By now, we're fairly clear that the eight-glasses-of-water-per-day rule is bunk, but a new study reveals how we keep our thirst in check. The good news is that the body has a built-in limit that stops us from drinking too much, which means that if you like downing eight or more glasses a day, you can go ahead, as long as you listen to your body when it tells you to stop.
The study, from the University of Melbourne and the Monash University, shows that the brain activates a "swallowing inhibition" when we've had enough to drink. This mechanism makes it up to three times harder for us to get more liquid down. "Here for the first time we found effort-full swallowing after drinking excess water which meant subjects were having to overcome some sort of resistance," said lead author Michael Farrell of the Monash Biomedicine Discovery Institute.
In tests, participants were asked to rate the effort required to swallow water. They were asked in two situations. When thirsty after exercise, and later, when they were forced to drink more than they needed. Liquid was administered in five-milliliter doses, and participants were asked at every stage to rate the pleasantness of the taste of the water (or sugar solution), as well as the subjective effort required to swallow it. They were also monitored by an fMRI brain-scanner throughout the experiment.
Subjects reported that the effort required to swallow increased as they drank when in the already-sated state—that is, after they'd already had a lot to drink. The fMRI scan also showed increased activity in the brain, which correlated with the subjects' reports of the effort required to drink. The sugary solution gave the same results as plain water, and the "pleasantness" rating showed no correlation with any brain activity.
This "hard-wired" mechanism could be considered the opposite of thirst, which is our motivation to drink and replenish fluids. The "swallowing inhibition," then, could be seen as thirst's natural counterpart, and between the two the body manages the level of hydration to a surprisingly accurate degree. "The regulatory mechanisms that have evolved to govern the cessation of drinking appear to be tightly calibrated," says the study, "with the amount of fluid ingested commensurate with the degree of fluid depletion."
What does this all mean, in practical terms? Well, you can forget about the eight-glasses rule, and just wing it. "If we just do what our body demands us to we’ll probably get it right," says Farrell. "Just drink according to thirst rather than an elaborate schedule."