Just add water

Take a look around. Right this second we bet there's a water bottle within a three-metre radius of where you are. How did it get there?

A drop of physiological need mixed with an ocean of marketing savvy. But, as with any megatrend, you have to stop and ask, "What's really going on?" When a water bottle becomes a must-have item on a planet that's 70 per cent liquid, some of the reasoning is bound to be hogwash.

MYTH You need eight glasses a day

FACT No one's sure where this "rule" came from, says Dr Heinz Valtin, a nutritionist and author of two studies on the origin of the theory that the human body works best on eight 235ml glasses of water a day. The truth is, your daily requirement depends on your diet, size and body chemistry. To determine how much water you should drink, weigh yourself each morning for three to four days in a row - pick a time other than your period to rule out hormone-induced water retention. If you lose half a kilo in a day, it means you came up short on liquids the day before. Drink half a litre of water first thing in the morning for every half kilo you've lost and adjust your daily intake until your weight is steady.

MYTH Drink only when you're thirsty

FACT Sedentary people might do fine using this mantra, but anyone who occasionally feels the urge to be active need not subscribe. "Exercise blunts your thirst mechanism," says sports dietitian Leslie Bonci. "You lose fluid so rapidly that the brain can't respond in time." In fact, a recent study from Maastricht University in the Netherlands found that women lose more water during exercise than men. An hour before you hit the gym, grab an extra 600ml so you can hydrate before you dehydrate. "It takes 60 minutes for the liquid to travel from your gut to your muscles," Bonci says.

MYTH Tea and coffee dehydrate you

FACT Down two long blacks and you'll visit the ladies' room often enough to earn a VIP pass. But despite its speedy exit, the liquid in your favourite morning caffeine boost still counts towards your hydration goal. After all, it's basically water, unless you muck it up with flavoured syrups. "Caffeinated beverages do not dehydrate you when consumed in moderation, that is, five cups or less per day of coffee, tea or cola," says Dr Lawrence Armstrong, author of Performing in Extreme Environments. In fact, Dr Armstrong says that any fluids you ingest will help keep your cells saturated, including juice, iced tea or soft drinks. (Just keep an eye on sugar levels and kilojoule count.)

MYTH Bottled water is better than tap

FACT Unless you're travelling through South East Asia, what comes from the tap is as nutritious as water gets. Tap water contains minerals like sodium, calcium, magnesium and zinc. Purified and distilled waters are boiled during processing to strip them of any trace minerals. Shop-bought H2O also lacks the fluoride that's sprinkled into the water supply to keep your teeth healthy. (If you want to know more, go to nhmrc.gov.au for info on the Australia Drinking Water Guidelines.) Plus you're hurting the planet every time you buy another bottle - according to the Bottled Water Alliance, Australia's current annual use of bottled water generates more than 60,000 tonnes of greenhouse gas emissions. (You're also wasting cash - the cost of one litre of bottled water is up to 2500 times the cost of tap water. For some smart water choices, see right.)

MYTH Drinking water before a meal helps you lose weight

FACT The water you drink before or during a meal won't keep you from overeating, and it won't flush food out faster from the body, says nutrition and weight-loss expert Dr Barbara Rolls, author of The Volumetrics Eating Plan. "Water doesn't bind to food, so it empties out of the gut really quickly," she says. You can pad your meals with water in other ways to help cut kJs. When water is contained in foods like vegetables, it travels through the stomach and into the intestines along with the rest of the meal, making you feel full without adding to the meal's kilojoule count. "If you just drink water, you only satisfy thirst mechanisms, whereas foods that contain a lot of water satiate hunger and hydrate you as well," Dr Rolls says. Choosing broth-based soups like chicken noodle or juicy fruits and vegies like watermelon, cucumber and tomato is an easy way to fill up on water.

MYTH I can't get dehydrated while swimming

FACT You're actually more likely to become dehydrated when you spend an extended period of time in the pool, Dr Armstrong says. "Part of the reason is psychological; when you come out of the pool, the last thing you want to look at is a glass of water," he says. But physiology also comes into play. "Thirst is controlled by the volume of blood at the centre of the body," Dr Armstrong says. So when the brain senses a lack of blood at your core, you reach for your glass. But water – in the pool, not the bottle – creates a hydrostatic pressure that pushes blood from your skin to the centre of your body, tripping up the system.