Don't forget that in hot weather, if you're physically active or if you have a fever, you'll lose more fluid from sweating and increased breathing rate. That means you'll need to drink far more fluid on a sunny day than in winter. As you get older you tend to be less active, but probably do need just as much fluid as more physically active younger people because your body is less efficient at preserving water by reabsorbing it in the kidneys.

Water tablets - what you need to know

Water tablets, or 'diuretics', are commonly given for high blood pressure or heart failure. They can increase your risk of dehydration in hot weather, so be vigilant for symptoms of dehydration. However, they're usually given because you have too much fluid on board, so don't overcompensate by drinking more.

Do you remember the days when every school playground had a water fountain? Admittedly, much of the water was squirted over other passing children, but I still remember queuing up for a cooling drink after a hot run around the playground. No longer, it would seem - in a recent survey, three quarters of parents said their children's school didn't have a water fountain and nearly two thirds said the school didn't provide water throughout the day, including banning it from the classroom. Yet there's good evidence that being dehydrated affects kids' concentration and behaviour.

Fizzy drinks - beware!

Most doctors these days advise avoiding sugary drinks because of the number of 'empty' calories they contain and the damage they can do to your teeth. A can of fizzy drink has over 140 calories - if you're eating all the calories you need, adding a single daily can of fizzy drink adds up. Diet drinks get around the calories but recent studies suggest they may trick your body into laying down more fat, so are best taken in moderation. Water carries no such risks, so drink it freely!

Non-alcoholic fluids, including tea, coffee and fruit juice, all count towards your fluid intake. A lot of people believe, mistakenly, that tea and coffee are diuretics and dehydrate you. In fact, below about 400 mg a day of caffeine, caffeine-containing drinks don't dehydrate you and can count towards your daily fluid intake. A mug of instant coffee contains about 100 mg and a cup of tea about 50 mg - so you'd need to pass the eight-cups-of-tea a day mark to have to worry!

Alcohol doesn't count!

All alcohol has a diuretic effect - that means that far from hydrating you, it makes you pass more water. "I'm keeping my fluid intake up with a pint or three" is one excuse you can't use to offset all the dangers of excess alcohol to your health.

Early symptoms and signs of dehydration include:

• Dark (rather than pale straw-coloured) urine.

• Headache.

• Tiredness.

• Not passing water often.

• Poor concentration.

• Feeling tired, dizzy or light-headed.

In more serious cases, you may feel physically weak - hence the term 'heat exhaustion', which is mostly due to dehydration. Hopefully you'll never see anyone with, let alone suffer yourself from, severe dehydration - this can be life-threatening. Symptoms include sunken eyes; confusion and irritability; rapid, weak pulse; sagging skin and cold hands and feet.

It might seem obvious that if you're dehydrated, you'll feel thirsty. But it's important to remember that as you get older, your body gets less good at reading the symptoms of dehydration and 'telling' you to drink. That makes you more prone to dehydration.

With thanks to 'My Weekly' magazine where this article was originally published.