Factoid: about 71% of the earth’s surface is covered with water and, in a neat bit of evolutionary symmetry, the percentage of water in the human body varies from 45-75% according to age, gender and body composition - fatty tissue contains less water than lean tissue. The average is about 60%. Also, crucially, the percentage of water depends on your hydration level.

You might think from the picture above that carrying a water bottle is a bit of a trendy accessory. Wrong. It’s not just during a heat wave we need it – although we certainly need to up the sipping and glugging when it’s hot – but actually, we need to keep up fluid levels all the time.

We feel thirsty when we have already lost around 2-3% of our body’s water. But being dehydrated by just 2% negatively affects our performance, both doing mental tasks and physical coordination.

Water serves multiple purposes: to function properly all the cells and organs of your body need water.

1. Water lubricates the joints Cartilage, found in joints and the disks of the spine, contains around 80% water. Long-term dehydration can reduce the joints’ shock-absorbing ability, leading to joint pain.

2. Water forms saliva and mucus Saliva helps us digest our food and keeps the mouth, nose, and eyes moist. This prevents friction and damage. Drinking water keeps the mouth clean and, if you drink water instead of sweetened beverages, it can reduce tooth decay.

3. Water delivers oxygen throughout the body Blood is more than 90% water, and blood carries oxygen to different parts of the body.

4. Water boosts skin health and beauty With dehydration, the skin can become more vulnerable to skin disorders and premature wrinkling.

5. Water cushions the brain, spinal cord, and other sensitive tissues Dehydration can affect brain structure and function. Water is also involved in the production of hormones and neurotransmitters. Prolonged dehydration can lead to problems with thinking and reasoning.

6. Water regulates body temperature Water stored in the middle layers of the skin comes to the surface as sweat when the body heats up. As it evaporates, it cools the body. Some scientists have suggested that when there is too little water in the body, heat storage increases and the individual is less able to tolerate heat strain. Having a lot of water in the body may reduce physical strain if heat stress occurs during exercise.

7. The digestive system depends on water The bowel needs water to work properly. Dehydration can lead to digestive problems, constipation, and an overly acidic stomach. This increases the risk of heartburn and stomach ulcers.

8. Water flushes body waste It’s needed removal of urine and faeces as well as for perspiration.

9. Water helps maintain healthy blood pressure A lack of water can cause blood to become thicker, increasing blood pressure.

10. The airways need water When you are dehydrated, airways are restricted by the body in an effort to minimise water loss. This can make asthma and allergies worse.

11. Water makes minerals and nutrients accessible. Many of these dissolve in water, allowing them to reach different parts of the body.

12. Water prevents kidney damage The kidneys regulate fluid in the body. Insufficient water can lead to kidney stones and other problems.

13. Water boosts performance during exercise Some scientists have proposed that consuming more water might enhance performance during strenuous activity. One review found that dehydration reduces performance in activities lasting longer than 30 minutes.

14. Water may also help with weight loss. “’Preloading’ with water before meals can help prevent overeating by creating a sense of fullness. Also, it obviously helps if you swap sweetened drinks for water.

15. Water may help prevent DVT Dehydration is also linked to conditions such as deep vein thrombosis as I discovered when my husband had a DVT; read my feature here. And please don't think that only couch potatoes get DVTs: Jasmijn Muller, known as the queen of endurance cycling, was diagnosed with one a few years ago after dehydrated cycling in the heat. Here is her story.

So how much water do you need? This varies from person to person and on what you’re doing and how hot it is – if you’re perspiring a lot then you need to drink more. Also if you have an infection or fever, a health condition like diabetes or you are vomiting or have diarrhoea, which all cause you to lose water. Pregnant and nursing mothers will also need more.

About 20% of water intake comes from food but the rest you need to do yourself. The commonly quoted 8x8 rule – an 250ml glass eight times a day – is useful as, combined with the water in food, it comes to about the recommended 2.7 litres a day for women, 3.7 litres for men. But remember you may need more as I explained in the last paragraph.

One common question is whether drinking cold or iced water is as useful to your body as room temperature. The answer is that both have benefits. Cold water will lower your body temperature and help you cool down quicker – so it’s great during a heatwave or when you get overheated for any reason - whereas room temp or warm water may help your body assimilate it quicker so having a glass by your bed for the night and drinking a room temp water when you wake up may be most helpful.

To make it a bit more tempting, add squeezed lemon or lime juice, chopped cucumber or ginger, and a few mint leaves.

But however you consume it, the most important thing is… drink water!

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