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Water: The Science of Nature's Most Important Nutrient
Len Kravitz, Ph.D.

Water is colorless, tasteless and odorless. Because of its numerous and diverse functions in the body, it is often regarded as the most important nutrient. Most people can survive no more than 7 days without water (Williams, 2005). Although there is rigorous proof of its need for optimal health, scientists still have a difficult time objectively advising people how much they need to drink daily to maintain this favorable health. This article will explore into the depths of knowledge and research on this mysterious nutrient called H2O.

Water 101: The Basic Facts About Water in the Body?
Water is the most abundant constituent of the body, accounting for 50% to 60% of its mass. It is an inorganic (contains no carbons) substance composed of two hydrogen atoms which are bonded to one oxygen atom. Water is intricately involved in numerous functions of the body including the transport of oxygen, nutrients and waste products into and out of the cells. Drinking water contains several electrolytes (substances in solution that conduct an electric current) including calcium, chloride, fluoride, magnesium, potassium and sodium. Water is necessary for all digestion and absorption functions, and lubricates mucous membranes in the gastrointestinal and respiratory tracts. Even though it contains no calorie content, water is the medium for most chemical reactions in the body, especially those metabolic reactions involved in energy production. The body uses water as a coolant, helping to regulate body temperature during exercise, fever and in hot environments. Water also serves as a cushioning component between joints, in the spinal cord and in the brain.

How is Water Stored in the Body?
Water is stored in either intracellular fluid (ICF) or extracellular fluid (ECF) compartments. The ICF accounts for about 65% of the body water while the ECF (35%) is the blood plasma and lymph (a transparent, slightly yellow fluid that carries lymphocytes), which serve as the medium of transport for wastes and nutrients throughout the body. Minerals such as chloride, potassium and sodium participate in the maintenance of the ICF and ECF levels; a process governed by hormonal messages from the brain and the kidneys. If any molecule becomes too concentrated in one fluid compartment, it will pull water from the other compartment to dilute itself. For instance, eating pizza often makes a person thirsty. This is because the sodium from the pizza sauce and cheese (and meats) accumulates in the ECF, pulling water from the ICF. Cell sensors detect this change and signal the brain that the cell is dehydrating. The brain (specifically the hypothalamus) sends a signal to drink more water. So, whenever any minerals or molecules become too concentrated in one compartment (ICF or ECF), the brain will signal the body to drink more water until the compartment is appropriately diluted for homeostasis (maintenance of the body's internal environment). If more fluid is present than desired at the cell, the kidneys proceed to make urine by filtering the excess fluid from the blood.

What is the Origin of the 'Drink Eight 8-Ounce Glasses of Water a Day' Advice?
For clarity, eight 8-ounce glasses is equal to 1,893 milliliters, or 2 quarts, or one-half a gallon, or approximately 1.9 liters. Most fitness professionals, nutritionists and personal trainers for years have encouraged clients to drink eight 8-ounce glasses of water a day. Surprisingly, no scientific evidence can be found that supports this “8 X 8” recommendation. In a superbly researched and written review, Heinz Valtin (2002) traces the origin of this recommendation to two possible sources. One source is an 'unreferenced' excerpt within a text (Nutrition for Good Health) authored by Drs. Fredrick Stare and Margaret McWilliams in 1974 which recommends around 6 to 8 glasses per 24 hours, and this can be in the form of coffee, tea, milk, soft drinks, beer, etc. However, Valtin highlights a much earlier origin in 1945 by the Food and Nutrition Board of the National Research Council which states 'A suitable allowance of water for adults is 2.5 liters (which is approximately eight 8-ounce glasses) daily in most instances…'

