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Water and Health. The effects of dehydration Hilary Forrester Research for Water UK. The fact that water is good for our health seems fairly obvious. Benefits were recognised by ancient physicians, eg Hippocrates. But the exact reasons why water keeps us healthy have not always been clear –

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water and health

Water and Health

The effects of dehydration

Hilary Forrester

Research for Water UK

The fact that water is good for our health seems fairly obvious.
  • Benefits were recognised by ancient physicians, eg Hippocrates.
  • But the exact reasons why water keeps us healthy have not always been clear –
  • Pisanelli, physician 1586 had a curious explanation for the health benefits of chilled drinking water:
    • He noticed that a glass of cold water becomes clouded “as if it were coated by expelled noxious humours” and concluded that drinking cold water would have the same ability to remove toxic substances from the body.


  • Water forms about 60% of body weight.
  • One third of body water is outside the cells, eg blood, lymph etc.
  • Two thirds is inside cells (intracellular)
  • Water can move freely between these compartments because cell membranes are permeable to water.
Water gains and losses
  • The main sources of water are:
    • fluid drunk and
    • any water contained in food.
  • Third source is metabolic water – which is created by the body as a result of the breakdown (oxidation) of food components, eg fats, proteins and carbohydrates.
  • Water losses occur via urine, faeces and evaporation from the skin and lungs.
    • The amount lost from skin is about 920 ml per day,
    • and from the lungs about 300 ml,
    • depending on exertion rates, temperature and humidity.
  • In order to remain healthy, water gains and losses must be balanced.
Daily water requirements
  • Normal daily turnover of water is about 4% of total body weight in adults.
  • For a 70kg adult this is about 2.5 to 3 litres per day under average conditions
  • The recommendation from the Food Standards Agency is that “average sized” adults should drink about 2.5 litres per day, (about 8 glasses) more in warm weather or if you exercise.
  • Pregnant women need about an extra 30ml of water per day due to the needs of the fetus and amniotic fluid.
  • Breastfeeding women need about an additional litre of water per day to replace that lost in milk.
  • In children the daily turnover of water is about 15% of total body weight. Compared to adults they have a larger surface area to body weight ratio and so are more likely to lose water by evaporation. In addition their kidneys are less efficient at concentrating urine so they can’t “save” as much water as adults can.
Estimated actual water intakes of populations
  • There aren’t many studies into people’s actual water intakes.
  • But those that exist, clearly show that we don’t drink enough.
  • One study found that on average, people drink less than one-third of the recommended daily amount, ie 800ml instead of 2,500 ml !
  • Another study showed average water consumptions to be as low as 1.5-1.7 litres per day.
  • Defined as a 1% or greater loss of body weight as a result of fluid loss.
  • This is mild dehydration, but even at this level, some physical functions are impaired.
  • Dehydration in excess of 3-5% of body weight decreases endurance, strength and mental abilities and can cause heat exhaustion.
  • Loss of more than 10% of body weight is life threatening.
Signs of dehydration
  • Some early signs of dehydration are:
    • Sunken features – particularly the eyes
    • Headache
    • Tiredness
    • Loss of appetite
    • Flushed skin
    • Heat intolerance
    • Light-headedness
    • Dry mouth and eyes
    • Loss of skin elasticity
    • Reduced urine output
    • Dark urine – healthy urine should be “very pale yellow, or “straw coloured”
  • The kidney’s play a key role in regulating the amount of water in the body. They help maintain water balance by adjusting the concentration of urine.
  • However, there is a limit to how much they can concentrate the urine because they require a minimum amount of water to excrete the body’s waste products.
  • Thirst is triggered when the kidneys cannot save enough water to prevent an increase in plasma sodium levels and osmolality.
  • But, the trigger of “thirst” only occurs at a point when a person is already dehydrated to a level of about 2% loss of body weight.
  • So in other words – “Don’t wait until you are thirsty before you drink” because by that time you will already be suffering the effects of dehydration.
  • Incidentally, tea, coffee and other caffeine drinks don’t count towards your total daily water intake because caffeine is a diruetic – which means that it causes the body to lose water. In fact it quenches your thirst without actually increasing your body’s water content. In one study the consumption of 750mg of coffee (over the course of a day) resulted in an overall decrease in total body water of 2.7%
Adverse health effects of dehydration
  • We have seen that we should be drinking about 2.5 litres of water per day,
  • and that it is very easy to become dehydrated.
  • Also that the majority of people do not drink enough.
  • But you may be thinking – slight dehydration, I can cope with that, it only causes a headache or feeling a bit tired.
  • This is where my research uncovered some quite startling findings about the potential damage that we could be doing to our health even by being only mildly dehydrated…
Adverse effects of dehydration
  • The research findings are detailed, so I’m going to run through them quite quickly without quoting the authors, just to give you a flavour.
  • Breast cancer
