Drinking too much water 'could kill you'

An investigation into the death of a Tasmanian hiker has turned the spotlight on a little-known condition caused by drinking too much.

Bushwalker Jonathan Dent became lost during a four-hour trek in Tasmania’s Dial Ranges in 2011.

He phoned his wife several times during the day to say he was lost but unfortunately he was not discovered until rescue crews found his body two days later.

A coronial enquiry this week found that the most likely died from hyponatremia, more commonly known as "water poisoning".

Water poisoning

So how is it that such a benign substance as water can kill you and why are we not more aware of the dangers?

Hyponatremia occurs when the body's sodium levels are diluted by excessive water consumption. The condition can be fatal because the symptoms feel to the victim a lot like dehydration.

A 2005 study, published in the New England Journal of Medicine, found that one sixth of marathon runners develop some degree of hyponatremia, and it has also been identified as a possible cause of six unexplained deaths of Australians hiking on the Kokoda Trail since 2006.

The dangers of 'over-hydration'

Dr Sean Rothwell was part of a research team who investigated the deaths and said people's understanding of the body's water requirements has become skewed.

"There is no scientific evidence that we need to drink eight glasses of water a day," he said.

"That has definitely been exaggerated … generally you should drink when you are thirsty.

Over-hydration does not do you any good, but it can definitely do you harm."

His team collected blood from nearly 200 people over a four-day period on the Kokoda Trail and found a small number were unwittingly suffering exercise-induced hyponatremia.

Worldwide, there have been 1600 reported cases of the condition and at least 16 deaths since it was identified. The condition most commonly affects ultra and endurance athletes, according to Dr Rothwell, because exertion releases a hormone that causes the body to retain water, further diluting bodily salts.

Problems also arise when amateur athletes are led to believe they need to drink too much, while not replenishing salts lost through sweat and urine.

"People who aren't athletes are in some ways more at risk, because they are inexperienced," Rothwell said.

Symptoms

He said people who drink more than a litre of water an hour and participate in heavy exercise could be at risk of hyponatremia. The symptoms can include nausea, weakness, headache and unsteadiness.

While these symptoms mimic dehydration, keeping track of fluid intake and urination is the key to determining if a person is suffering from water poisoning. Dr Rothwell's team also found the body could regulate on its own if patients stopped drinking water.

"If you drink a lot, then you should be passing a lot of fluid too, that is normal. The problems arise when the body retains all that water."

Sports drinks

The International Marathon Medical Directors Association (IMMDA) advocates 'drinking to thirst' for endurance runners, and no more.

In its guidelines for fluid replacement , the association recommends 0.03 litres per kilogram, or three litres for a 100 kg person, but suggests a sports drink for extended activity.

"If your event or workout is longer than 30 minutes you should be drinking a sports drink … There is actually decreased benefit to watering down or diluting sports drinks or alternating sports drinks with water," reads the IMMDA's advice. However, Dr Rothwell said relying on sports drinks is unwise as they are not designed for medical use.

"Sports drinks do not prevent Exercise Associated Hyponatremia. Although they do have some salt in them, it is not high enough in concentration to be preventative. They certainly won’t reverse the effects, they will actually make it worse."

"The important message is to only drink when you are thirsty."