So, What is the Current Daily Water Intake Recommendation?
The Institute of Medicine (IOM) published its Dietary Reference Intake for Water in February of 2004. This scientific committee established an adequate intake (AI) for total water to prevent the harmful (chiefly acute) effects of dehydration. It is essential to first note that every day close to a liter of water is lost from breathing, perspiring and in bowel movements. As well, the average urine output for adults is up to 1.5 liters a day. Consequently, the IOM AI for sedentary men and women (19-50 yr) is 3.7 liters and 2.7 liters per day, respectively. The committee explains that drinking fluids (water and beverages) represent about 81% of total water intake with 19% of water being provided by foods. So, the AI recommendation for actual fluid intake is 3.0 liters for men and 2.2 liters for women. Since 1 liter = 33.8 fluid ounces, men are recommended to drink 101.4 fluid ounces of beverages and drinking water (which is 13 cups {a cup is 8 fluid ounces}), and women are recommended to drink 74.4 fluid ounces (9 cups) of drinking fluids daily. The AI for girls 14-18 years is 2.3 liters/day (77.7 ounces or 9.5 cups) and 2.4 liters (81.1 ounces or 10 cups) for boys 14-18 years.

The Influence on Hydration on Health and Disease
Kidney Stones
Portis and Sundaram (2001) summarize several factors that may contribute to kidney stone formation including age (it is more common in adults versus elderly, but more common in elderly versus children), gender (it is two to three times more common in males then females), race (it is more common in Whites versus those of Asian ethnicity, who are more often affected than Blacks), climate (it occurs more frequently in hot, arid climates), and medications (drugs that treat swelling such as for congestive heart disease and cirrhosis of the liver). Portis and Sundaram continue that the most important factor influencing kidney stone formation is decreased fluid intake. To help prevent kidney stones, Hughes and Norman (1992) recommend moderation in the intake of calcium, oxalate (beverages such as beer, chocolate milk, teas, and fruit juices), protein, sodium and alcohol while increasing the intake of water and fiber.

Cancer of the Bladder and Lower Urinary Tract
The causes of bladder cancer include cigarette smoking and occupational exposure to aromatic amines (air contaminated by wild fires or coal tar). However it has also been clearly established that decreased water consumption is associated with bladder and lower urinary tract cancer (Altieri, La Vecchia and Negri, 2003). Altieri and colleagues theorize that the decreased fluid intake results in a greater concentration of carcinogens in the urine and/or prolonged contact with bladder mucous membranes.

Colorectal Cancer
Colorectal cancer is cancer that develops in either the colon or the rectum, which are parts of the body's digestive system. From the small intestine, partly digested food enters the colon (the first five feet of the large intestine), which removes water and nutrients from the food and turns the rest into waste. The waste then passes from the colon into the rectum (the last six inches of the large intestine) and then out of the body. In most cases, colorectal cancers develop slowly over a period of several years. Researchers have theorized that low fluid intake (in conjunction with the fact that we excrete about 80-200 ml of water a day in waste) may increase the risk of colorectal cancer risk by increasing bowel transit time, thus increasing the carcinogen contact within mucous membranes in the colon and rectum (Altieri, La Vecchia, and Negri, 2003).
Breast Cancer
The association with fluid intake and breast cancer remains uncertain at this time. More research is needed to establish if any association exists.
Clinical Health
From a clinical point of view, any health-related body water deficit (e.g., sickness, diarrhea, vomiting or climatic stress) that challenges the ability of the body to maintain homeostasis can negatively impact physiological function and health. If these conditions continue for &Mac179; 24 hours a health practitioner should be consulted.

Hydration and Mental Performance
The research on hydration and mental performance is in its formative years. However, the science is clear that decrements in visuomotor (visual perception by the brain), psychomotor and cognitive performance can occur when 2% or more of the body weight is lost due to water restriction, heat or physical exertion (Grandjean and Grandjean, 2007).

