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Harvard Medical School
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What's in Your Food
Make a splash with this silent nutrient.
InteliHealth Medical Content

Reviewed by the Faculty of Harvard Medical School

Water, often called the "silent nutrient," may be overlooked or taken for granted. Yet, next to oxygen, water is the most needed element for life. You could live for weeks without food but only a few days without water. The balance of water in your body is regulated by a sensitively controlled mechanism, which gives rise to thirst. The kidneys play a vital role in water balance by conserving or excreting water as necessary.

Water acts as a solvent, coolant, lubricant and transport agent. The amount of water in your body varies with body fat. Water, as a percentage of body weight, is higher in lean individuals due to the nearly water free characteristics of fat tissue. Besides keeping body temperature stable, water carries nutrients, eliminates toxins and waste products, maintains blood volume and provides the environment in which cellular chemical reactions take place. The body has three sources of water: the fluid you drink, the water content of the food you eat, and the metabolism of proteins, carbohydrates and lipids.

Even if you're inactive, your body loses up to 80 ounces of water a day — mainly in urine, perspiration (even invisible sweat) and bowel movements. Every time you exhale, for example, you lose water vapor that adds up to 1 or 2 glasses of water a day. To replace this loss, you should consume at least 6 full glasses of water a day — a minimum of about 48 ounces. Of course, foods are 70% to 90% water, so don't count just the glasses you drink in a day.

The amount of water you need varies depending on the climate where you are and the amount of activity you engage in. For instance, intense exercise in hot, humid weather can cause excess water loss of a quart (32 ounces) or more an hour through sweat. That water must be replenished immediately to prevent serious dehydration, even death. If too much fluid is lost through sweating, blood pressure falls and the amount of oxygen getting to the brain decreases.

Here are a few guidelines to getting your daily fluid quota of six to eight glasses:

  • Thirst is not always a good indicator of the body's need for water. It's important to drink regularly, even if you don't feel thirsty. This is especially true in the elderly because the sensation of thirst becomes weaker in very old people. Also, intense dehydration may impair the usual strong desire to drink.
  • Drink 1 or 2 glasses of water 30 minutes to 1 hour before exercising. Drink a half of a glass again 10 minutes before your workout. Finally, drink at least 1 glass of water after you finish exercising.
  • You don't need sports drinks, unless you do strenuous aerobic exercise for at least 60 minutes. These beverages contain sugar or fruit juice and electrolytes (mainly sodium and potassium). But for most of us, ordinary water is fine.
  • Increase your fluid intake if your diet is high in fiber, protein or salt.

Remember, fluid comes in a variety of forms, including the water in solid foods. Many fruits, vegetables and soups are at least 80% water, so they can count toward your daily intake. Milk, diet sodas, unsweetened carbonated waters, most herbal teas, decaffeinated teas and decaf coffee can substitute, cup-for-cup, for pure water. The same is no true of fruit juices and sugar-sweetened drinks. Although theses other beverages still count as fluid sources, there sugar content can slow down your body's water absorption. Finally, don't count coffee or alcohol as a water substitute: Caffeine is a mild diuretic (a substance that helps remove water from the body via urine). This makes coffee and other caffeinated beverages poor sources of water.

It is possible, but very rare, for a healthy person to drink too much water to the point of toxicity. Even if you drink a great deal of water, there's no danger of flushing nutrients out of your body — or excess calories, for that matter. Nutrients dissolved in water are absorbed into the bloodstream from the digestive tract, long before water goes to the kidneys for excretion.

Hard water vs. soft water

Hard water is high in calcium and magnesium. Soft water is high in sodium. Because soft water is better for cleaning, many people (and cities) soften their water by removing the calcium and magnesium and adding sodium. There's no nutritional benefit to softening water, however, since most Americans don't need more sodium in their diets.

Bottled water

It may taste better, but bottled water is held to the same legal safety standards as tap water. And that store-bought water may be coming from a municipal water source that is no cleaner than the water from your own faucet.

Water pollution

The Environmental Protection Agency, which is responsible for the safety of public water in the United States, sets maximum allowable levels for regulated contaminants, monitors the water supply for contaminants and resolves problems. Water utility companies are responsible for monitoring contaminant levels in local tap water and reporting the results to the state and federal government. Many people feel, however, that existing laws are not strong enough (or enforced enough) to protect the water supply from contamination. The result has been a boom for companies that bottle water and make water filtration systems.

Water pollution can come from city sewage, industrial waste, pesticides, bacteria and radioactivity. Water can be contaminated by heavy metals, such as lead and mercury, from old, corroded pipes. Hundreds of different contaminants have been identified in public drinking water, but only when they reach dangerous levels do authorities step in.

Many municipalities use chlorine to protect tap water from bacterial contamination. To find out how your water is treated, contact your local water department or state board of health. Then check your own pipes. Older homes may have lead pipes, and even newer, copper pipes can have lead solder at their joints. If you suspect a problem of any kind, have your water tested by your local water utility company or a private lab.

If your water contains lead, let the water run for a few minutes before using it; and use cold water for drinking and cooking, because lead leaches more easily into hot water. Bottled water may be the best short-term solution if your water is contaminated. Ultimately, you may want to consider installing a water purification system.

If you have questions about water safety or want to find a certified lab in your area to analyze your tap water, call the Environmental Protection Agency's Safe Drinking Water Hotline at 1-800-426-4791.



7095, 24100,
Last updated September 30, 2013

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