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About Water: How much should you drink every day? Water is essential to good health, yet needs vary by individual. These guidelines can help ensure you drink enough fluids.
How much water should you drink each day? — a simple question with no easy answers. Studies have produced varying recommendations over the years, but in truth, your water needs depend on many factors, including your health, how active you are and where you live.
Though no single formula fits everyone, knowing more about your body's need for fluids will help you estimate how much water to drink each day.
Health benefits of water 
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Functions of water in the body
Water is your body's principal chemical component, comprising, on average, 60 percent of your weight. Every system in your body depends on water. For example, water flushes toxins out of vital organs, carries nutrients to your cells and provides a moist environment for ear, nose and throat tissues.
Lack of water can lead to dehydration, a condition that occurs when you don't have enough water in your body to carry out normal functions.
How much water do you need?
Every day you lose water through your breath, perspiration, urine and bowel movements. For your body to function properly, you must replenish its water supply by consuming beverages and foods that contain water.
A couple of approaches attempt to approximate water needs for the average, healthy adult living in a temperate climate.
- <LI class=doublespace>Replacement approach. The average urine output for adults is 1.5 liters a day. You lose close to an additional liter of water a day through breathing, sweating and bowel movements. Food usually accounts for 20 percent of your total fluid intake, so if you consume 2 liters of water or other beverages a day (a little more than 8 cups) along with your normal diet, you will typically replace the lost fluids.
- Dietary recommendations. The Institute of Medicine advises that men consume roughly 3.0 liters (about 13 cups) of total beverages a day and women consume 2.2 liters (about 9 cups) of total beverages a day.
Even apart from the above approaches, it is generally the case that if you drink enough fluid so that you rarely feel thirsty and produce between one and two liters of colorless or slightly yellow urine a day, your fluid intake is probably adequate.
Factors that influence water needs
You may need to modify your total fluid intake depending on how active you are, the climate you live in, your health status, and if you're pregnant or breast-feeding.
- <LI class=doublespace>Exercise. The more you exercise, the more fluid you'll need to keep your body hydrated. An extra 1 or 2 cups of water should suffice for short bouts of exercise, but intense exercise lasting more than an hour (for example, running a marathon) requires additional fluid. How much additional fluid is needed depends on how much you sweat during the exercise, but 13 to26 ounces (or about 2 to 3 cups) an hour will generally be adequate, unless the weather is exceptionally warm.
During long bouts of intense exercise, it's best to use a sports drink that contains sodium, as this will help replace sodium lost in sweat and reduce the chances of developing hyponatremia, which can be life-threatening. Fluid also should be replaced after exercise. Drinking 16 ounces of fluid per pound of body weight lost during exercise is recommended.
<LI class=doublespace>Environment. Hot or humid weather can make you sweat and requires additional intake of fluid. Heated indoor air also can cause your skin to lose moisture during wintertime. Further, altitudes greater than 2,500 meters (8,200 feet) may trigger increased urination and more rapid breathing, which use up more of your fluid reserves. <LI class=doublespace>Illnesses or health conditions. Signs of illnesses, such as fever, vomiting and diarrhea, cause your body to lose additional fluids. In these cases you should drink more water and may even need oral rehydration solutions, such as Gatorade, Powerade or Ceralyte. Certain conditions, including bladder infections or urinary tract stones, also require increased water intake. On the other hand, certain conditions such as heart failure and some types of kidney, liver and adrenal diseases may impair excretion of water and even require that you limit your fluid intake. - Pregnancy or breast-feeding. Women who are expecting or breast-feeding need additional fluids to stay hydrated. Large amounts of fluid are lost especially when nursing. The Institute of Medicine recommends that pregnant women drink 2.4 liters (about 10 cups) of fluids daily and women who breast-feed consume 3.0 liters (about 12.5 cups) of fluids a day.
Beyond the tap: Other sources of water
Although it's a great idea to keep water within reach at all times, you don't need to rely only on what you drink to satisfy your fluid needs. What you eat also provides a significant portion of your fluid needs. On average, food provides about 20 percent of total water intake, while the remaining 80 percent comes from water and beverages of all kinds.
For example, many fruits and vegetables — such as watermelon and cucumbers — are nearly 100 percent water by weight. Beverages such as milk and juice are also comprised mostly of water. Even beer, wine and caffeinated beverages such as coffee, tea or soda can contribute, but these should not be a major portion of your daily total fluid intake. Water is one of your best bets because it's calorie-free, inexpensive and readily available.
Dehydration and complications
Failing to take in more water than your body uses can lead to dehydration. Even mild dehydration — as little as a 1 percent to 2 percent loss of your body weight — can sap your energy and make you tired. Common causes of dehydration include strenuous activity, excessive sweating, vomiting and diarrhea.
Signs and symptoms of dehydration include:
- Mild to excessive thirst
- Fatigue
- Headache
- Dry mouth
- Little or no urination
- Muscle weakness
- Dizziness
- Lightheadedness
Mild dehydration rarely results in complications — as long as the fluid is replaced quickly — but more-severe cases can be life-threatening, especially in the very young and the elderly. In extreme situations, fluids or electrolytes may need to be delivered intravenously.
Staying safely hydrated
It's generally not a good idea to use thirst alone as a guide for when to drink. By the time one becomes thirsty, it is possible to already be slightly dehydrated. Further, be aware that as you get older your body is less able to sense dehydration and send your brain signals of thirst. Excessive thirst and increased urination can be signs of a more serious medical condition. Talk to your doctor if you experience either.
To ward off dehydration and make sure your body has the fluids it needs, make water your beverage of choice. Nearly every healthy adult can consider the following:
- Drink a glass of water with each meal and between each meal.
- Hydrate before, during and after exercise.
- Substitute sparkling water for alcoholic drinks at social gatherings.
If you drink water from a bottle, thoroughly clean or replace the bottle often. Refill only bottles that are designed for reuse.
Though uncommon, it is possible to drink too much water. When your kidneys are unable to excrete the excess water, the electrolyte (mineral) content of the blood is diluted, resulting in a condition called hyponatremia (low sodium levels in the blood). Endurance athletes — such as marathon runners — who drink large amounts of water are at higher risk of hyponatremia. In general, though, drinking too much water is rare in healthy adults who consume an average American diet.
If you're concerned about your fluid intake, check with your doctor or a registered dietitian. He or she can help you determine the amount of water that's best for you.
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May 23, 2006 © 1998-2008 Mayo Foundation for Medical Education and Research (MFMER). All rights reserved. A single copy of these materials may be reprinted for noncommercial personal use only. "Mayo," "Mayo Clinic," "MayoClinic.com," "EmbodyHealth," "Reliable tools for healthier lives," "Enhance your life," and the triple-shield Mayo Clinic logo are trademarks of Mayo Foundation for Medical Education and Research. NU00283
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