Brian Walker, a general practitioner and specialist in sports medicine, said distilled water was useless for replacing minerals lost through sweating.

'Distilled water has no minerals ... it can suck minerals out of your body and cause muscular cramps when you are dehydrated.'

Dr Walker said mineral water and filtered tap water were better options. Alternatively, he suggested dehydrated hikers lick a small amount of salt before gulping down a bottle of distilled water.

As the heatwave continues - with the mercury touching 37 degrees Celsius on Wednesday in some parts of Hong Kong and topping 33 yesterday, and an increasing number of people hit by heatstroke - doctors have urged people to drink at least eight glasses of water a day to stay hydrated.

Dietician Mimi Sham said sports drinks, such as Gatorade and Pocari, are also good cooling drinks, especially after strenuous exercise. 'There are lots of electrolytes, such as sodium and potassium, in these drinks.

'They speed up the gastric emptying rate, meaning that water leaves the stomach and reaches the intestine for absorption faster than normal,' she said.

Fruit juice is a healthy drink, but it can also do more harm than good to dehydrated people. Cindy Lam Lo-kuen, associate professor of the family medicine unit at University of Hong Kong said juice could do the opposite of quenching thirst as its osmolarity - the amount of dissolved substances in the liquid - could be higher than that of blood.

'If you prefer the taste of fruit juice, you have to dilute a glass of juice with three glasses of water to achieve the purpose of rehydration,' she said.

Ms Sham warned that drinking too much fruit juice with a high concentration of sugar could lead to diarrhoea.

Soft drinks are also bad choices. 'The high sugar concentration in soft drinks slows down water absorption in the body. The gas is another big disadvantage, as it makes you feel like you have drunk a lot of liquid but in reality you have not,' she said.

Tea, coffee, and the popular energy drink Red Bull should not be taken if you are dehydrated because of the high caffeine content. 'Caffeine is diuretic and can cause water loss,' Ms Sham said.

If you are a tea addict, she suggests drinking weak green tea.

Alcoholic drinks should also be avoided because they have a similar effect.

Dr Lam said people had to beware of the high humidity as well as the heat. 'High humidity can inhibit perspiration, which is vital for releasing heat,' she said.

'When your body keeps heating up, it will have a detrimental impact on all different sorts of chemical reactions taking place inside.'