In a nutshell: Difficulty swallowing is confirmed as a way of metering water intake, and the parts of the brain involved are identified.

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It seems there’s always room in our stomach for that extra bite of dessert, but that’s not the case for water. Healthy people exquisitely meter the water they drink to match their hydration levels, although this ability may be lost in old age and certain diseases.

This brain imaging study by Pascal Saker from the Florey Institute of Neuroscience and Mental Health at the University of Melbourne and colleagues confirms that “swallowing inhibition” — the switch from effortless gulping to forced swallowing — is one lever that prevents overhydration.

In the study, twenty people worked up a thirst on an exercise bike and then climbed into a brain scanner for functional magnetic resonance imaging (or fMRI), which measures blood flow as a proxy for brain activity. The volunteers’ brains were scanned as they held for 7 seconds and then swallowed sips of water squirted into their mouth via a thin plastic tube. After each sip, they rated the effort it took to drink the water on a 0-10 scale.

They then drank as much water as they could — typically just over a litre — before jumping back into the scanner to repeat the experiment.

The scans showed that when people try to swallow after over-drinking — which required three-times as much effort as when thirsty — parts of the prefrontal cortex, a brain region that controls goal-directed behaviours, are more active. That’s because swallowing — usually an automatic, unconscious act — becomes a deliberate task requiring will, says co-author Michael Farrell from Monash University. Not even spiking the water with sugar lessened the effort.

Swallowing inhibition exists, says Farrell, because “if you over-drink, there’s a very real risk that you could kill yourself.”

Next steps:
The team will test whether stimulating the prefrontal cortex using transcranial magnetic stimulation affects how easy it is to swallow.

Saker, P., Farrell, M.J., Egan, G.F., McKinley, M.J., & Denton, D.A. (2016). Overdrinking, swallowing inhibition, and regional brain responses prior to swallowing. Proceedings of the National Academy of Sciences, 113(43), 12274–12279.

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