Doggedly following advice to drink at least eight glasses of water a day is said to be of no health benefit to most people and only enriches bottling companies.

And it makes people go to the toilet a lot.

Public health researcher Rob Quigley said yesterday adults typically needed around 2 litres of fluids a day, but did not need to obtain all of this by drinking water.

"We get a lot of fluid from food. Fruit and vegetables are upwards of 90-95 per cent water. Eating an apple a day is a little bit like drinking a glass of water."


Australian university lecturer Spero Tsindos, who examines water consumption in the latest edition of the Australian and New Zealand Journal of Public Health, said encouraging people to drink large amounts of water was driven by vested interests.

Pricier brands of bottled water can cost more than $3 a litre at New Zealand supermarkets.

A Pennsylvania University research review published in 2008 found no evidence that drinking eight glasses of water a day improved skin tone, aided dieting or prevented headaches (except those induced by hangovers).

A person's daily fluid needs, in addition to the quantities derived from food and plain water, can be supplied from tea, coffee and even moderate amounts of mildly alcoholic drinks such as beer - despite their mildly diuretic effects - and various other drinks.

"Out of all the fluids to drink," Mr Quigley said, "water is one of the best, then it's trim milk. Whatever you do, don't touch all the sugary drinks. Tea and coffee are fine; you've just got to watch your sugar.

"Your body is very good at thirst. Listen to your body's thirst mechanism. If you're thirsty, have something to drink."

But people who engaged in a lot of vigorous activity in hot conditions needed to ensure they drank enough, he said.

Excessive water consumption can be dangerous, causing the brain to swell. Two cases of fatal water intoxication associated with use of the illegal drug Ecstasy have been reported in New Zealand.