Drinking too much water may cause potentially fatal water intoxication, claims a new study which has for the first time identified the mechanism that regulates fluid intake in the human body and stops us from over-drinking.
The study, led by researchers from Monash University in Australia, challenges the popular idea that we should drink eight glasses of water a day for good health.
It 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.
"If we just do what our body demands us to we will probably get it right - just drink according to thirst rather than an elaborate schedule," said Michael Farrell, associate professor at Monash.
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 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. This was compatible with our notion that the swallowing reflex becomes inhibited once enough water has been drunk," Farrell said.
Researchers 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 much effort, suggesting the frontal cortex steps in to override the swallowing inhibition so drinking could occur according to the researcher's 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.
Elderly people, however, often did not drink enough and should watch their intake of fluids, said Farrell.
The study was published in the journal Proceedings of the National Academy of Sciences.
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