Hydration and Physical Performance
According to Murray (2007) the literature discussing physical performance and hydration began in the late 1800's. Murray summarizes that a decrease in body water below normal can stimulate inhibiting alterations in central nervous activity (reduced motivation and effort), cardiovascular function metabolic reactions and themoregulatory control mechanisms. A water loss, such as through dehydration, exceeding 2% of body weight (as little as 3 lbs {water} in a 150-lb athlete) can provoke these negative consequences. These deleterious physiological events are more severe warm in environments as compared to cold environments.

What are the Signs and First Aid of Dehydration and Heat Disorders?
The initial signs of dehydration may include light-headedness, headache, loss of appetite, flushed skin, dry sticky mouth, fatigue, dry eyes, muscle weakness, burning sensation in the stomach and a dark urine with a strong odor (Kleiner, 1999). As the dehydration worsens, Kleiner states that symptoms may include difficulty swallowing, clumsiness, sunken eyes, dim vision, numbness of the skin and muscle spasms. The one effective treatment for dehydration is to replace lost fluids with cool water. Sports drinks containing electrolytes and a carbohydrate solution may also help.

The three heat syndromes related to dehydration are heat cramps, heat exhaustion and heat stroke. Heat cramps are painful, brief muscle cramps that occur during exercise or work in a hot environment. Heat cramps usually involve the muscles fatigued by heavy exertion such as the calves, thighs, abdomen, and shoulders. Kleiner (1999) theorizes that the cramping is most likely due to the high sweat rates and dehydration disrupting the sodium and potassium ICF and ECF balance. Gradually cool down and begin to rest. Drink an electrolyte-containing (sports) drink while gently massaging and stretching the affected muscle groups. If the cramps continue for an hour seek medical assistance.

With heat exhaustion a person may go into hypovolemic shock (a state of decreased blood plasma and volume, characterized by pale, cool, clammy skin with a rapid heart rate and shallow breathing) and have some of the following symptoms: low or undetectable blood pressure, nausea, heavy sweating, low-grade fever, headache, and diminished consciousness. If you suspect heat exhaustion, get the person into a shady or air-conditioned location. Lie the person down and elevate the legs and feet slightly. Cool the person by spraying or sponging him/her with cool water and fanning. Have the person drink cool water. Heat exhaustion can quickly lead to heatstroke. If the symptoms start to worsen call immediately for medical assistance.

Heat stroke is an escalation of heat cramps and heat exhaustion. It is a life-threatening condition occurring when body temperature is 104 degrees (F) or higher. Sweating often stops as the body temperature is so high. Pulse rate may start to increase to about 130 b/min or higher (what is referred to as a sinus tachycardia). Seizures, lack of consciousness or hallucination may also occur. Lastly, weak muscles may become either more rigid or limp. Immediate medical intervention is needed to prevent brain damage, organ failure and/or loss of life.

What is the Proper Fluid Replacement to Sustain Endurance Exercise?
Failure to hydrate appropriately during exercise is a chief contributing factor to poor performance during endurance events, particularly in hot and humid conditions. The American College of Sport's Medicine (ACSM) recently released its newest position stand on exercise and fluid replacement in an effort to guide exercisers towards safe and enjoyable participation in endurance exercise (Sawka et al., 2007). The next three sections, prehydrating before exercise, hydrating during exercise, and rehydrating after exercise, summarize key points from this ACSM position paper.

Prehydrating Before Exercise
The prehydration goal is to make certain that any fluid and electrolyte insufficiency is corrected prior to starting the cardiovascular exercise bout. Hydrating before the exercise can begin progressively about 4 hours before the workout session. About 5-7 mL/kg body weight (1 kg = 2.2 lbs) should be sufficient. So, if a person weighs 150 lbs, that weight is 68 kg; therefore 7 mL/kg X 68 kg = 476 milliliters of fluid. Since 8 ounces is equivalent to 237 milliliters, 476 millilers is about 16 ounces, or two glasses of water. Consuming some sodium-containing foods or snacks with the two glasses of water may help retain the fluid. A beverage with very light sodium (20¬-50 mEq per liter or 460-1150 milligrams per liter) would also suffice (note that mEq stands for milliequivalent).

