Water is the largest single component of an individual's body. Total body water (TBW) constitutes 60% of body weight in adult males and 50% in adult females. It decreases with age and is higher in athletes than in non athletes. Of the TBW, 38% is located intracellularly (referring to the inside of the cell) and 62% extracellularly (referring to outside the cell including blood plasma and lymph, saliva, fluids in the eyes, skin, tendons, muscles, skeleton, kidneys, etc.). Water is a vehicle for the transportation of nutrients, regulation of body temperature and participation in metabolic reactions.

WATER BALANCE: INTAKE VERSUS OUTPUT

The maintenance of a constant normal volume of water in the body is essential to health. There is no provision for water storage in the body; therefore, the amount lost every 24 hours must be replaced to maintain health and efficiency.

The sources of water intake and output are shown in the following diagram:

• Normal daily intake (2.5 liters)
• Normal daily water output (2.5 liters)
• Liquids (1.2 liters)
• Foods (1.0 liters)
• Metabolism (0.3 liters)
• Urine (1.5 liters)
• Skin (0.6 liters)
• Lungs (0.3 liters)
• Feces (0.1 liters)

Water Intake

Liquid intake averages 1.2 to 1.5 liters per day. During exercise and thermal stress, water intake must increase.

Foods, especially fruits and vegetables, contain large quantities of water.

Metabolic water is derived when molecules are degraded for energy and represents about 12 % of daily water.

Water Output

Under normal conditions, the kidneys eliminate 1.5 liters per day.

Water is lost in the form of sweat, a refrigeration mechanism to cool down the body.

Note: Water balance is directly related to the concentration levels of sodium and potassium. Excess sodium lowers potassium and causes water retention. Therefore, it is prudent to read labels on prepared food and not to over salt your meals. Keep in mind that restaurants usually use large amounts of salt to flavor food.　

WATER IN EXERCISE

Exercise and sweating places demands on the body's fluid reserves and creates a relative state of dehydration. If sweating is excessive and fluids are not continually replaced, it severely compromises cardiovascular function & work capacity; it can also lead to a reduction in blood volume, sweat and core temperature which may rise to lethal levels. Exercise in hot and humid environments poses a great challenge to temperature regulation. In extreme conditions, fluid intake should be up to five or six times above normal.　

WATER AND THE KIDNEYS

The main function of the kidney is to maintain a balance with respect to fluids, electrolytes (sodium, potassium and chloride) and various other molecules. A fixed amount of waste products from the normal breakdown of food is sent to the kidney and must be eliminated. Water intake of less than 0.5 liters makes it impossible for the kidney to eliminate all of the daily waste. The amount of water taken in affects the amount of urine in which waste products can be excreted. The output of urine form the kidney can range from a volume of as little as 0.5 liters to as much as 12 liters. Therefore, the more water ingested, the higher the volume of urine and the less concentrated it will be.

HELPFUL TIPS

• Drink a glass of water first thing in the morning.
• Drink another glass of water just before bed.
• Match each cup of coffee with two cups of water.
• Match each glass of wine, beer and hard liquor with two glasses of water.
• Carry a water bottle around with you all day long.
• Put lemon or orange slices in your water to make it more palatable.
• Drink from a fancy glass to heighten the pleasure of drinking. This trick works for children and adults.
• When exercising, drink water before, during and after.

References:

Clinical Nutrition (2nd Edition), David M. Page; Washington, D.C .:C.V. Mosby Co. Publishers 1998.Modern Nutrition in Health and Disease (7th Edition), Maurice E. Shils, M.D., Sc.D.& Vernon R. Young; Ph.D.Exercise Physiology: Energy, Nutrition, and Human Performance (3rd Edition) Williams D. McRadle, Frank I. Katch, Victor L. Katch; Philadelphia: Lea and Febiger Publishers, 1991.