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How Dehydration Affects Performance

By Alison McGinnis DPT, FAFS

Water is vital to keep your body functioning optimally and efficiently. Why? Well let’s start with a few numbers:

  • Muscles are 75% water
  • Blood is 82% water
  • The brain is 76% water
  • Lungs are 90% water
  • Even your bones are 25% water!

The average person loses about 10-15 cups (3-4 liters) of water a day through sweat, urine, exhaled air and bowel movements. You lose 1-2 liters of water each day just from breathing alone.

What is lost must be replaced by the fluids you drink and the foods you eat just to maintain a hydrated state. If you are thirsty, your body is telling you that your cells are already dehydrated. This is because thirst does not develop until body fluids are depleted below levels required for optimal functioning.

During exercise, it is not uncommon to lose 6-10% of your body weight from sweating (weigh yourself before and after exercise to see how much you’ve lost). It has been shown that physical performance falls off when you’re dehydrated by as little as 2%.

When running, water consumption doesn’t necessarily enhance performance, but it does prevent the decline in performance from dehydration.

Sweating reduces blood volume, which causes the heart to work harder to pump oxygen throughout the body and to muscles. When mildly dehydrated, an athlete can experience decreased endurance and increased fatigue as well as an increase in perceived effort.

When exercising in hot weather or for prolonged periods, inadequate rehydration can also lead to hyperthermia, reduced blood pressure and decreased blood flow to muscles. Drinking water during exercise can limit this decline in blood volume and maintain performance.

Here are a few signs of various stages of dehydration:

  • EARLY signs of dehydration: Fatigue, anxiety, irritability, depression, cravings, cramps, headaches
  • MATURE signs of dehydration: Heartburn, joint pain, migraine/headaches, fibromyalgia, constipation/colitis, angina pain
  • Signs of EMERGENCY dehydration: Asthma, allergies, mature diabetes, hypertension, autoimmune diseases (lupus, psoriasis, etc.)

Essentials of daily hydration:

  • A good formula to use is: 0.6 x body weight (in pounds) = # of ounces of water to drink per day. So for example, if you weigh 150 lbs., you’d need to drink 90 oz. of water a day or just over 11 8 oz. glasses.
  • Use ¼ tsp of celtic sea salt or himalayan salt for every 32 oz. of water to increase electrolytes.
  • Avoid diuretics, caffeinated or alcoholic drinks and prescriptions. Every 6 oz. of each requires an additional 10-12 oz. of water to rehydrate.

Can you drink too much water?
Electrolytes are important to move water and nutrients in and out of a cell. During intense exercise, the kidneys have a decreased ability to excrete excess water. This water then moves into the cells and flushes electrolytes out of your system.

The important thing to remember is to balance your water intake with your sodium intake. Symptoms of water intoxication include nausea, vomiting, fatigue and confusion.

To counteract this problem while exercising:

  • Consider adding a little more salt to your diet. Throughout the day, drink mineral water or make your own (add ¼ tsp of celtic sea salt or himalayan salt for every 32 oz. of water). Gatorade and other sports drinks contain a ton of sugar (7 tsp. in a 16 oz. bottle of Gatorade!!) so it’s a better option to make your own by adding lemon, lime or orange slices to your newly made water to add flavor.
  • If you cannot make your own salted water during exercise or a race, consume a sports drink in combination with plain water to take in enough electrolytes to balance the added water in your system. Make sure not to drink too quickly to cause gastrointestinal issues.

Most people only need to hydrate during an event lasting longer than 60 minutes or during intense heat. If you plan on exercising longer than 60 minutes, it is recommended to drink water every 20 minutes to minimize dehydration throughout the duration of the activity. And always remember to rehydrate after any amount of exercise.

Need some help getting creative with drinking enough water? Here are a few ideas to increase your water consumption:

  • Start your day with a glass of water. This is when your body is most full of toxins and dehydrated.
  • Add fresh lemon or lime to water if you’re more likely to drink something flavored.
  • Drink hot herbal tea (caffeine free) or hot water with lemon instead of coffee.
  • Replace soda with sparkling water. It still has the carbonated feel without the sugar.
  • If you can’t replace your coffee or soda, make sure to alternate with a glass of regular water in between to decrease the effects on your body.
  • Keep a water bottle with you and drink it throughout the day.
  • Drink a glass of water with each meal (your stomach needs water to help digest food, and a lack of water makes it harder to break down nutrients and use them as energy).
  • Take regular water breaks instead of coffee breaks throughout the day (set an alarm if you need a reminder).
  • Eat your water by consuming foods high in water. Tomatoes, cucumbers, watermelon, celery, green peppers and spinach are all foods with a high water content.
  • Substitute sparkling water for alcoholic drinks at social gatherings.


Alison McGinnis graduated from Northwestern University with her Doctorate of Physical Therapy and has been with Finish Line PT since 2011. She is recognized as a Fellow of Applied Functional Science (FAFS) through the internationally acclaimed Gray Institute. She attended the University of North Carolina–Chapel Hill for her undergraduate work and competed as a pole vaulter on the varsity track and field team for all four years.

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