Popular notion holds that eight glasses of water is the optimal amount to drink each day, but many people find this hard to achieve. Now, a new study may shed light on why, after identifying a swallowing mechanism that stops us from consuming too much liquid when we are not thirsty.
But how much water do we need to drink on a daily basis?
While you may have heard that eight 8-ounce glasses of water each day - known as the "8x8 rule" - is the aim, there is no scientific evidence that pinpoints precisely how much fluid is the optimal amount.
Based on studies to date, the Institute of Medicine (IOM) recommend that women should aim to drink around 2.2 liters of total beverages daily (around 9 cups), while men should aim to consume around 3 liters of total beverages daily (around 13 cups).
However, contrary to these recommendations and the so-called 8x8 rule, the new study suggests we should only drink when we are thirsty, after discovering a mechanism that makes drinking excess water challenging.
Study co-author Michael Farrell, of the Biomedicine Discovery Institute at Monash University in Australia, and colleagues publish their finding in the Proceedings of the National Academy of Sciences.
Right prefrontal cortex 'overrides' swallowing inhibition for excess water
For their study, the team enrolled a number of participants and asked them to drink large amounts of water immediately after exercise, when they were thirsty, and later on in the day, when they were not thirsty.
In each condition, the researchers asked the participants to rate how difficult it was to swallow water.
Compared with water consumption just after exercise, the participants found it three times more difficult to drink water later on when they were not thirsty.
"Here, for the first time, we found effort-full swallowing after drinking excess water which meant they were having to overcome some sort of resistance," says Farrell. "This was compatible with our notion that the swallowing reflex becomes inhibited once enough water has been drunk."
The team used functional magnetic resonance imaging (fMRI) on each of the participants, which allowed them to measure brain activity just before they swallowed water in each experimental condition.
They discovered that certain areas of the right prefrontal cortex of the brain showed significantly higher activity when participants had to make an effort to swallow the water, suggesting that this brain region "overrides" the swallowing inhibition to allow excess water consumption.
'Drink according to thirst'
Farrell and team stress that drinking too much water can cause significant harm, it can lead to hyponatremia, for example, where blood sodium levels are abnormally low.
"There have been cases when athletes in marathons were told to load up with water and died, in certain circumstances, because they slavishly followed these recommendations and drank far in excess of need," notes Farrell.
The researchers say their findings suggest that when it comes to water intake, we may fare better by listening to the body's needs.
"If we just do what our body demands us to we'll probably get it right - just drink according to thirst rather than an elaborate schedule."
Still, the team points out that water intake remains essential to human health, and there are certain groups - such as elderly individuals - who do not consume enough water.