Learn all about the fundamental nutritional importance of water, its role in the body, and a nutritionist's advice on maintaining proper hydration.
Jill Weisenberger, MS, RD, CDE, CHWC, FAND
Jill Weisenberger is a nutrition writer for Innerbody Research. She is a registered dietitian nutritionist, certified diabetes care and education specialist, certified health and wellness coach, and author of four wellness books.
Eight glasses of water a day, right? Maybe not. For some time now, experts have told us to drink eight 8-ounce glasses of water each day. Because we've heard this mantra for so long, it's difficult to imagine that it wouldn't be the truth. There isn't any evidence for it, however, concluded scientific reviews in 20021 and 2008.2 What's more, in 2004 the Institute for Medicine (IOM) issued new fluid guidelines stating that average healthy Americans should let thirst be their guide and that even caffeinated beverages like cola, tea and coffee count toward our fluid requirements.
That's not to say that water is not important. In fact, at least 50% of your body is water. Take a look at what it does for you.
Water in the Body
Water has the following functions within the human body.
Transports nutrients and other compounds. As a major component of the blood, water helps move glucose, water-soluble vitamins, minerals, other nutrients and some medications throughout your body.
Cools you. When you become too hot, your blood vessels dilate and you start sweating. The sweat evaporating on your skin is cooling. Water also has a high heat capacity. This means that a lot of energy is needed to increase its temperature. Since the body contains more water than anything else, it takes quite a significant amount of heat to raise your body temperature.
Protects and lubricates. It would be hard to swallow a peanut butter sandwich or practically any food without saliva, wouldn't it? Fortunately, our salivary glands produce ample saliva, which is largely water. Tears lubricate and clean the eyes, and synovial fluid lubricates your joints. Cerebrospinal fluid protects your brain and spine from trauma. Similarly, amniotic fluid surrounds and shields the fetus. Fluids throughout your body protect and lubricate your organs and tissues.
Participates in metabolism. Most chemical reactions in your body use water in at least one of three ways: a solvent, a reactant or a product of the chemical reaction. Water produced in chemical reactions is called metabolic water, and your body uses it in the same ways it uses the water you drink.
Even though the myth of drinking eight glasses of water a day may not be true, water is still an essential ingredient in your body's system.
Water in the Diet
If you are the typical American adult, you consume about 20 to 25% of your daily water from solid foods. Fruits, vegetables, cooked grains and even meats and cheese provide water. The rest of your water intake comes from beverages of all types.
Water Content of Selected Foods and Beverages (food and its percentage of water by weight):
The IOM report does not specify water requirements. Rather it includes guidelines for total fluid intake. From both food and beverages, women should consume, on average, 91 ounces of total water, and men should have 125 ounces of total water daily. This should cover your fluid losses in urine and feces, and the normal, but continual losses from the lungs and skin. The water loss from the skin and the respiratory tract is referred to as insensible water loss. If you are sick, you may lose additional water from nasal secretions, through vomiting or diarrhea or from sweating with a fever. Thus, you'll need to drink additional beverages.
How should you measure your fluid intake?
There is usually no reason to measure your water intake or impose a water requirement on yourself. Healthy people will meet their fluid needs by paying attention to their thirst. If you have heard and believed that thirst is a poor indicator of hydration status, you have fallen prey to another myth. Though we often hear such things as, "by the time you're thirsty, you are already dehydrated," the truth is that experts define dehydration when the concentration of blood has increased by at least 5%, but thirst begins much sooner than that, usually before the concentration of blood rises 2%. Thirst may not indicate hydration status, however, for individuals with medical conditions requiring fluid control, individuals taking some medications, athletes or those involved in other strenuous activities, or people living in especially hot climates.
What are some liquid sources of water?