    • Women who drink an adequate amount of water can cut their risk of breast cancer by 79%
  • Colon cancer
    • The risk of developing colon cancer was reduced by 45% in women and 32% in men who were adequately hydrated.
    • Prevention of this type of cancer is particularly important since research has shown that sufferers of colon cancer are more likely to go on to develop other forms of cancer.
  • Bladder, kidney, prostate and testicular cancer
    • are all associated with dehydration and research shows that adequate intake of water can protect against such cancers.
  • The main theory about why increased water consumption has a protective effect against cancer is that because water balance is closely linked to cell metabolism, dehydration may adversely affect the cells ability to get rid of carcinogens.
Cardiovascular disease
    • Controlling high blood pressure is important for the prevention of cardiovascular disease. Essential hypertension is usually viewed as being due to inappropriate excretion of sodium from the body (hence low salt diets). However, the body uses water to get rid of excess sodium – and this function is impaired by dehydration. Adequate water intake is therefore also important in controlling hypertension.
    • In addition, there is some evidence to suggest that the mechanical function of the heart may be affected by dehydration. Cells shrink when dehydrated and this may alter the distances between cardiac muscle cells and consequently affect the transmission of electrical impulses that stimulate contraction of the heart. In addition, cell shrinkage may disturb intracellular calcium levels which are also important for muscle contraction.
  • Gallstones
    • Inadequate daily water intake is associated with the formation of gallstones since they are more likely to if the bile is too concentrated.
  • Oral health
    • Saliva is important for good oral health, however saliva production is impaired by dehydration.
Pressure sores
    • Particular important in older people. Dehydration leads to a loss of padding especially over bony points and hence increases the risk of ulcers forming.
  • Falls in the older people
    • Falls in older people can result in factures and even death. Dehydration is one of the causes of falls as it can lead to dizziness and fainting. Older people are particularly susceptible to dehydration because they have a diminished thirst response.
  • Urinary tract
    • Dehydration leads to the formation of concentrated urine and this is associated with the formation of urinary tract stones (kidney stones). Urinary tract stones can also increase the chances of getting urinary tract infections.
    • Urinary tract infection can themselves cause serious damage to the kidneys, particularly in children. Drinking adequate amounts of water and emptying the bladder regularly can help protect against UTIs.
    • Inadequate intake of water during pregnancy can lead to a serious condition called oligohydramnios, where there is too little amniotic fluid surrounding the baby. This may result in too few nutrients and oxygen reaching the baby and prevent the baby turning to the cephalic position (head first) before birth. In very severe cases it can impair fetal lung development. Drinking adequate amounts of water has been shown to increase amniotic fluid volume in such cases.
  • Skin
    • The skin acts as a water reservoir and participates in the fluid regulation of the whole body. Skin thickness depends, among other things, upon its fluid content. In mild dehydration the skin may appear flushed (because it is thinner and blood vessels are more obvious). It may also be dry and loose due to loss of elasticity. Since loss of skin elasticity is also a sign of ageing – dehydration may make your skin appear older than it actually is.
    • The cosmetics industry would do just as well promoting drinking water as anti-ageing creams!
Sport and exercise
  • There is a lot of money in sports research and hence a lot of data on the effects of dehydration on athletic performance.
  • Briefly, mild dehydration of as little as a 2% decrease in body weight can reduce aerobic endurance and cause increased body temperatures, heart rate, and perceived exertion
  • At a dehydration level of only 2%, physical performance is reduced by 20%.
  • Performance become substantial impaired when fluid losses exceed 5% of body weight.
  • Swimmers are particularly susceptible to dehydration because water immersion depresses the thirst response.
Mental performance
  • And finally… dehydration can also affect mental performance to quite a surprising degree.
  • Studies have shown that mental performance is adversely affected even at very low levels of dehydration (1% loss of body weight), but the effects become highly significant at a mere 2% loss of body weight.
  • Functions that were affected included, short-term memory, hand-eye motor coordination, symbol recognition and classification, arithmetic efficiency, attention and concentration.
  • At greater levels of dehydration the decline in mental performance becomes progressively worse.
So we have seen that whilst it might seem obvious that water is good for our health, medical science is only just beginning to uncover the extent of it’s protective role.
  • Dehydration has now been implicated in a surprisingly large number of conditions, some of which can be very serious, eg cancer.
  • We are all aware of the dangers of driving when tired, but driving when dehydrated could be just as dangerous in view of the detrimental effects on mental performance.
  • Future research will no doubt strengthen our knowledge of water’s vital health protection role.
  • So, now that you know the risks… make sure you drink enough water – aim for 2.5 litres per day, and don’t wait until you are thirsty before you drink because by then it will be too late!