The mechanism that regulates fluid intake in the human body also stops us from over-drinking, said a multi-institute study led by Monash University in Australia. The study also challenges the popular idea we should drink eight glasses of water a day for health.
This study has revealed, for the first time, that too much fluid intake can cause potentially fatal "water intoxication." 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.
Drinking too much water puts the body in danger of water intoxication or hyponatremia. This is a condition where vital levels of sodium in the blood become abnormally low, potentially causing symptoms ranging from lethargy and nausea to convulsions and coma.
Associate Professor Michael Farrell from the Monash Biomedicine Discovery Institute oversaw the work by University of Melbourne PhD student Pascal Saker as part of a collaboration with several Melbourne institutes.
"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 Farrell.
Building on a previous study, 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," noted Farrell.
"This was compatible with our notion that the swallowing reflex becomes inhibited once enough water has been drunk."
Farrell, who works in the university's Department of Medical Imaging and Radiation Sciences, 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 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.
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" was published online in the journal, Proceedings of the National Academy of Sciences in the United States of America.
It was carried out in collaboration with the Florey Institute of Neuroscience and Mental Health, University of Melbourne and Baker IDI & Diabetes Heart Institute.