With the weather getting hotter, it's time to think not just about summer clothes, cleaning air-conditioners and planning a holiday, but also about drinking. Let me clarify that I mean drinking water - and maybe water with some salts and sugars. Alcohol - while quite good in small amounts for staving off heart disease - can be a disaster when it comes to dehydration. If you're drinking alcohol on a hot day, make sure you match every alcoholic drink with twice as much water. There shouldn't be much more to say this week. Unfortunately, dehydration and water-salt balances are more complex than that. If it were simple, we wouldn't have hundreds of cases of dehydration every summer. In Hong Kong, the issue is complicated by habits and culture. A study of blood chemistry a few years ago found that many Hong Kong women are continually mildly dehydrated. When the investigators checked what the women in this study were eating and drinking, they found that they were deliberately drinking very little. The toilets they had to use during the day at work were dirty, so they didn't want to have to use a toilet until they got home. Taxi drivers, who face a parking fine if they stop to find a toilet, often do the same thing. (Unfortunately, some use other methods, resulting in those half-filled 'water bottles' found in gutters and back streets.) Restricting your fluids, unless you're in kidney failure or on a specific medical regimen, is dangerous, especially during Hong Kong's summer months. But how much should you drink? This varies, depending on how much exercise you're doing, whether you're indoors or outdoors, doing physical work, or in air-conditioning (which dehydrates you). A good rule is to drink between one and two litres per day. That's a couple of large bottles or six to eight glasses of water. Extra fluid comes from your food or soup. Obviously, when you're exercising in hot weather, you should drink more. But you should drink before you feel thirsty. Every five minutes or so, you should take a gulp of water (about 10 to 20 millilitres), rather than down a whole bottle of water an hour. If you keep feeding the water into your system in small amounts, you'll keep all your systems ticking over efficiently and not risk overloading yourself with water when things have begun to go seriously wrong. Some people doing heavy exercise - such as army recruits on a full-gear run - drink too much water too often. Massive intakes can dilute the vital salts in your blood so much that your body chemistry becomes dangerously disturbed. A number of US Army recruits have died this way, so drink water in small amounts and often. Some trainers give athletes drinks with salts and sugar in them to keep their biochemistry balanced and provide regular energy boosts, but don't drink something too sugary or you'll have higher sugar levels and lower water levels than your body needs. Lastly, how can you tell if you're getting dehydrated? The first sign is lack of urine. Your kidneys kick in to help keep fluid in your body at the first sign that water levels are dropping in your blood. If you normally need to use the toilet every two hours, and find that time has lengthened to five or six hours, you're dehydrated. If your urine is much darker - closer to a tea colour - then you're dehydrated. If you, or your child, are producing no urine at all, you're seriously dehydrated and need to seek medical help - as well as drink fluid right away.