A new study led my Monash University has identified a mechanism that regulates fluid intake in humans for the first time. Their results show that it works to prevent humans from over-drinking, and challenges the popular eight glasses of water a day rule.

Image credits Tomasz Sienicki.

The paper, led by associate professor Michel Farrell of Monash’s Biomedicine Discovery Institute working in collaboration with University of Melbourne PhD student Pascal Saker, has shown that the brain kicks in a “swallowing inhibition” after excess liquid is consumed. This mechanism helps maintain water levels in the body — taking an active part in maintaining homeostasis.

Building on previous research, the team asked participants to rate how much effort it took to take a swig of water under two different conditions — first after exercise when they were thirsty, and the second time after they’ve already had their fill, so they drank an excess amount. They report it was three times as difficult for the participants to swallow after over-drinking. They used fMRI imaging to peer into the activity of various parts of the subjects’ brains, focusing on the period just before swallowing.

We found effort-full swallowing after drinking excess water which meant they were having to overcome some sort of resistance,” Farrell said.

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

The fMRI studies showed strong activity in the right prefrontal areas of the brain in participants who were trying to swallow in effort-mode. The team believes this shows that the area has to step in and overcome the swallowing inhibition so drinking could occur.

While it may sound a bit insignificant, these findings are pretty important — you can actually poison yourself if you drink too much water. Called hyponatremia, it causes sodium levels in the blood to drop, leading to lethargy-like states and nausea to full-blown convulsions, coma, even death. While this doesn’t happen very often, people undergoing heavy exercise are particularly at risk of over-hydrating.

“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,” Farrell said.

So how can you make sure you won’t overdrink? Well luckily, we have an in-build prevention system, as the study found.

“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,” Farrell concluded.

He does point out that people — most usually the elderly — often don’t drink enough, and should watch their intake of fluids.

So just let the thirst guide you, people.

The full paper “Overdrinking results in the emergence of swallowing inhibition: an fMRI study” has been published in the journal Proceedings of the National Academy of Sciences.