Athletes and heavy sweaters may benefit from sports drinks. They are ideal for individuals who are very active for at least 60 minutes. They contain fluid and electrolytes lost in sweat, as well as carbohydrates to refuel and prevent fatigue. For casual exercisers who sweat little, sports drinks likely offer little more than water and extra calories. Added vitamins in sports drinks are unnecessary because we do not lose vitamins in sweat. Sodas, sweet tea and fancy coffee drinks also provide you with necessary water, but they give you plenty of unnecessary added sugars and calories, so have them only rarely.
To keep your body hydrated, you can do more than just drink water. Many healthy foods contain water, as well as some of the other liquids you consume, such as milk. Eat and drink a variety of foods and liquids to make sure your fluid intake is where it should be.
Water and Electrolytes: A Critical Balance
Electrolytes are substances that, when dissolved in water, dissociate into positively and negatively charged ions. This makes them capable of carrying an electrical current. For example, when you dissolve salt (sodium chloride) in water, sodium and chloride separate. Sodium provides a positive charge and chloride brings a negative charge. Electrolytes help maintain fluid balance because they draw water to them. Through osmosis, water will cross cell membranes to make the concentration of dissolved particles the same on both sides. The major electrolytes are sodium, potassium, chloride and phosphorus.
This is the major positively charged electrolyte in the extracellular fluid (fluid outside the cells).
Functions: Sodium helps regulate blood pressure; assists with acid-base balance; aids in muscle contraction; assists with the transmission of nerve signals; and participates in the active transport of glucose and some other nutrients.
Recommended intakes of sodium: Our actual daily requirement for sodium is very small, just a few hundred milligrams. It would be hard, perhaps impossible, to eat so little sodium and still maintain a diet adequate in other nutrients. For this reason, the IOM set the Adequate Intake (AI) at 1500 mg. The Tolerable Upper Intake Level (UL) is 2300 mg. The 2010 Dietary Guidelines for Americans is more strict and recommends a daily limit of 1500 mg of sodium for anyone age 51 years or older, all African Americans, and all people with high blood pressure, diabetes or chronic kidney disease. This includes almost half of the population - including children! The rest of the population is advised to limit sodium consumption to 2300 mg/day. Most Americans consume significantly more sodium than health experts consider safe. According to the Centers for Disease Control and Prevention (CDC), the average daily sodium intake for Americans over the age of two is more than 3400 mg.4
Sources of sodium: Though it is not the saltiest food per serving, bread is the greatest contributor of sodium to the American diet because it is so frequently consumed.5 Approximately 10% of our sodium intake is from sodium that occurs naturally in our food. Three-quarters of our intake comes from processed foods and restaurant foods. The rest comes from what we add in cooking or at the table. Thus, the most important steps to reducing sodium intake are to eat few highly processed foods and to prepare many of our meals from scratch.
When you get too much or too little sodium: High sodium diets are linked to high blood pressure, bone loss and kidney stones. Low sodium intakes relative to sodium losses in sweat can cause overhydration, which can be deadly.
This is the major positively charged electrolyte in the intracellular fluid (fluid within the cells).
Functions: Potassium assists with managing blood pressure by blunting the effects of excess sodium; aids in muscle contraction; and assists in nerve transmission.
Recommended intakes of potassium: The AI for potassium for adults is 4700 mg per day. For children, the AI ranges between 3000 and 4700 mg, depending on age and gender. Unfortunately, less than 3% of Americans meet the AI for potassium.6
Sources of potassium: The most notable sources of potassium are fruits, vegetables and dairy, though potassium is present in all food groups. High-potassium foods include white and sweet potatoes, beans, leafy greens, tomatoes, oranges, halibut and clams.
When you get too much or too little potassium: A severe dietary imbalance of potassium is rare in healthy people. Individuals with healthy kidneys excrete excess potassium readily. If people take large doses in the form of supplements or if they have impaired kidney function, potassium can build up in the blood and alter the electrical rhythm of the heart causing heart attack and death. Because potassium helps to balance sodium, low potassium intakes are linked to high blood pressure, especially when sodium intake is excessive.