Hydrating During Exercise
The hydration goal during exercise is to prevent excessive water loss and disparities in electrolyte balance in the working muscle cells. Hydration recommendations during exercise can be quite variable depending on a person's sweat rate, mode of exercise, exercise duration, weather conditions, opportunities to hydrate, training status, heat acclimatization and exercise intensity. Because of the above circumstances, a customized hydration strategy is recommended that includes periodic hydration segments during the workout session. Sawka and colleagues (2007) clarify that prolonged (&Mac179; 3 hours) exercise is difficult to balance electrolyte and water deficits. Exercisers are encouraged to monitor their pre- and post-workout body weights during different workouts and try to match the weight loss (via sweat) with fluid replacement during the exercise. To sustain endurance exercise performance &Mac179; 1 hour, carbohydrate consumption (with a mixture of sugars such as glucose, fructose, maltodextrine, and sucrose) may be beneficial. Carbohydrate consumption at a rate of ~30-60 grams per hour has been shown to be quite effective in maintaining glucose levels for continuous aerobic performance beyond one hour (Sawka et al.). Sawka and fellow researchers add that the carbohydrate concentration should be up to 8%, and not beyond, because a higher concentration may impede gastric (stomach) emptying. Electrolyte needs during prolonged exercise are best replenished with fluids containing ~20-30 mEq per liter (460-690 milligrams per liter) of sodium and ~2-5 mEq per liter (80-200 milligrams per liter) potassium.

Rehydrating after Exercise
After exercise, the goal is to replenish any fluid or electrolyte shortfall. Sawka et al. (2007) suggest a resumption of normal of meals and snacks (that contain adequate sodium) with sufficient water to restore the body. The authors state the sodium losses are quite different between individuals and difficult to assess, but a variety of food choices supply the depleted electrolytes. Lastly, fluids are best absorbed by the cells of the body post-exercise when ingested gradually, as opposed to in single large amounts. As a general rule of thumb, for each kilogram (2.2 lbs) of weight post-exercise below the pre-exercise weight the body will need about 1.5 liters of fluid (Sawka et al.). Converting kg to lb, for each pound of sweat you lose in exercise, drink about 25 ounces of fluid post-exercise for replenishment.

Water is the most omnipresent substance on our planet. Life as we know it could not exist without water. Yet some unique physical properties are poorly understood. For instance, why does water expand, instead of contracting, when it freezes?, or why does water store heat better than virtually any other fluid?, or how is it that two atoms of hydrogen and one atom of oxygen, both flammable gases, are able to combine and become a liquid. The unique properties of water place limits on our physiology and anatomy while simultaneously providing the opportunities for physical activity, exercise and life as we know it. Yet, there is so much more yet to learn about this mysterious molecule we call H20.

