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Daily water requirements

Hydration / dehydration

Water for health and performance.

By Dr. L. Lee Coyne, the Healthy Professor

Dr. L. Lee CoyneNearly all the bio-chemical reactions that occur in body cells depend on water and electrolyte (sodium, potassium, calcium, chloride, phosphorous, magnesium, etc.) balance. These balances are not only vital to maintaining life but also affect physical and mental performance.

Water is the most abundant component of the body (60% + by weight). I believe it was Mike Colgan of the Colgan Institute who referred to the body as a "Hairy protein bag full of water". This bag of water has many holes which allow for leakage. These holes include skin pores which allow for perspiration (skin leakage), the kidney / bladder system which expels wastes carried by water and the respiratory system which must be moist or breathing would be very dry and painful.

Adequate hydration is very important in the maintenance of body temperature. When muscles contract they generate heat which must be dissipated from the core to the body surface and adequate water to maintain adequate blood volume is vital.

Blood, kidney, heart and lungs are made of 80% or more water. Muscle, spleen, brain, intestines, & skin are 72 - 75% water. Even bones are 22% and fat tissue is 10% water. On a normal, moderate temperature, inactive day you would lose 1.5 liters (6 glasses) of water through kidney filtration (urine production) and another 0.750 - 1 liter (3 - 4 glasses) through the skin and respiration.

So an average person needs 8 - 9 glasses per day just to replace average losses. It is true you get some of that from fruits, vegetable, other beverages and food. My "rule of thumb" for water requirements has long been - weight in pound / 2 = oz. of water / day. Caffeinated, alcoholic and many carbonated beverages have a diuretic effect and actually increase the daily fluid requirements. One should choose pure (that would take up another column) water or high quality sport beverage in some circumstances.

Naturally, daily fluid requirements will vary with environmental conditions, clothing and exercise intensity and duration.

Even mild dehydration - 1% of body - which would represent approximately .75 to 1 litre of water (1% of 75 Kg = 750 ml.) can create a reduction in muscle performance and start to show dehydration symptoms. Early symptoms are headaches, dry eyes (ask any contact lens wearer what happens after a couple of glasses of wine), drowsiness, loss of concentration, irritability.

If the dehydration is 2 - 3 % , serious performance inhibition occurs. Dr. David Costill demonstrated that at these low levels of dehydration 1 - 3% even the time for 1500 meters was inhibited. The time for a competitive 10 K was reduced by 2.5 minutes which is serious in a 30 min 10 K.

Muscle cramps are also a sign of inadequate fluid replacement and electrolyte loss, particularly calcium and magnesium. Even "Lactate threshold" - an indicator of maximal work performance ability is lowered which is not a good thing in high intensity, endurance competition. Thicker blood, fast heart rate, negative changes in blood pressure are other symptoms.

Don't wait until you are thirsty to decide to drink. Fluid replacement is part of a daily plan. Thirst is a sign - too late - of dehydration, performance is already impaired.

You actually lose significant fluid just sitting in an air conditioned car or office. Frequent drinks of water during a long automobile trip will reduce apparent road fatigue. The same applies to sitting at your desk. A friend has a water bottle holder mounted on the dash of car to encourage convenient hydration while driving.

Here are typical water losses during exercise : 1 hour of weight training = 8 oz; 45 minutes of swimming = 10 oz, a softball game = 16 oz; 5 mile run = 24 oz, 45 minutes of full court basketball = 24 oz; bicycling for 1 hour = 33 oz. and a marathon = 116 oz.

As the environmental temperature, exercise intensity and / or duration increases, you need to drink more and may want to switch to a quality sport drink (one made with a glucose polymer like maltodextrin rather than table sugar and 6 - 8 electrolytes rather than just sodium and potassium) to avoid a condition known as Hyponatremia or water intoxication caused by electrolyte loss and excess water intake.

During the famous Daedalus man powered flight over the Aegean Sea (energy equivalent of 3 non-stop marathons) in 1988, the athlete lost only 1.5 Kg and had normal blood chemistry at the end. He drank a cup of high quality sport drink every 15 minutes for just over 4 hours.

One last point - cool beverages are absorbed better than room temperature or warm beverages.


Lee Coyne, Ph.D. is a nutritional consultant, lecturer and author of Fat Won't Make You Fat and the Lean Seekers coaching program. He may be reached at 1-800-668-4042 or by e-mail