When a person is thirsty, a drink of water can be very satisfying, but after the thirst has been quenched, drinking more can be unpleasant. New research reveals the root of these experiences in the brain.

Researchers scanned the brains of people as they drank water. Brain areas involved in emotional decision-making lit up in the scanner when people drank in response to feeling thirsty, whereas regions involved in controlling movement kicked in when people forced themselves to keep drinking after quenching their thirst.

These brain circuits probably evolved to prevent people from drinking too much water, resulting in dangerously low sodium levels, the researchers reported March 24 in the journal Proceedings of the National Academy of Sciences. [7 Foods You Can Overdose On]

The instinct for thirst in humans and other animals likely evolved when vertebrates colonized land during the Ordovician period, about 400 million years ago. Thirst ensures that creatures maintain a balance of hydration and nutrients, such as sodium, that are vital to the healthy functioning of cells.

But what's going on inside the human brain when a person drinks to satisfy a thirst?

To find out, Pascal Saker, a neuroscientist at the University of Melbourne, Australia, and colleagues recruited 20 healthy men and women, and had them exercise on a stationary bike for one hour. Then they scanned the volunteers' brains using magnetic resonance imaging (MRI), which measures changes in blood flow to different brain areas.

During the first scan, the volunteers drank water to quench their thirst, and during a second scan, they were told to keep drinking water even though their thirst was quenched. The volunteers reported that drinking to satisfy their thirst felt pleasurable, whereas drinking excess water felt unpleasant.

In the scans taken while the participants drank water to quench their thirst, their brains lit up with activity in the anterior cingulate cortex and the orbitofrontal cortex — regions that play a role in emotional decision-making.

By contrast, the scans taken while the volunteers continued drinking water after they no longer felt thirsty showed activity in the brain's putamen, cerebellum and motor cortex, areas involved in controlling and coordinating movement. These areas may be involved in forcing oneself to keep drinking, even when the brain tells one to stop.

The post-drinking scans also showed activity in the midcingulate cortex, insula, amygdala and periaqueductal gray — areas involved in emotion, motivation and more basic brain functions.

It's a good thing the brain knows when to tell the body when it has had enough to drink. Drinking too much water can lead to dangerously low sodium levels, a condition known as hyponatremia, or cerebral edema (excess fluid in the brain).

People with schizophrenia sometimes drink too much fluid and develop these conditions, suggesting the brain disorder may affect the body's ability to regulate its balance of water.
 

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