Side Bar 1: 15 Frequently Asked Questions on Water
1) Why do you need to drink more water with air travel?
The re-circulated air on airplanes has less moisture. The travel time at high altitudes also increases your water loss through evaporation. As a general guide, drink one 8-ounce glass of water or juice for each hour of flying time.
2) Why do men have a higher percent of water than women?
Men have a slightly high percent of muscle mass in the human body and muscle is composed of about 75% water.
3) Why does your sweat vary on workouts during the week.
The total 'rate' of sweating as well as the total 'sweat loss' can be markedly different from day to day due to differences in the environment (heat and humidity), exercise intensity, exercise duration, mode of exercise (the less accustomed to the activity, usually the more work and sweat loss) and type of clothing (water absorbency). {see answer to question #7 to learn how to calculate exercise sweat rate}
4) How much water can be lost in an hour of exercise?
Doing light exercise in a cool or moderate environment the sweating rate might be as little as 100 ml/hour, which is about 3 ounces. However, during vigorous exercise in a hot environment the sweat loss can be over 3,000 ml/hour, which is about 100 ounces (Murray, 2007).
5) How does age effect your ability to hydrate?
Drinking only in a response to the body's thirst signals increases an older adult's risk of becoming dehydrated; because with age, thirst becomes a less effective indicator of the body's fluid needs. Seniors who have relocated to locations where the weather is warmer or dryer than the climate they are accustomed are also more susceptible to become dehydrated. They need to drink water regularly. Dehydration in children usually results from losing large amounts of fluid (such as from play) and not drinking enough water to replace the loss. An infant can become dehydrated only hours after becoming ill. Dehydration is a major cause of infant illness and death throughout the world.
6) Physiologically, why don't colder environments impair physiological function as much as hot environments?
There is performance impairment associated with colder environments. However, dehydration is not as deleterious because cardiac output (heart rate x stroke volume) is higher in colder environments (enhancing cardiovascular performance) while core temperature is lower (Murray, 2007).
7) How can you determine your sweat rate.
To determine sweat rate, measure body weight before and after exercise (wearing no clothes), the amount of fluid consumed during exercise, and the amount of urine excreted (if any) during exercise.
Follow the example below to calculate sweat rate (Williams, 2005):
a. Pre-exercise body weight 130 lbs
b. Post-exercise body weight 126.5 lbs
c. Change in body weight -3.5 lbs (or 56 ounces)
d. Drink Volume 16 ounces
e. Urine Volume 0 ounces
f. Sweat loss (c + d - e) 72 ounces
g. Exercise time 45 minutes
h. Sweat Rate (f divided by g) 72 ounces/45 minutes = 1.67 ounces/minute
Sweat rate varies from person to person due to body weight differences, genetic factors, heat acclimation ability and metabolic (energy production) efficiency (Sawka, 2007).
8) Please explain about the different types of water, including herbal, vitamin, purified, spring, mineral and artesian.
a. Herbal water features flavors derived from herbs that tout health benefits associated with antioxidants.
b. Vitamin water is fortified with various vitamins and other additives, including a sweetener that adds calories to the drink.
c. Purified water is usually produced by some type of distillation process.
d. Spring water flows naturally from an underground source.
e. Mineral water comes from a protected underground source and must contain some minerals.
f. Artesian water is drawn from a well that taps a confined aquifer (underground layer of water permeable rock, sand, clay or silt).
9) What is the composition of sweat?
Although this varies from person to person, the composition of sweat is approximately 99% water and the electrolytes sodium and chloride (Williams, 2005). Williams notes that other minerals lost in tiny amounts include calcium, potassium, magnesium, iron, zinc, copper and some water-soluble vitamins.
10) Besides counting daily cups of water intake, is there a way to monitor whether you are drinking enough or too much water?
As a general rule of thumb, urine color is a good 'marker' of water intake. Urine is composed of water, urea (metabolic waste), organic materials (including small amounts of carbohydrates, enzymes, fatty acids and hormones) and some electrolytes. Normal urine should be clear to amber (light yellow) in color. It is often more yellow if taking vitamins and some medications. Urine that is a dark yellow and lower volume output (than usual for you) are indicators of dehydration.

