Q: Can you drink too much water?

A: Yes, drinking too much water could be fatal. But it's very rare.

At some point, most of us have experienced mild dehydration – it makes you feel thirsty, gives you a dry mouth and possibly a headache. But severe dehydration is very serious and can kill you.

So when it comes to drinking water, most of the health messages we hear are about making sure we get enough, especially in hot weather, when you've had a tummy bug or when you're exercising. But is it possible to drink too much water?

While it is possible, says Pennie Taylor a research dietitian at CSIRO, "it's rare in the general healthy population that someone would drink too much water".

"We are more concerned in the clinical settings where somebody might have chronic heart disease or heart failure or might have renal failure where they might have a little too much water that their body can't cope with, which increases their risks of overhydration."

When you consume too much water, the level of an important mineral, sodium, can drop too low. This can lead to a condition known as hyponatremia, which can lead to the body retaining too much water. Hyponatremia can cause your cells to swell, which is particularly harmful to the brain and can lead to seizures and even death.

Another group at risk of hyponatremia are elite athletes who can be at risk of heat stroke and feel they need to rehydrate, but if they overdo it they become water intoxicated. Even in this group however, hyponatremia is still unusual.

Symptoms of hyponatremia include confusion, disorientation, nausea and vomiting.

"It's just about knowing where your boundaries are and how much water you actually need. The biggest thing for people is to monitor fluid loss and intake if they are concerned," says Taylor.

Why we need it

Water makes up around 50 to 80 per cent of your lean body mass. As well as helping to maintain the balance of essential minerals, your body needs adequate water to help with:

  • temperature regulation, especially keeping you cool by allowing you to sweat when you get hot.
  • digestion and processing of food, by keeping your gastrointestinal tract moist to aid in the passing of the food through the gut.
  • absorption of nutrients and helping you to pass waste.

When you lose more fluid than you take in, you become dehydrated, this is a particular problem when you are:

  • very young – young children have a lower body weight and are more affected by fluid loss, especially when sick.
  • older – as you age you tend to not respond to thirst as quickly as you used to.
  • living with certain medical conditions, such as kidney disease.
  • unwell with a viral infection, such as gastroenteritis.
  • living in a hot climate.
  • physically active, especially in hot weather.

"If you're exercising out in the high heat and humid weather, for example, sometimes people think if it's overcast they're going to be okay, but if it's humid you're going to sweat more and lose more fluids," Taylor says.

Another group at risk of dehydration is people who have also had weight loss surgery, as they can find it difficult to take in an adequate amount of fluids throughout the day.

Eight glasses a day?

We've been told we need to drink at least eight glasses of water a day, but Taylor says there's no scientific evidence to support this. Health professionals agree men's bodies need around 2.6 litres and women's around 2.1 litres of water a day (pretty much eight glasses a day). But it turns out you get some of this from other drinks and food, Taylor says.

"Approximately four glasses of water, or a litre a day, is obtained from the food that we eat. So we get that from solid foods and things such as watermelon, tea, coffee, milk and yogurt, which all contribute to our total fluid load," Taylor says.

You can also include caffeinated drinks in your fluid intake. While these drinks do have a mild diuretic effect (ie they promote water loss) you get more fluid from them than you lose. However, alcohol makes you dehydrated.

On top of what you get from food and drinks, your body produces another 250mls of water a day when metabolising substances in the body, such as fat, protein and starch.

Taking all this into consideration, Taylor suggests you drink four to six standard glasses (250ml) of water per day. Obviously, you will need more if you are living in an extremely hot climate and are very active. Likewise, if you live in a very cold climate you may need to drink a little less.

So how do you know when you need to drink more water? It isn't rocket science, your thirst can be a guide or check the colour of your urine – it should be pale yellow.

Pennie Taylor is an accredited practising dietitian. She is the Senior Research Dietitian for the Clinical Research Unit at CSIRO Animal, Food and Health Sciences, she spoke to Jenny Pogson.

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