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  • Writer's pictureMelissa Mathes


Hydration, why is it so important?  Sixty-seventy percent or our body is composed of water and functions in this aqueous solution.  The purpose of water; transports nutrients (and oxygen) to tissues, elimination of waste products, regulates and maintains body temperature, maintains blood circulation and pressure, lubricates joints and body tissues as well as helps facilitate digestion. Basically we can’t live without it!  In terms of it’s influence on exercise, as dehydration increases there is a gradual decrease in both physical and mental performance.

Exercise, Water Loss and Fluid Balance

Exercise increases body temperature dependent on the intensity, duration, elevation (environmental conditions), clothing and metabolic rate.  Our body gets rid of the excess heat by sweating (water loss).  Sweat is composed of water and electrolytes like sodium (the electrolyte that is secreted the most).  Sweat evaporation is the heat loss during exercise.   Too much loss can lead to dehydration.  Studies show that a body weight loss equal to 2% is sufficient to cause a detectable decrease in athletic performance.  Dehydration greater than 2% of body weight increases the risk of nausea, vomiting, diarrhea, and other gastro-intestinal problems during exercise.


The loss of fluids and salts that are essential to maintain normal body function.  This occurs when the body loses more fluids than it takes in..  This can be from too little fluid, during exercise, not adequately hydrated before exercise, ambient temperatures and elevation.

Detriments of dehydration:

  • Muscle fatigue

  • Inability to regulate temperature

  • Heat illness (cramps, heat exhaustion, heat stroke)

  • Decrease energy and athletic performance

Each individual has a different rate of sweat loss; fluid and electrolyte requirements are crucial from person to person to avoid dehydration.

Fluid Balance

Thirst means your body is headed toward dehydration don’t use it as an indicator for proper hydration.  The goal is to be euhydrated (not dehydrated, but adequate hydration status) prior to your workout.    You can check your hydration status by doing a sweat rate.  This means you weigh yourself naked in the morning prior to your workout, then you weigh yourself after.  Comparing the difference in weight and making sure you account for the fluid you drank during your workout will give you an idea how much fluid you need to replace your losses.  Weight loss is likely from fluid loss, make sure you don’t urinate before you weigh yourself after exercise.

% Body weight change:

  • Well hydrated -1 to 1%

  • Dehydration 1 to 2%

  • Significant 3-5%

  • Serious dehydration > 5%

Correct your losses: Consume 20-24oz. of fluid for every one pound of fluid you lost.  If you were in the minimal dehydration zone or worse, this is an indicator that you probably need more fluid during your exercise then what you are currently consuming.

Daily Fluid Needs

Your daily needs outside of exercise are important to maintain.  The fluid needs below doesn’t take into account individuals with health issues or the obese.

  • Average healthy adult 30-35ml/kg

  • Adult 55-75 30ml/kg

  • Adult >75 years 25ml/kg

Pre-Exercise Hydration

Quick tip:  If you urine is not lemonade to clear in color in the toilet, consume enough fluid to make it that way.

  • Consume 16-20oz. of fluid (water or sports beverage) approximately 3-4 hours prior to exercise

  • Consume 8-12oz. of fluid 10-20 minutes before exercise

During Exercise

  • Drink 3-8oz. of water or sports drink every 15-20 minutes

  • When exercising for more than 1 hour, the drink you choose should be a sports drink that contains carbohydrate in it and electrolytes . The same amount every 15-20 minutes is recommended

  • Don’t drink more than 1 quart/hour of exercise

Post Workout

  • Drink 20-24 oz. of water or sports beverage (ideally a recovery drink!) for every 1 pound lost. If no weight was lost, you should still consume at least 24oz. of fluid the first hour after your workout.

  • Ice your drink so it is absorbed quicker and helps cool off core temperature

Sports Drinks/Electrolytes

  • The purpose of these drinks is to help replenish your sugar (glycogen) stores in your muscles so you have plenty of energy to complete your workout and not bonk! Having that extra energy is helpful in having a more productive workout/training session.

  • Sports drinks help with rehydration, because the electrolytes help you retain the fluid and let it get into the muscle cells to provide the required nutrients and to facilitate the elimination of waste.

  • The need for carbohydrate in sports beverages increase with prolonged activity.

  • Electrolytes, specifically extra sodium needs are dependent per individual, and if they are a “salty sweater”. This is the person who notices salt crystals on their face, or white smudges on their hats, visors, clothes, etc.  These people should look for sports drinks that contain >150mg of sodium per serving


Water intoxication, where the body contains too much water and dilutes the electrolytes.  This condition is called hyponatremia and is a serious situation.  The symptoms include:  behavioral changes, confusion, drowsiness, nausea/vomiting, weight gain (fluid retention), muscle cramps, weakness/paralysis, coma and risk of death.   If you notice any of these signs, go to your nearest emergency room/doctor/urgent care and get treated immediately.  Drinking more than 1 liter an hour can encourage this situation without electrolytes.

Keep your body well hydrated and you will train at a higher level of performance, and reduce your risk of many detriments and injury!

Melissa A. Mathes, MPH, RDN, CSSD

Simpson, M, Howard, T.  Selecting and Effectively Using Hydration for Fitness,  American College of Sports Medicine, updated 2015

Position Stands, Joint Position Statements and Opinion Statements are published in Medicine & Science in Sports & Exercise®, the official journal of ACSM.   Exercise and Fluid Replacement

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