Drinking too much water can lead to a "water intoxication". However, this instance is rare in healthy persons because the kidneys can produce a large quantity of urine in a brief time period to correct this imbalance. A surplus of water intake may also lead to greater exposure to pollutants in the water, if sustained for an extended period of time.
11) Will drinking water help with weight loss?
There is some evidence for men and women that water intake with a meal may help to promote satiety and take the edge off hunger (Valtin, 2002). Water has no caloric value and when substituted for sweetened (usually with high-fructose corn syrup or sucrose) beverages, that add calories with few other nutrients, it will provide a positive modification to a weight management plan.
12) Why do some athletes wet their body during endurance competition events?
Sponging the head and torso with cold water or a water spray is a skin wetting technique. Although perceived to be performance enhancing, this practice has not been demonstrated to reduce core temperature or improve cardiovascular performance.
13) Do women who are pregnant or breast feeding need to drink more water?
Yes, expectant mothers and those who are breast-feeding need additional fluids daily to stay hydrated. Women at risk of gaining too much weight are encouraged to consume more water (no calories) and limit their consumptions of sweetened fluids (with calories).
14) What is hyponatremia?
Hyponatremia ("natremia" comes from the Latin word for sodium, and means "sodium status") means subnormal levels of sodium in the blood. This may occur in prolonged cardiovascular events such as a marathon. Symptoms include vomiting, headache, bloating, swollen feet and hands, disorientation, undue fatigue and wheezy breathing. Fluid intake overload is the main cause of exercise-induced hyponatremia. An excessive loss of total body sodium is another cause or contributing reason. Medical intervention is necessary in order to clearly discern whether symptoms are from a heat disorder or hyponatremia.
15) Is cold water absorbed faster in the body than warm water?
No, however cool or lukewarm water is soothing to the taste and absorbed faster in the body.

Side Bar 2: Definition of Terms Associated with Hydration
Electrolyte: A substance in solution that can conduct an electric current. Electrolytes in the human body include calcium, chloride, fluoride, magnesium, potassium and sodium.
Euhydration: A normal state of body water content also called normohydration.
Dehydration: The loss of water (due to exercise, illness, environment, medications {such as diuretics}, or fluid deprivation) and salts essential for normal body function.
Hydration: To supply water to in order to restore or maintain fluid balance.
Hypohydration: The removal of water from the body.
Hyperhydration: A state of excess fluids in the body, also called overhydration.
Hyperthermia: An acute condition which occurs when the body produces or absorbs more heat than it can dissipate.
Hyponatremia: An abnormally low concentration of sodium in the blood. Too little sodium can cause cells to malfunction, and extremely low sodium can be fatal.
Hypovolemic shock. A state of decreased blood plasma and volume, characterized by pale, cool, clammy skin with a rapid heart rate and shallow breathing. Also called physical collapse.
Osmolality: The amount or concentration of dissolved substances (known as solute) in a solution.
Osmosis: Diffusion of fluid through a semipermeable membrane from a solution with a low solute concentration to a solution with a higher solute concentration until there is an equal concentration of fluid on both sides of the membrane.

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Grandjean, A.C. and Grandjean, N.R. (2007). Dehydration and cognitive performance. Journal of the American College of Nutrition, Vol. 26(90005), 549s-554s.
Hughes, J. and Norman, R.W. (1992). Diet and calcium stores. Canadian Medical Association Journal, 146(2), pp. 137-143.
Institute of Medicine. Executive Summary. Dietary reference intakes for water, potassium, sodium, chloride, and sulfate. National Academy of Sciences, Food and Nutrition Board, National Academy Press, 2004.
Kleiner, S.M. (1999). Water; An essential but overlooked nutrient. Journal of the American Dietetic Association, 99, 200-206.
Murray, B. (2007). Hydration and physical performance. Journal of the American College of Nutrition, 26(5), 542s-548s.
Portis, A.J. and Sundaram, C. P. (2001). Diagnosis and initial management of kidney stones. American Family Physician, Vol. 63(7), pp. 1329-1338.
Sawka, M.N., Burke, L.M., Eichner, E.R., Maughan, R. J., Montain, S.J., and Stachenfield, N.S. (2007). American College of Sports Medicine Position Stand: Exercise and fluid replacement. Medicine & Science in Sports & Exercise, Vol. 39, pp. 377-390.
Valtin, H. (2002). “Drink at least eight glasses of water a day.” Really? Is there scientific evidence for “8 X8”? American Journal of Physiology - Regulatory, Integrative and Comparative Physiology, 283: R993-R1004.
Williams, M.H. (2005). Nutrition for Health, Fitness & Sport 7th Edition. McGraw Hill Higher Education.