This is the major negatively charged electrolyte in the body.
Functions: Chloride is used to form hydrochloric acid (HCl) secreted into the stomach; aids in the immune function; and assists in nerve transmission.
Recommended intakes of chloride: For men and women aged 19-50 years, the AI for chloride is 2300 mg.
Sources of chloride: Chloride is usually found as sodium chloride, common table salt.
When you get too much or too little chloride: Since chloride is usually consumed as table salt, there are no known toxicity symptoms for chloride alone. Deficiencies of chloride do not occur from lack of intake, but rather from severe vomiting or dehydration.
This is the major negatively charged electrolyte in the intracellular fluid, usually combined with oxygen in the body to form phosphate.
Functions: Phosphorus makes up part of DNA, RNA, cell membranes and the mineral complex in the bone; assists in providing the body with energy, as a component of ATP (adenosine triphosphate); and helps regulate biochemical reactions by activating or deactivating various enzymes.
Recommended intakes of phosphorus: The Recommended Dietary Allowance (RDA) for adults is 700 mg/day. For adolescents and teens, the RDA is 1250 mg/day.
Sources of phosphorus: Where there is protein, there is usually phosphorus. Some of the best sources of this mineral are meats, eggs and dairy. Highly processed foods, especially sodas and processed meats, also provide ample phosphorus to the diet.
When you get too much or too little phosphorus: Phosphorus is ubiquitous in the diet, so dietary deficiencies are extremely rare. Low blood phosphorous levels may come from parathyroid disorders, vitamin D deficiency or overuse of phosphate binding antacids. In the short-term, low phosphorus levels can cause muscle weakness. Weakening of the bones occurs when low phosphorus levels occur long term. High phosphorus levels in the blood can occur when individuals take excessive vitamin D supplements or phosphorus-containing laxatives. Individuals with kidney disease may also experience high phosphorus levels. Very high blood levels of phosphorus can cause muscle spasms and convulsions. Knowing whether you have too much or too little vitamin D in this case can be very helpful; a vitamin D home test allows you to find out conveniently from home.
Electrolytes are no less important than fluid for maintaining proper hydration status. And as you can see, each of the above minerals plays multiple roles in the body - from bone density to immune function, muscle contraction and nerve transmission. Eating a balanced diet with an abundance of fresh fruits and vegetables will help assure the proper intake of these minerals.
Maintain Safe Hydration Levels
You can see how intricately tied water and electrolytes are in the body. A problem with one easily leads to a problem with the other. It might be hard to imagine that water, something so pure, so critical for life can be toxic.
Overhydration: Overhydration or water intoxication can occur when athletes replace excessive sweat losses with plain water. Remember that sweating causes loss of water and electrolytes. When water only is replaced, the concentration of sodium in the blood becomes too low, a condition called hyponatremia. Severe hyponatremia is lethal. It can also occur when large quantities of water are consumed so rapidly that the kidneys cannot excrete the excess.
Dehydration: Dehydration occurs when too much fluid is lost from the body through sweating, vomiting or diarrhea. The treatment for mild dehydration is water or water and salts. Severe dehydration requires medical attention with intravenous fluids and careful monitoring of electrolytes.
Water is an essential part of the human body. Make sure you drink enough fluids and eat foods that contain water. You may not be able to measure the exact amount of ounces you consume during a day, but if you eat healthy foods, and drink fluids that contain more water than sugars or additional calories, you should maintain safe hydration levels.
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Valtin Heinz. "Drink at least eight glasses of water a day." Really? Is there scientific evidence for "8 x 8"? Am J Physiol Regul Integr Comp Physiol 283: R993-R1004, 2002.
Kolso J, Jeckel K, Wildman EC. Water, Hydration and Health: What Dietetics Practitioners Need to Know. SCAN's Pulse Winter 2012 Vol.31, No. 1.