New research is questioning the old ‘eight glasses of water a day’ adage.

An Australian study has revealed the mechanism that regulates fluid intake in the human body and stops us from over-drinking.

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

Associate Professor Michael Farrell says the body has a system for regulating water intake – thirst.

“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,” A/Prof Farrell claims.

In the latest experiment, 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 already drinking an excess amount of water.

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

“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,” Associate Professor Farrell said.

“This was compatible with our notion that the swallowing reflex becomes inhibited once enough water has been drunk.”

They then used functional magnetic resonance imaging (fMRI) 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 a lot of 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,” he said.

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

Associate Professor Farrell said elderly people, however, often didn’t drink enough and should watch their intake of fluids.

The study, ‘Overdrinking results in the emergence of swallowing inhibition: an fMRI study’ is available in PDF form, here.