What water does in the body: seven jobs you will notice

How much water is in you

An adult is 50 to 65 per cent water, children more, older people less. Muscle contains around 75 per cent water, fatty tissue only about 10 per cent, which is why the share varies with body build. The brain is at about 75 per cent, blood at over 90 per cent. So water is not merely “present”: it is the substance in which almost everything else takes place.

Seven jobs

1. Transport: Blood is just over 90 per cent water. It carries oxygen, sugar, vitamins and hormones to where they are needed and takes away carbon dioxide and waste.
2. Cooling: Sweat evaporates and takes heat with it. Without water there would be no temperature control, and heat would become dangerous within hours.
3. Detoxification: The kidneys filter around 180 litres of blood fluid a day and excrete about 1.5 litres of it as urine. Too little water means more concentrated urine and more work for the kidneys, and over time a higher risk of kidney stones.
4. Joints and tissue: Joint fluid, intervertebral discs and cartilage need water to cushion and glide.
5. Digestion: Saliva, stomach acid and intestinal mucus are watery. When water is short, stools become hard, and constipation is often a drinking problem.
6. Brain: Even a loss of one to two per cent measurably impairs reaction time, short-term memory and mood. Headaches are a common first sign.
7. Skin: Water keeps the cells plump and the skin elastic. Drinking is no miracle cure, but a lack of water shows in the face first.

What happens when water runs short

At a loss of one per cent, thirst makes itself known. At two per cent, performance and concentration drop, the blood thickens and the pulse rises. At three to four per cent come headaches, dizziness and dry mucous membranes. From five per cent it becomes serious: confusion, cramps, circulation problems. Ten per cent is life-threatening. For a person weighing 70 kg, one per cent is already 0.7 litres, and a hot afternoon without drinking is enough to get there. How to spot the signs early is covered in Signs of dehydration.

What water cannot do

It does not detox “better” if you drink more than you need, and it does not burn fat. Water does not replace minerals you take in with food, and it does not cure diseases. It does something much simpler: it keeps the body in the state in which it can help itself. The Water intake calculator works out how much you need for that.