Physiological Functions

Water is considered an essential nutrient because it must be consumed from exogenous sources to satisfy metabolic demand. Water constitutes approximately 60% of adult body weight. It is a catalyst for a majority of enzymatic reactions including those involved in nutrient digestion, absorption, transport, and metabolism. It is also required for facilitating excretion of metabolic waste by the kidneys. Inadequate intake of water compromises cell functions by contributing electrolyte imbalances, contraction of plasma volume, and inability to regulate body temperature.

Factors Affecting Availability

Water is not consumed in sufficient amounts by most individuals since thirst does not develop until body fluids are depleted well below levels required for optimal functioning. Mechanisms that trigger thirst sensations are stimulated by increased osmolality or decreased extracellular volume which are not detected until significant contraction of plasma volume has occurred. Groups most vulnerable to dehydration, infants, elderly adults, and athletes, are either not able to adequately express thirst sensations or to detect them. With extreme heat and excessive perspiration, thirst may lag behind actual water requirements. To prevent dehydration, a minimum of eight cups of fluid is required daily from beverages and foods.

Deficiency

Loss of body water amounting to 10% of the body weight impairs work performance and is associated with nausea, weakness, delirium, and hyperthermia. Signs of dehydration include poor skin turgor, skin tenting on the forehead, decreased urine output, concentrated urine, sunken eyes, dry mucous membranes in the mouth and nose, orthostatic blood pressure changes and tachycardia. Water losses exceeding 20% of body weight are life-threatening.

Toxicity

Water intoxication may develop if large amounts of water are provided to patients to replenish fluids lost with surgery, trauma or other conditions associated with fluid and electrolyte losses, especially if compromised renal function or hormonal imbalances are also present. The ensuing increase in intracellular fluid volume can cause swelling of brain tissue accompanied by headaches, nausea, vomiting, muscle twitching, convulsions, and even death.

Requirements

Consumption of approximately 2.5 to 3 liters (10.4-12.5 cups daily) or half your body weight in pounds of water is recommended to maintain optimal hydration. Both foods and beverages can satisfy this requirement. The equivalent of 8 cups of water (64 fluid ounces or 2 liters)

Dietary Sources

Water requirements are most effectively met by consumption of plain water or beverages which are > 90% water by volume. Water may also be obtained from solid foods such as fruits and vegetables which have a high water content. Low moisture foods such as grains and meat products do not contribute significantly to water intake. A Few Facts

About Hydration: Dehydration is easier to prevent than to treat.

Water and the Body: A Job Description

• Transports nutrients and oxygen to cells
• Ensures adequate blood volume
• Protects against heat exhaustion
• Acts as insulation in the cold
• Regulates body temperature
• Cushions joints
• Suppresses appetite
• Assists the body in metabolizing stored fat
• Relieves fluid retention problems
• Reduces sodium buildup in the body
• Helps to maintain proper muscle tone
• Rids the body of waste and toxins
• Relieves constipation
• Helps convert food into energy
• Maintains strength and endurance
• Protects organs

Progressive Effects of Dehydration and loss of body water

0-1%: Thirst
2-5%: Dry mouth, flushed skin, fatigue, headache, impaired physical performance
6%: Increased body temperature, rate of breathing and pulse rate; dizziness; weakness
8%: Dizziness, increased weakness, labored breathing with exercise
10%: Muscle spasms, swollen tongue, delirium
11%: Poor blood circulation, failing kidney function
The American Dietetic Association's Complete Food & Nutrition Guide (Minneapolis: Chronimed Publishing; 1996), p.168

• The National Research Council (NRC) uses a sliding scale of 1 milliliter of water for every calorie burned. The NRC says the average man — who burns about 2,900 calories daily — needs 2,900 milliliters, or about 12 cups, of water each day. The average woman — who burns 2,200 calories daily — needs about 2,200 milliliters, or about 9 cups, of water each day. For your own calculations: One measuring cup (8 ounces) of water equals 236 milliliters of water.
Mayo Clinic, Consumer Health Tips and Products, June 25,2002.

• The American College of Sports Medicine (ACSM) recommends drinking about 17 ounces of liquid 2 hours before exercise and drinking early and at regular intervals during exercise (5-8 oz every 15-20 minutes).
“Exercise and Fluid Replacement,” ACSM, Vol.28, No.1, 1-1996.

• Mild to severe dehydration commonly occurs among athletes, even when fluid is readily available. Consequently, it is in the athlete’s best interest to adopt fluid-replacement practices that promote fluid intake in proportion to sweat loss.
Int J Sport Nutr 1995 Jun;5 Suppl:S62-73.

• Vigorous exercise may delay the thirst mechanism, making it difficult to replace fluid loss without a plan for periodic consumption. Athletes should become accustomed to consuming fluid at regular intervals (with or without thirst) during training sessions so that they do not experience discomfort during competition.
Position Stand, J Am Diet Assoc 1993 Jun;93(6):691-6.

• Increasing dehydration, due to inadequate fluid consumption, directly impairs stroke volume, cardiac output, and skin blood flow, which results in larger increases in body core temperature, heart rate, and ratings of the difficulty of exercise.
Med Sci Sports Exerc 1992 Sep;24 (9 Suppl):S324-30.

• Hyponatremia (low blood sodium levels) as a result of over-hydration, has become more and more common in endurance athletes, and many governing bodies are changing their hydration recommendations accordingly. To prevent complications, endurance athletes are advised to be aware of the effect of dehydration as well as over-hydration and plan their fluid intake according to their individual needs.
The Physician and SportsMedicine - Vol 31 - No. 7 - July 2003

• The average American is chronically dehydrated and consumes only 4.6 servings of water per day.
Survey of 3003 Americans, Nutrition Information Center, New York Hospital-Cornell Medical Center (April 14, 1998).

• DRINK LOTS OF water and keep yourself on schedule” is an old health adage. Recent studies have demonstrated that drinking water is, indeed, associated with a substantial physiological response. Drinking 500 ml of water increased metabolic rate by 30%. The increase occurred within 10 min and reached a maximum after 30–40 min. The total thermogenic response was about 100 kJ (which equals about 96 kcal per day or a loss of 5.5 lbs per year).
Journal of Clinical Endocrinology & Metabolism 88(12):6015–6019, 03'.

• Dehydration is one of the ten most common causes for hospitalization among Medicare patients. In 1991, 6.7% (731,695) of Medicare hospitalizations had dehydration listed as a principal diagnosis, costing Medicare more than $446 million in hospital payments. Most importantly, the study revealed that about half of the people over age 65 who were hospitalized with illnesses accompanied by dehydration die within one year of admission.
Am J Public Health 1994 Aug;84(8)1265-9.

• Nursing facility residents are particularly at risk for dehydration. As many as 75% of residents have average fluid intakes below 1,500 cc per day, while a minimum recommendation is between 1,500 to 2,000 cc of fluid a day. These residents may need more opportunities and reminders to drink.
Practical Solutions to Preventing Dehydration, Illinois Council on Long Term Care, 1997 Feb, 7 (#198).

• New research indicates that fluid consumption in general and water consumption in particular can have an effect on the risk of urinary stone disease; cancers of the breast, colon, and urinary tract; childhood and adolescent obesity; mitral valve prolapse; salivary gland function; and overall health in the elderly. Dietitians should be encouraged to promote and monitor fluid and water intake among all of their clients and patients through education and to help them design a fluid intake plan.
J Am Diet Assoc 1999 Feb;99(2):200-6

By Feinberg School of Medicine, Northwestern University, HydraCoach Inc.