The ‘eight glasses of water a day’ rule isn’t backed by evidence, so how much should we be drinking?
Many of us have heard the generic advice telling us to drink eight glasses of water every day, but that figure doesn’t appear to be rooted in any scientific facts.
That’s according to the authors of a new study, which looked out how much water people around the world drank each day.
And it turns out that there’s no blanket rule for how much water we should all be drinking — how much we need depends on different factors.
Is the eight-glasses-per-day rule bogus?
“The current study clearly indicates that one size does not fit all for drinking water guidelines,” researchers say in the study, published by Science late last month.
“The common suggestion that we should drink eight, 8-ounce glasses of water per day [about 2 litres] is not backed up by objective evidence.”
However, we already knew this. In fact, Australia’s water intake guidelines say there’s no one rule that applies to everyone.
Pennie Taylor — a senior research scientist and clinical dietitian for the CSIRO — explains it this way.
“The study continues to validate and reconfirm what we know about water being essential for survival and that the level of our daily water intake need is dependent on several factors: age, height, weight, general health and environmental factors — weather, altitude and humidity, for example,” Dr Taylor says.
“We need more fluids if we are unwell, have increased needs for pregnancy and lactation, or have high-exercise levels.
“These needs also increase if the weather is hot and humid.”
What’s really interesting about the study is that researchers produced an equation to predict a person’s water needs, taking in factors such as activity and humidity levels into account.
However, the researchers point out that water intake needs change, depending on people’s diets and say that no study — not even theirs — has been able to determine exactly how much water we get from food.
“If people consume a higher-energy-density diet with lower water content, then they may need more water from drinks and beverages,” they recommend.
“Without measured water intakes from food, it was not possible to assess the relative contributions of food and drinking water or beverages to water turnover in this study.”
Where did eight-glasses-per-day come from?
The eight-glasses-of-water-per-day rule is most commonly traced back to decades-old literature, with the nuance of the original messages lost over the years.
One of possible origins is a quote from the US National Research Council’s Food and Nutrition Board in the Nutrition Reviews journal, back in 1945:
“A suitable allowance of water for adults is usually 2.5 litres daily. Most of this quantity is contained in prepared foods.”
Another often-cited origin for the rule is a passage from a book called Nutrition for Good Health by Frederick J Stare and Margaret McWilliams, which was published in 1974:
“How much water each day? This is usually well regulated by various physiological mechanisms, but for the average adult, somewhere around 6 to 8 glasses per 24 hours and this can be in the form of coffee, tea, milk, soft drinks, beer, etc. Fruits and vegetables are also good sources of water.”
You’ll note that both quotes talk about water intake from food, suggesting the amount of water people need to drink each day is dependant on their diet.
Both suggest a general estimate of how much people should drink, but some people have interpreted the advice as a strict prescription.
What’s the point of guidelines then?
Dr Taylor says guidelines are a good starting point for people.
“Determining each person’s total fluid needs and considering all the factors that go into predicting needs is complex and, for the general person wanting to be healthier or drink more water, this information can be overwhelming and, therefore, not applied,” she says.
“This is where general guidelines can be helpful.
“They are simple enough to start someone on their way towards increasing their water intake, without complicating it.
“But guidelines can be too broad, and if a person is needing more or less — as they identify they are exercising more, become ill or are pregnant — these changes may warrant being more precise.
“Seeking support from an accredited practising dietitian can help to individualise one’s daily water or total fluid needs.”
The researchers also point out that guidelines can help inform decisions about water resources.
“Improved guidelines are of increasing importance because of the explosive population growth and climate change the world currently faces, which will affect the availability of water for human consumption and non-ingestive uses such as irrigation, cooling and manufacturing,” the researchers say.
“Presently, 2.2 billion people lack access to safe drinking water.
“The water turnover measures provided here can help to shape strategies for drinking water and water-enriched food management as the global population increases and the climate changes.”
What are the Australian guidelines for fluid intake?
Basically, they say that it’s different for everyone, but they do have vague guidelines.
“There is no single level of water intake that would ensure adequate hydration and optimal health for half of all the apparently healthy people in the population, in all environmental conditions,” the federal Health Department’s Eat for Health website says.
It says the water intake from foods versus fluids changes depending on people’s ages and points out that more active people in hotter climates may need more water.
It also changes depending on whether someone is pregnant or breastfeeding.
Here’s a breakdown of the department’s “adequate intake” guidelines for fluid intake:
|
Age range |
Female |
Male |
|---|---|---|
|
1-3 years |
1.0 L/day (about 4 cups) |
1.0 L/day (about 4 cups) |
|
4-8 years |
1.2 L/day (about 5 cups) |
1.2 L/day (about 5 cups) |
|
9-13 years |
1.4 L/day (about 5-6 cups) |
1.6 L/day (about 6 cups) |
|
14-18 years |
1.6 L/day (about 6 cups) |
1.9 L/day (about 7-8 cups) |
|
19 years and older |
2.1 L/day (about 8 cups) |
2.6 L/day (about 10 cups) |
Is anything wrong with drinking eight glasses a day?
“No, but you will need to monitor if this is too much or too little for you,” Dr Taylor says.
“If you are starting out, aim to stop when you feel full, sipping across the day is more likely to hydrate you than having two glasses in one sitting.
“If you find you are forcing yourself to drink up to eight to 10 glasses a day, speak to your GP or dietitian to identify if you fall into the category where more or less is needed.
“Keep note of intake for a few days of your intake, exercise and even the weather to discuss.”
Is it possible to drink too much water?
Yes.
But the Australian guidelines don’t have an upper level of intake for water consumption. Here’s what the website says:
“Excess water intake can cause hyponatraemia, but this is a rare occurrence in the general population.
“There are no data on habitual consumption resulting in specified hazards in apparently healthy people.
“In addition, there is a significant self-regulation of excess water consumption in healthy people in temperate climates.”
Hyponatraemia is a low concentration of sodium in the blood, Better Health Victoria’s website says:
“In severe cases, low sodium levels in the body can lead to muscle cramps, nausea, vomiting and dizziness.
“Eventually, lack of salt can lead to shock, coma and death.”
But again, those federal government guidelines say hyponatraemia is rare.
Does my fluid intake have to be just water?
The Australian guidelines say the definition of fluids incudes “plain water, milk and other drinks”.
“Ideally, water is best, but we all know it can be difficult to drink consistently,” Dr Taylor says.
“Milk and juice do count, in fact it is said that up to 20 per cent of water is consumed through core foods such as vegetables, naturally whole fruits, herbal tea, milk, yoghurt.
“However, aim not to have sugar-sweetened drinks, and juice can fall into this category as does flavoured milks, [so] choose wisely.”
What are some tips for boosting my water intake?
If you’ve monitored your water intake and don’t think you’re drinking enough, here some advice from Dr Taylor on how to boost it:
- Carry a water bottle with you
- Keep a water bottle on your desk or kitchen table and each time you pass it, take a sip
- If you don’t like the taste of water, slice up your favourite fruits or herbs — such as mint — and add them to the water. Letting them infuse overnight will add more flavour and adding a slice of lemon will enhance the taste of the fruit
- Keep a bottle of water on your bedside table and take a few sips before bed and when you wake up in the morning
- Set reminders in your mobile phone.
What foods boost our fluid intake?
Dr Taylor listed the following foods, not only because they are naturally 90 per cent more fluid, but also they’re also a good source of fibre:
- berries (fresh or frozen)
- cucumber
- celery
- iceberg lettuce
- spinach
- tomato
- watermelon
- zucchini
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