Water makes up a large proportion of the body – on average 60% of body weight in men and 50-55% in women (because women have a higher percentage of body fat). Water has many functions in the body including regulating temperature, transporting nutrients and compounds in blood, removing waste products that are passed in the urine and acting as a lubricant and shock absorber in joints. Water is lost in urine and in sweat and is also being lost throughout the day when you breathe and when small amounts of water evaporate through the skin. To avoid dehydration you need to replace this fluid regularly with fluids from food and drinks. ‘Fluid’ includes not only water from the tap or in a bottle, but also other drinks that provide water such as tea, coffee, milk, fruit juices and soft drinks. You get water from the food you eat as well – on average it’s estimated that food provides about 20% of your total fluid intake.
If you don’t consume enough fluids, over time the body will become dehydrated. Studies have shown that at about 1% dehydration (the equivalent of 1% of body weight water loss) there are negative effects on mental and physical function and these become more severe as dehydration gets worse. Symptoms of mild dehydration include a dry mouth, headaches and poor concentration. When the body detects that more water is needed the first thing that happens is that the kidneys reduce the amount of water lost in the urine. This means that the colour of the urine becomes darker and you can use the colour of your urine to tell if you are well hydrated – if you are drinking enough your urine should be a straw or pale yellow colour. If it’s darker then you probably need more fluid. Thirst kicks in when the body is already a little dehydrated, so it is important to drink when you are thirsty.
Adults need to drink around 1.5–2 litres of fluid a day. A typical mug or glass is about 200 millilitres (ml) so this equates to 8-10 drinks a day. Children need slightly less and should aim for around 6-8 drinks a day, but once they reach teenage years their requirements are similar to adults. Don’t forget that fluid needs can vary depending on various factors including level of physical activity and climate, so it is best to remember to drink regularly to keep thirst at bay.