Glug glug glug. I’m drinking a big glass of ice water after getting thirsty, and it’s flowing easily down my throat like a river. But a study of thirsty and well hydrated people suggests this isn’t always the case.
We rarely pay attention to the business of swallowing, but it may play a subtle role in controlling our fluid intake, on top of our conscious feelings of thirst. If we are dehydrated, swallowing is effortless; if we are overhydrated, swallowing feels more difficult, putting us off drinking, according to a study by Michael Farrell at Monash University in Melbourne, Australia, and his team.
“Normally it’s something we are not really conscious of – away it goes,” says Farrell. But when his team asked volunteers to rate the sensation of taking a small sip of water, they found that people who had recently drunk a lot of water said it took much more effort to swallow than those who were mildly hydrated – their difficult ratings rose from one out of ten to nearly five.
Is eight really great?
When people were overhydrated, brain scans showed that swallowing was linked with more activity in certain regions of the brain, including the prefrontal cortex, which is responsible for conscious thought processes.
“It suggests a mechanism for inhibition of drinking that we don’t usually think about,” says Zachary Knight at the University of California, San Francisco.
To put it to the test, after my first thirst-quenching drink I go on to have two mugs of tea over the next hour and a final large glass of water, making me unpleasantly bloated. Now comes the final “test sip”: true enough, the fluid seems to linger more at the roof of my mouth before I can force it down. I could just be imagining it, but my subjective experience does chime with the findings.
Farrell says the discovery is further evidence that controversial advice to deliberately drink fluids is wrong. “It shows we have several very subtle mechanisms for regulating the amount we drink. If left to your own devices, you will drink the requisite amount of water to maintain balance.”
Farrell says even people doing exercise just need to drink according to their thirst. “These are well refined mechanisms forged on the anvil of evolution.”
Journal reference: PNAS, DOI: 10.1073/pnas.1613929113