Forget eight glasses a day — just drink when you're thirsty

Results from a study led by Monash University challenge the popular idea that humans should drink eight glasses of water a day for health.

The research showed that a ‘swallowing inhibition’ is activated by the brain after excess liquid is consumed, helping maintain tightly calibrated volumes of water in the body.

Building on a previous study, the researchers asked participants to rate the amount of effort required to swallow water under two conditions: following exercise, when they were thirsty; and later, after they were persuaded to drink an excessive amount of water.

The results showed a three-fold increase in effort after overdrinking.

Functional magnetic resonance imaging (fMRI) was used to measure activity in various parts of the brain, focusing on the brief period just before swallowing.

The fMRI showed the right prefrontal areas of the brain were much more active when participants were trying to swallow with much effort, suggesting the frontal cortex steps in to override the swallowing inhibition so drinking could occur according to the researchers’ instructions.

“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,” said Associate Professor Michael Farrell, who oversaw the collaborative study.

Drinking too much water puts the body in danger of water intoxication or hyponatremia, where vital levels of sodium in the blood become abnormally low, potentially causing symptoms ranging from lethargy and nausea to convulsions and coma.

“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,” said Associate Professor Farrell, who cautioned that elderly people often didn’t drink enough and should watch their intake of fluids.

‘Overdrinking results in the emergence of swallowing inhibition: an fMRI study’ is published online in the US journal Proceedings of the National Academy of Sciences. It was carried out in collaboration with the Florey Institute of Neuroscience and Mental Health, the University of Melbourne and the Baker IDI & Diabetes Heart Institute.