Water and Hydration

Keeping ourselves hydrated is probably the most important part of our daily lives. We can only survive for a few days without water while we can live for around 8 weeks without food. Our body weight consists of around 65% water. The majority of this is contained within our cells. The remainder is contained in the lymph and circulatory systems and some is used for assisting excretions and secretions.

What does water do?

  • It assists with the absorption, excretion and circulation of nutrients into and out of cells. It also assists with carrying the nutrients in the blood
  • Aids digestion
  • Is a medium for all chemical exchanges
  • Is a solvent for the body’s substances
  • Assists in removing cellular waste
  • It is used for building and repairing the body
  • It is needed for body temperature regulation and distributing heat around the body from areas where it’s produced to cooler sites such as the skin’s surface
  • It lubricates the moving joints and eyes

Fluid depletion

We lose water in the body mainly by urinating, breathing and sweating. Dehydration happens when lost fluid is not replaced. Changes in the fluid balance of as little as 1-2% can result in dehydration which can have negative consequences. A 10% loss of body fluid through dehydration is life threatening.

The effects of chronic dehydration include:

  • Constipation
  • Lethargy
  • Dental disease (dehydration is associated with salivary dysfunction and saliva is essential for maintaining oral health)
  • Headaches
  • Impaired cognitive function
  • Confusion in the elderly – can result in dizziness and fainting and can increase the risk of falls
  • Skin changes – flushed skin, dry skin and loose skin with loss of elasticity
  • Increased risk of urinary tract infection and renal stones

Why do we sweat?

The purpose of sweating is to keep your core temperature at around 37 degrees Celsius. This is why we sweat when we exercise as the heat we generate whilst exercising has to be dissipated in order to maintain this temperature. There are several factors which can determine how much each individual sweats including:

  • The exercise intensity
  • Length of exercise
  • Environmental factors (temperature and humidity)
  • Body size
  • Fitness levels
  • Gender (women tend to sweat less than men)

During a normal 1 hour exercise session, the average person should be expected to lose around 1 litre of fluid.

How much do we need?

The amount of fluid needed for each individual person will depend on various factors such as environmental temperature, humidity, individual metabolism, activity levels and general state of health.

The World Health Organization (WHO) provide general recommendations for the average person:

  • Men (based on a 70kg male) – 2.5 litres a day
  • Woman (based on a 58kg female) – 2.2 litres a day
  • Children – 1 litre a day (for a 10kg child) or 0.75 litres a day (for a 5kg child)

The water requirements can be met through food as well as fluid. Some foods such as fruit and vegetables contain high amounts of water.

Thirst is not the best indicator of fluid status as thirst is a response to dehydration so you are technically dehydrated once you feel thirsty. The best method of measuring your hydration level is through the colour of your urine. As seen in the chart below, your urine should be pale or pale yellow. The darker it is, the more dehydrated you are.


If you experience sluggishness, fatigue, loss of appetite, headaches, feeling excessively hot, light-headedness and nausea, start drinking immediately.

You will need to increase your fluid intake if you are ill with sickness or diarrhoea to replace lost fluids and you may need to up the intake the more active you are due to sweating.

Different Drinks and Their Effects on Hydration

Many of us rely on drinks other than water such as tea or soft drinks. This is how useful they are in rehydrating our bodies:

  • Tea & Coffee – Approximately 40% of the nation’s fluid intake will come from tea. 80% of UK adults also regularly drink coffee. Both of these drinks have caffeine in them which can have a mild diuretic effect (causes increased output of urine). However, a recent review of the evidence linking caffeine with increased urine production observed that this generally only happens when at least 300mg of caffeine (or 5-6 cups of tea or 4 cups of coffee) is ingested. As a cup of tea is 99.5% water, it can be a useful way of hydrating ourselves as long as we don’t go overboard. However, it is advised that people with high blood pressure and pregnant women should limit their caffeine consumption.
  • Carbonated Soft Drinks – These include drinks such as Coca Cola, Lucozade, Ribena, 7Up, etc. These types of drinks are very high in sugar which add a lot of unwanted calories to our diets and can cause damage to our teeth. Even drinks labelled ‘sugar free’, ‘reduced sugar’ or ‘low sugar’ can still cause damage to our teeth because they have the same acids as the standard carbonated drinks. They are best kept to a minimum.
  • Hypertonic Soft Drinks – Hypertonic describes a drink that has a sugar (glucose) concentration which is higher than the glucose concentration of the blood (this is typically greater than 8g per carbohydrate per 100ml). If the concentration in the drink is higher than that of the blood, the body will have to provide fluids to dilute the drink before absorption can take place. This means that a considerable amount of water is initially removed from the body in order to process the high levels of sugar. This will delay swift fluid replenishment. Soft drinks are also very acidic.
  • Fruit Juices – Pure fruit juices can often have as much sugar as carbonated soft drinks. However, pure fruit juices also contain the same vitamins, minerals and phytochemicals as fresh fruit (although not the fibre or pectins). It is recommended to only have one glass of unsweetened fruit juice a day. They are also hypertonic as the sugar concentration is more than that of the blood. You can dilute fruit juices with water to make it a more effective rehydrating drink.
  • Fruit Juice Drinks – These are different to the pure fruit juices mentioned previously. Unlike ‘pure fruit juices’ and ‘freshly squeezed juice’ which contain 100% fruit juice, fruit juice drinks can contain as little as 5% real fruit juice. The rest is made up of a combination of sugar, water and often flavourings, colourings and sweeteners. These drinks won’t have the same health benefits and the added sugar in them can lead to dental problems. Check the labels to see how much sugar and real fruit juice they contain.
  • Dairy Drinks – Milk, milk-flavoured drinks and yoghurt drinks provide us with calcium which helps to strengthen our bones. Milk also contains 5g of carbohydrates per 100ml. This is similar concentration to that of the blood so the water content is easily absorbed into the body. A glass of milk is 85-90% water. Milk-flavoured drinks and yoghurt drinks may have added sugar so it’s best to check the labels. It is advised for the general population to use semi-skimmed or skimmed milk rather than whole milk as they are lower in fat but still contain the same amount of calcium. For children under five, whole milk should be used although semi-skimmed milk can be used from the age of two as long as the rest of the diet provides adequate amounts of energy.
  • Alcohol – Alcohol can have a dehydrating effect on our bodies because it depresses the production of the anti-diuretic hormone called vasopressin (the hormone which normally acts on the kidneys, concentrating the urine by promoting the re-absorption of water and salt into the body). Other side effects of alcohol consumption include tiredness, attack of free radicals (harmful molecules formed in the liver as it struggles to break down ethanol), headache and loss of co-ordination. Alcohol provides 7kcal per gram so can be a large provider of empty calories in our diets. The Department of Health advises that men should not regularly drink more than 3-4 units of alcohol per day and women should not regularly drink more than 2-3 units of alcohol per day. When drinking alcohol, alternate alcoholic drinks with water to counter-act the dehydrating effects.

The Bottom Line

The best way to keep yourself hydrated is to drink around 6-8 glasses of water a day. This can also include other types of hydrating drinks such as tea (including fruit teas and green tea), coffee (moderately), pure fruit juice and milk. Try to limit any drinks which have added sugar and alcohol to prevent diuretic effects and added calories which can contribute to obesity and health problems. Take a bottle of water with you wherever you go so you can easily top up your water supply.

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