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Water - why should we drink 8 glasses of water per day?

Sunday, 12 June 2016  |  Editor

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Water - why should we drink 8 glasses of water per day?

By Maria Esposito, BSc (Hons)

Nutritional Therapist

Normally, approximately 2.5 litre of water is required each day for a fairly sedentary adult in a normal environment to replace the total loss of water that occurs though urine, faeces, skin and lungs (Naghii).

To replace the water loss about 1 to 1.2 litre needs to be in drinks, (mainly water or cold herbal teas), 0.9 ml is taken though food and 0.3ml will be made by the body (oxidative metabolism) (William A. et al).

Effect of water restriction - What happens if you don’t drink enough?
The following are a series of changes in a case of water restriction:

  • Increase in pulse rate and in rectal temperature
  • Increase in respiration
  • Tingling and numbness of fingers and feet
  • Increase in concentration of blood
  • Diminution of blood volume and more difficult circulation

The difficulty and inadequacy of the circulation of blood under these conditions leads to:

  • difficult breathing
  • gastro-intestinal upsets
  • nausea and appetite failure
  • and eventually to difficulty in muscular movements and emotional instability (Lloyd)
  • In an animal study, where water was restricted the animals were highly irritable and in some cases bad tempered (Lloyd)


It has been shown that under intense activity a man will voluntarily drink only about half as much water as what is needed to replace his losses from sweating and urine. Only after food and rest does a person crave the water needed to replace the deficit. (Lloyd)

Herbal teas

Ingestion of hot drinks may cause internal body temperature to rise, which will result in sweating in order to cool the body down. Hot drinks are sometimes used in medical practice when it is desired either to temporarily increase body temperature of induce sweating (Lloyd). Therefore cold or cool drinks are preferred to hot drinks to replace fluid during the day, especially if the hot drinks are the only fluid daily intake.

In high intensity exercise and in competitive sports

In a sport or activity with duration of at least 30 minutes of continuous exercise, there is a risk of impaired performance and even health damage due to hypo hydration (low water). Sweating is usually the form accounting for most water loss during exercise or high environmental temperatures. The major danger of low fluid intake during a high performance is dehydration. This fluid deficit can rapidly reach level that impede heat dissipation, reduce heat tolerance and severely compromise cardiovascular function and exercise capacity. Prolonged dehydration will lead to cell death and multiple cell losses can lead to the person’s death (Naghii).

The need for replacement of water will depend on the extent of the losses incurred during exercise. Ingestion of plain water for a short duration (less than 90 minutes) is enough to replace the water lost.

In high intensity activity and competition ingestion of plain water in the post-exercise period results in a rapid fall in the plasma sodium concentration and in plasma osmolality (Maughan). The changes have the effect of reducing the stimulus to drink (thirst) and of increasing urine output, both of which will delay rehydration. Rehydration after the high activity can only be achieved if the sodium lost in sweat is replaced as well as the water (Maughan). There is no need to take any extra sodium tablets, unless dehydration is so severe, for most individuals the normal dietary intake of minerals is adequate to maintain sodium and electrolyte balance during repeated days of training (Wiliam A). It is sufficient to eat a salty food or food high in natural salt.

Sports drinks typically provide quantities of sodium, chloride and potassium (electrolytes), which can be drunk if in high activity or in competing in a hot environment.

Sugary drinks

During physical activities lasting less than 90 minutes, water is generally the beverage of choice, however, the inclusion of small concentrations of carbohydrate (6%-8% carbohydrate) have been shown to better sustain power output over water alone during physical activities that produce fatigue in 60 minutes or longer (William A.).

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