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Stay Hydrated: How Much Water Do You Need?

Elec3What you need to know:
  • Adequate water intake or hydration is determined by many factors.
  • Common advice such as "Drink 8 cups of water a day" or "Drink half your body weight in ounces" are far too simplistic and may not provide you with the water you need.
Why Water is Essential to Health and Performance
"Dehydration of as little as 2% loss of body weight results in impaired physiological and performance responses."
As your body's principal chemical component, water makes up roughly 60 percent of your total body weight. Every system and cell in your body depends on water. Water is essential to normal human function.

Lack of water leads to dehydration, a state that occurs when you don't have enough water to allow your body to perform normal functions. A review published in the Journal of the American Dietetics Association states that "Dehydration of as little as 2% loss of body weight results in impaired physiological and performance responses."

To put that into perspective, this would equate to losing almost 3lbs of water for an individual who weighs 150lbs. That may seem like a lot of water to lose, assuming that one must exercise a lot or resort to extreme measures to lose that much water. But let's take a closer look and see just how easy it is to lose water without exercising.

According to the Guyton Textbook of Medical Physiology, the same 150lb individual will lose about 2.3L of water daily from urine, feces, sweat, and insensible water loss through the skin and breathing. 2.3L equals almost 5lbs of water loss per day from normal body function. This estimation does not factor in a warmer climate. Naturally, sweat rates increase in warmer weather, so this same individual could lose up to 3.3L of water a day.

It should be clear that becoming dehydrated is not a difficult task. There is a common misconception that dehydration only occurs in people that sweat a lot due to exercise or warmer weather. This couldn't be further from the truth. Normal bodily functions can lead to dehydration if water intake is not addressed appropriately.

The importance of adequate water intake has important health considerations as well. There are studies that have demonstrated individuals who stay well hydrated are less likely to experience:
  • Cancers of the breast, colon, and urinary tract
  • Urinary stone disease
  • Mitral valve prolapse
  • Childhood and adolescent obesity
Meeting Your Basic Daily Needs
In sedentary individuals, it appears that men require about 12 cups of water per day and women require about 9 cups of water per day. Whole foods are estimated to provide 4 cups of that daily water total. Another 1 cup of that daily water recommendation comes from 'metabolic water' or water that your body makes from metabolic processes, thus making this water you don't have to worry about consuming.

So for the sedentary individual, they will require about 7 cups of water/fluid per day since the remaining 5 cups of water will come from food and normal metabolic function. This is assuming that one is eating enough to meet their calorie needs.

It's important that any fluid you count toward your daily total is non-caffeinated or non-alcoholic. Caffeine and alcohol raise water needs in the body. If you consume either of these, you will need more water.

Water Needs in Response to Physical Activity
As for athletes, there is strong evidence in the research showing that dehydration will have major impacts on endurance, strength, intensity, and mood. When it comes to athletes, little research has been done to determine exactly how much water intake is needed to prevent dehydration. This likely explains the wide variety of answers one can potentially be given when attempting to figure out how much water they need to rehydrate. The honest answer is, "It depends." The reality is, there are a number of factors that play into understanding how much fluid intake an athletes needs to appropriately rehydrate.

Athletes or active individuals will generally require greater amounts of water due to increased muscle mass, metabolic activity, and sweat rates. So how much water will an athlete require? For starters, we can make a safe assumption that athletes eat more food during the day than the average person and that they have a higher metabolic rate. With this in mind, they will be getting more water from food sources and metabolic function. Depending on the climate an athlete exercises in, daily water intake may need to increase to an additional 2-4L (8-16 cups) on training days. Water intake must be based on factors such as activity level, body mass, sweat rates, and climate.

Bottom Line
A safe, general guideline for athletes and water intake would be to consume 1/2 gallon of additional water on non-training days. When it comes to training days, an athlete may require a gallon or more of water per day to maintain adequate hydration levels.


Kleiner, S., Water: An essential but overlooked nutrient. Journal of the American Dietetics Association. Volume 99, Number 2, 200-206, 1999.
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