How much water do we need to drink a day?

When it comes to drinking water, we are drowning in advice and recommendations.

Most of the health messages we hear are about making sure we drink enough water, especially in hot weather, when you’ve had a tummy bug or when you’re exercising.

But how much is the right amount? And is it possible to drink too much?

Eight glasses?

You’ve probably heard that eight glasses a day is what you should aim for.

This advice appears to have come from a 1945 recommendation of the Food and Nutrition Board of the United States National Research Council, which stated: “A suitable allowance of water for adults is 2.5 litres daily in most instances … most of this quantity is in prepared foods.”

But really there’s no hard and fast rule on how much water you need to drink, said CSIRO dietitian Pennie Taylor.

How much water each one of us needs depends on a range of factors, such as our sex, bodyweight and how much physical activity we do.

Another thing to consider is where you live, those who live in a warmer and more humid climates tend to sweat more and lose more fluid.

Australia’s current dietary guidelines don’t recommend a specific amount of water, but simply recommend we ‘drink plenty of water’. The guidelines also encourage us to opt for water over juices, soft drinks, cordials or the like.

There are Nutrient Reference Values advising that adult men should drink 2.6 litres of water per day (about 10 cups) and adult women should drink 2.1 litres per day (about eight cups).

But these figures are based on the average weight of men and women, so if you’re underweight or overweight you may consider adjusting your fluid intake — Ms Taylor says a good rule of thumb is 35 millilitres of fluid per kilogram of bodyweight.

Also pregnant or breastfeeding women (who require more fluid), people who live or work in extremely hot climates, and people with high protein diets (the kidneys may need more fluid to help process the increased amount of protein) are encouraged to drink more water.

It’s also worth noting that other fluids can be counted towards your daily fluid intake. So juice, tea, coffee, and alcohol can all count.

On a hot day?

It’s on hot days that most of us notice we’re thirstier than normal.

This is because we’re sweating more, and we lose fluid through sweat — anywhere from 100 millilitres to several litres per day, depending on our activity levels and the temperature.

“We can lose between 1 to 3 per cent of our fluid quite easily,” Ms Taylor said.

Those who work or exercise in hot climates lose the most fluid — up to 2.5 litres of sweat in an hour in extreme circumstances, Associate Professor Ben Desbrow from Griffith University said.

“You need to replace those fluids pretty quickly, otherwise it’s going to fairly rapidly have an effect on your subsequent performance.”

How can you tell you’re not getting enough?

Your body will give you some pretty clear signs that you’re not getting dehydrated. So keep an eye out for symptoms such as a dry mouth, headache and feeling dizzy.

How much water why we need

Why do we need water?

Water makes up about 50-80 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.

Also pay attention to your toilet habits, the colour of your urine and how frequently you go to the toilet.

“Your kidneys do a great job in fluid regulation, so frequency of urination and colour of urination are your two best guides,” Associate Professor Desbrow said.

(It’s worth noting certain vitamin supplements and foods can change the colour of your pee, but this won’t affect how often you go.)

“My advice is never go to the toilet and not pay attention to what you’re seeing — it may seem simple, but it’s giving you messages,” Associate Professor Desbrow said.

Can we drink too much?

There is a thirst control centre in our brain that controls water intake, says Dr Michael McKinley, Senior Fellow at Florey Neuroscience Institute.

When we drink water, this part of our brain stops us feeling thirsty long before the water has been fully absorbed into the bloodstream.

“Usually if we take in too much water, it’ll suddenly feel like hard work to drink,” he said.

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However, in some circumstances when people drink a large volume of water, they can over-ride the thirst control centre in the brain.

When this happens, their sodium levels can drop too low. This can lead to a condition known as hyponatremia, where the body also starts to retain the excess water.

Our kidneys also help to keep things in balance.

“Normally if we drink too much water, our kidneys would excrete it [as urine],” Dr McKinley said.

But sometimes, factors like heat, physical stress or certain drugs can switch off the hormonal signal that causes the kidneys to excrete excess water.

Then there is a double whammy effect. Not only have you drunk a lot of water, but you start to hang onto all the water in your body. Drinking more just makes things worse.

“This is when things can get dangerous,” Dr McKinley said.

Hyponatremia, also known as water intoxication, can cause your brain cells to swell. It can lead to confusion, seizures, coma and even death.

While this condition is rare, one group at risk is endurance athletes.

Activity, especially in warm weather, can raise their body temperature too high. They are usually advised to drink plenty of fluid prior to an endurance event such as a marathon and feel they need to be hydrated. But if they overdo it, and quickly guzzle too much water, they can get into trouble.

“I remember reading about a woman who was running a marathon, she was drinking a lot of fluid and around the 18-mile mark, she started to feel confused and unwell. She had seizures, went into a coma and was dead within four hours due to hyponatremia,” says Dr McKinley.

Others at risk of hyponatremia include people with schizophrenia, who may have a compulsion to drink water.

“The condition is called psychogenic polydipsia and the people who have this have been known to drink 20, 30, 40 glasses a day. Hyponatremia-related deaths tend to be more common in this demographic, more so than athletes,” he said.

The drug ecstasy can also trigger water intoxication.