Drinking enough water is vital for life and to maintain short and long term health. Water regulates our temperature, transports nutrients and compounds in blood, removes waste in the urine and is involved in joint lubrication.

Fluid loss is affected by age, genetics , body size, fitness , environment and activity

Marketing of individual bottles of water has resulted in many myths about fluid intake, such as :


  • If you re thirsty you are already dehydrated
  • Drinking water prevents overeating
  • Drinking water flushes out fat
  • Students need to drink water during daytime classes to maintain their concentration
  • Water is a tonic for the skin
  • Drinking extra water prevents constipation
  • Flushing the colon prevents bowel cancer
  • Drinks containing caffeine cannot be included in fluid intake assessment
  • Drinking 8 glasses of water per day boosts metabolic rate

Adults are good at regulating their fluid requirements, but the mechanisms involved are not fully developed in children and may not function effectively in older people, some of whom also ignore thirst due to difficulties with toilets. Those who are ill with diarrhoea and vomiting may also need further advice , as do those taking diuretic medication and those  exercising over more than 30 -60 minutes, particularly in hot and humid weather and at high intensity.

The 8 glasses of water advice is thought to have started with a recommendation from the Food and Nutrition Board of the National Research Council  in America in 1948. An intake of 1ml of water for each calorie of food was suggested as the average daily requirement. The average person requires 2000 calories daily ( equated to 2000ml) . The Nutrition Council went on to also state that " most of this quantity is contained in prepared foods".

No studies since then have supported the idea of 8 glasses of water daily for health, particularly in addition to that contained in foodstuff,  but have provided guidelines in illness, in surgical procedures and resuscitation, for athletes and to replace fluids when dehydration is diagnosed. Several surveys have demonstrated that the average person actually consumes less than 8 glasses of water daily with no evidence of ill effects.

Many studies have demonstrated that tea and coffee can be counted in the fluid requirements. Caffeine acts as a weak diuretic at doses greater that 500mg ( about 4 espresso coffees), but taken as a fluid did not cause dehydration. Studies of athletes undertaking high intensity exercise in hot conditions showed no dehydration effects from caffeine. A recent study reviewed the effects of 4 cups of coffee or water and found no differences in haematological or urinary markers of hydration status in people.

Constantly pushing people to drink extra fluids can be harmful, causing "water intoxication" in those unable to excrete enough in their urine. Blood sodium levels drop too low, causing headache unconsciousness and coma.


references : Valtin H. " Drink at least eight glasses of water a day. Really? Is There Scientific Evidence for 8?. American Journal of Physiology, Regulation, Integration and Comparative Physiology 2002 283:R993-1004

Exercise Sport Sci Rev. 2007 Jul: 35 ( 3) Caffeine, fluid - electrolyte balance

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