Available at: . Accessed July 3, 2012.

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